The distinct functional roles of the inner and outer chloroplast envelope are revealed by
proteomic approaches
Elain Gutierrez-Carbonell1, Daisuke Takahashi2, Giuseppe Lattanzio1, Jorge
Rodríguez-Celma1, Jürgen Soll3, Katrin Philippar3, Julia Kehr4*, Matsuo Uemura5,
Javier Abadía1, Ana Flor López-Millán1
1
Plant Nutrition Department, Aula Dei Experimental Station, CSIC, P.O. Box 13034, 50080
Zaragoza, Spain; 2United Graduate School of Agricultural Sciences, Iwate University,
Morioka 020-8550, Japan; 3Plant Biochemistry and Physiology, Department Biology I,
Ludwig-Maximilians-Universität München, Großhadernerstr. 2-4, D-82152
Planegg/Martinsried, Germany; 4Center for Plant Biotechnology and Genomics (UPM-INIA),
Montegancedo Campus, Freeway M40, km. 38, 28223-Pozuelo de Alarcón, Madrid, Spain; *
Current address: Department Molecular Plant Genetics, University Hamburg, Biocenter Klein
Flottbek, Ohnhorststrasse 18, 22609 Hamburg, Germany 5Cryobiofrontier Research Center,
Faculty of Agriculture, Iwate University, Morioka 020-8550, Japan.
Corresponding author:
Ana Flor López-Millán
Avenida de Montañana 1005, 50059 Zaragoza, Spain
Tel; 34 976 716068
Fax: 34 976 716145
email: anaflor@eead.csic.es
Running title: Proteomic profile of inner and outer chloroplast envelope
Summary (300 words)
The protein profiles of inner (IE) and outer (OE) envelope membrane preparations from P.
sativum were studied using two different proteomic approaches, shotgun nLC-MS/MS and
two-dimensional electrophoresis. In this study, we used the relative enrichment of each
protein in the IE/OE pair of membranes to provide an integrated picture of the actual
composition and dynamics of the system. A total of 589 protein species were identified in
chloroplast envelopes using the two different proteomic approaches. From the 546 proteins
identified with the shotgun approach 321 (59%) show significant changes in allocation
between OE and IE, and from these 295 (92%) were found only with this technique, whereas
26 (8%) were also found in the 2-DE gels. Two groups of proteins were clearly separated
with the shotgun approach, with 180 and 141 of them having larger signal intensities in IE
and OE preparations, respectively. Regarding the 2-DE approach, 69 proteins were found,
with 43 not found with the shotgun approach. Functional classification of identified protein
species reveals that IE is clearly enriched in proteins related to transport, protein metabolism
and photosynthesis, whereas proteins preferentially associated to OE are more heterogeneous
and participate in a wide range of metabolic processes. Relative enrichments clearly indicate
that in the envelope the IE is the selective barrier for the transport of a wide range of
metabolites, and also plays a major role in controlling membrane protein homeostasis.
Furthermore, based on our data, metabolic processes previously described to occur in the
envelope, including chlorophyll and tocopherol biosynthesis, can be ascribed preferentially to
IE whereas others such as carotenoid or lipid biosynthesis occur in both IE and OE. Our
results allow assignation of envelope proteins to the IE and/or the OE and also revealed new
protein species in envelope preparations that had not been previously found in the
chloroplast.
Introduction
Chloroplasts, the green photosynthetic plastids responsible for energy capture in plants, are
surrounded by a double membrane system, the chloroplast envelope. The chloroplast
envelope consists of an inner (IE) and an outer (OE) membrane and controls the bidirectional
traffic flow of a diverse range of substances between the cytosol and the biochemically active
chloroplast interior, and therefore is essential for proper cell functioning. Because the
chloroplast is a semiautonomous organelle, nuclear encoded proteins need to be transported
across this membrane system and this process is achieved via the joint action of translocons
located at the OE and IE [TOC and TIC, respectively; reviewed in (1-3)]. In addition, the
envelope controls the exchange between the stroma and the cytosol of many ions and
metabolites needed for metabolic processes. For instance, inorganic nitrite and sulphate need
to cross the envelope to be reduced within the chloroplast stroma (4-6), whereas some amino
acids and fatty acids synthesized in the chloroplast need to cross the envelope for their
distribution within the cell (7). Accordingly, transporters of amino acids, dicarboxylic and
fatty acids, sugars and inorganic ions, have been found in the chloroplast envelopes of several
plant species (4, 8-11). Other organic compounds such as cofactors and vitamins are also
likely to cross the envelope, although the molecular mechanisms of transport remain
uncharacterized (12).
The envelope is not only a selective barrier between the chloroplast and the cytosol, since
several unique biosynthetic pathways are present in this membrane system [reviewed in (12,
13)]. The chloroplast envelope plays an important role in certain aspects of fatty acid and
glycerolipid metabolism, including biosynthesis and desaturation, and also contains enzymes
involved in the formation of isoprenoid plastid constituents such as carotenoids, chlorophylls
and prenyl-quinones (12). Envelopes are also involved in signaling, since they are the site for
the production of molecules such as jasmonate and abscisic acid involved in plastid-to-
nucleus communication (12). On the other hand, the presence in the envelope of antioxidant
enzymes such as ascorbate and gluthatione peroxidases has lead to hypothesize the
participation of the envelopes in redox defense mechanisms (12, 14).
A substantial body of knowledge about envelope proteins has arisen from proteomic
approaches using spinach and Arabidopsis (4, 8, 15). In spinach, a proteomic approach
identified 54 proteins, 27 of them not previously reported in this plant compartment (4).
Later, a combined proteomic and in silico study reported more than a hundred envelope
proteins in Arabidopsis (8). A curated list, including 226 envelope proteins was later
published using a combination of the data reported until then (12), and AT_Chloro, a
database dedicated to sub-plastidial localization of Arabidopsis chloroplast proteins, was later
released (1, 16). A total of 1,323 proteins were identified in chloroplasts and statistical
analysis allowed the accurate localization of 819 proteins after cross-contamination analyses
(1, 16). From this subset, “the whole envelope proteome” included 460 proteins whereas “the
specific envelope proteome” (that containing proteins with increased abundance in the
envelope when compared to other sub-plastidial compartments) included 298 proteins (1, 16).
Their functional classification indicated that unknown, metabolism, transport, translation,
chaperone, and protease protein classes comprised more than 80% of the chloroplast
envelope proteins (1, 16). Recently, the chloroplast envelope proteomes of pea, Arabidopsis
thaliana and Medicago sativa were compared, and 247 envelope proteins were identified
(17). In addition, comparative proteomics between pea and maize has shown that C3- and C4type envelopes have qualitatively similar but quantitatively different membrane proteins (18).
The allocation of chloroplast envelope proteins to the OE and IE has been mainly
performed based on in silico studies and extensive literature search (1, 16, 19). This approach
has some limitations, especially for the identification of OE proteins, most of which are
synthesized without a chloroplastic targeting sequence (16). Recently, the proteome of the
OE and IE of pea has been studied using nHPLC-MALDI/QTOF, and 49 and 50 proteins
were assigned to the IE and OE, respectively (17). Overall, the proteomic characterization of
the IE and OE membranes is challenging due to: i) the difficulties in obtaining highly purified
IE and OE membrane fractions, 2) the inherent problems involving the separation and
solubilization of highly hydrophobic proteins within these fractions, and 3) the relative low
abundance of the envelope proteome, which accounts for only 1-2% of the chloroplast
proteins (16). A detailed protein profile of these membrane subsystems will thus help to
clarify their respective roles in chloroplast functioning and biogenesis.
The aim of the present study was to characterize the protein profiles of IE and OE
membranes from the chloroplast envelope of Pisum sativum, using two complementary
proteomic approaches, 2-DE and label-free shotgun proteomics. 2-DE electrophoresis is
widely used in plant proteomics (20), and label free shotgun proteomics has been successfully
used to study the proteome of membranes such as that of the plant plasma membrane (21,
22). Pea was chosen as a model system because the method for the separation of IE and OE
membranes is well established (23) and because the available EST sequencing data (24)
facilitates annotation and comparison with Arabidopsis thaliana genes.
Experimental Procedures
Plant material and envelope membrane preparation
Pea plants (Pisum sativum L., cv. ‘Arvica’, Prague, Czech Republic) were grown on sand in
the greenhouse under a 16 h light (220 µmol m-2 s-1)/8 h dark regime at 21 °C. Preparations of
OE and IE membrane fractions were obtained from intact, Percoll-purified chloroplasts,
isolated from leaves of 10-day-old pea plantlets as described in (25) and (23). Chloroplasts
were ruptured in a Dounce-homogenizer after hypertonic treatment in 0.65 M sucrose on ice,
and re-adjusted to isotonic conditions by two-fold dilution. Intact chloroplasts and thylakoid
membranes were removed by low speed centrifugation (4150g, 4 °C, 10 min), followed by a
second two-fold dilution and centrifugation at 196000g, 4 °C for 1 h. The light-green
supernatant consisting of crude envelope membranes was removed, and IE and OE
membranes separated on a discontinuous sucrose gradient (0.996, 0.800 and 0.465 M) by
centrifugation (137000g, 4 °C, 3 h), resulting in an opaque (OE) and a light yellow-green (IE)
fraction. Each fraction was diluted four-fold, centrifuged (137000g, 4 ° C, 1 h), the pellet resuspended in 0.5 M NaiPO4 buffer (pH 7.9) and frozen in liquid N2. Final protein
concentration was approximately 2.5 µg µL-1. For details and buffer compositions during
isolation, see (23). In order to obtain a preferably broad coverage of proteins occurring in the
envelope, including integral membrane proteins as well as those associated with the
membrane, we did not carry out further purification steps. One-dimensional gels of the IE and
OE (silver-stained) are shown in Supplementary Figure 1. Specific proteins occurring in the
IE and OE preparations were detected using specific antibodies and used to assess membrane
enrichment in the different fractions.
Label free nano-liquid chromatography-tandem mass spectrometry (nLC-ESI-MS/MS)
Label free nLC-ESI-MS/MS analysis was made with four independent biological replicates
(i.e., each one from a different batch of plants) for the two membrane types (IE and OE).
Sample preparation for label free nLC-ESI-MS/MS was carried out according to (21).
Briefly, purified OE and IE fractions were subjected to 1-DE to remove non-protein
compounds. The resulting gel bands were cut into four pieces, proteins were in gel digested
with trypsin, and subsequently peptides were extracted.
Peptide solutions were concentrated in a trap column (Lcolumn Micro 0.3 x 5 mm; CERI,
Japan) using an ADVANCE UHPLC system (Michrom Bioresources, Aubum, CA, U.S.A.).
Elution was carried out with 0.1% (v/v) formic acid in ACN and concentrated peptides were
separated in a Magic C18 AQ nano column (0.1 x 150 mm; Michrom Bioresources) using a
linear gradient of ACN (from 5% to 45%) and a flow rate of 500 nL min-1. Ionization of
peptides was carried out with a spray voltage of 1.8 kV using an ADVANCE spray source
(Michrom Bioresources). Mass spectrometry analysis was carried out on an LTQ Orbitrap
XL (Thermo Fisher Scientific, Waltham, MA, U.S.A.) equipped with Xcalibur software
(version 2.0.7, Thermo Fisher Scientific). Under the data-dependent scanning mode, full scan
mass spectra were obtained in the range of 400 to 1800 m/z with a resolution of 30000.
Collision-induced fragmentation was applied to the five most intense ions at a threshold
above 500.
Mass data analysis was performed according to (22). For semi-quantitative analysis of IE
and OE membrane proteins, Progenesis LC-MS software (version 4.0, Nonlinear Dynamics,
Newcastle, U.K.) was applied to experimental data files according to the software’s
instructions. Briefly, raw MS/MS files were imported to Progenesis software. In this process,
compared peptides were analyzed with ANOVA (p<0.05). The resultant peptides were
analyzed and identified with the Mascot search engine (version 2.3.02, Matrix Science,
London, U.K.) by comparison with the NCBInr Green Plants database (version 20120616;
18538238 sequences; 6354146082 residues). Search parameters were: peptide mass tolerance
±5 ppm, MS/MS tolerance ±0.6 Da, one allowed missed cleavage, allowed fixed modification
carbamidomethylation (Cys), and variable modification oxidation (Met) and peptide charges
were set to +1, +2 and +3. Positive identification was assigned with at least two unique topranking peptides with scores above the threshold level (p < 0.05). Only proteins identified in
all four biological replicates were considered for this analysis. The list of peptides identified
and quantified by shotgun proteomic and Progenesis LC-MS analyses are listed in
Supplementary Table 1.
2-DE
Gels were made from two independent IE and OE membrane preparations (i.e., each from a
different batch of plants) with four technical replicates each (n=8). Preliminary 2-DE
experiments were carried out using a first dimension IEF separation with a linear pH gradient
3–10; in these conditions most of the spots were located in the central region of the 2-DE gel
in the OE preparations and evenly distributed throughout the gel for the IE preparations
(results not shown); therefore, to prevent protein co-migration and improve resolution a
narrower pH gradient (5-8) was chosen for the OE samples. First dimension IEF separations
were carried out in a Protean IEF Cell (BioRad, Hercules CA, USA) on 7 cm ReadyStrip IPG
Strips (BioRad) with linear pH gradients 5-8 for OE and 3-10 for IE preparations. Strips were
rehydrated for 16 h at 20ºC in 125 µL of rehydration buffer containing 80 µg (for OE) or 45
µg (for IE) of envelope proteins and a trace of bromophenol blue, and then transferred onto a
strip electrophoresis tray. IEF was run at 20ºC, for a total of 14000 V h (20 min with a 0-250
V linear gradient, 2 h with a 250-4000 V linear gradient and 4000 V until 10000 V h). After
IEF, strips were equilibrated for 10 min in equilibration solution I [6 M urea, 0.375 M TrisHCl, pH 8.8, 2% (w/v) SDS, 20% (v/v) glycerol, 2% (w/v) DTT] and for another 10 min in
equilibration solution II [6 M urea, 0.375 M Tris-HCl pH 8.8, 2% (w/v) SDS, 20% (v/v)
glycerol, 2.5% (w/v) iodoacetamide].
For the second dimension SDS-PAGE, equilibrated IPG strips were placed on top of
vertical 12% SDS-polyacrylamide gels (8 x 10 x 0.1 cm) and sealed with melted 0.5%
agarose in 50 mM Tris-HCl, pH 6.8, containing 0.1% SDS. SDS-PAGE was carried out at 20
mA per gel for approximately 1.5 h, until the bromophenol blue reached the plate bottom, in
a buffer containing 25 mM Tris, 192 mM glycine, and 0.1% SDS, at 4 ºC. Gels were
subsequently stained with Coomassie-blue G-250 (Serva, Barcelona, Spain).
Gel Image and statistical analysis
Stained gels were scanned with an Epson Perfection 4990 Photo scanner at 600 dpi (Epson
corporation, Barcelona, Spain). Experimental Mr values were calculated by mobility
comparisons with Precision Plus protein standard (BioRad) run in a separate lane on the SDSgel, and pI was determined by using a 3-10 linear scale for IE and 5-8 for OE preparations.
Spot detection, gel matching and interclass analysis were performed with PDQuest 8.0
software (BioRad). First, normalized spot volumes based on total intensity of valid spots
were calculated for each 2-DE gel and used for statistical calculations of protein abundance;
for all spots present in the gels, pI, Mr, and normalized volumes (mean values, SD, and CV)
were determined. Only spots consistently present in 100% of the replicates (all eight gels)
were considered as consistent and used in further analysis. The spots were also manually
checked, and a consistent reproducibility between normalized spot volumes was found in the
different replicates.
Protein in gel digestion of 2-DE spots
Spots were excised automatically using a spot cutter EXQuest (BioRad), transferred to 500
µL Protein LoBind Eppendorf tubes, destained in 400 µL of 40% [v/v] acetonitrile (ACN)
and 60% [v/v] 200 mM NH4HCO3 for 30 min and dehydrated in 100% ACN for 10 min. Gel
pieces were dried at room temperature and then in gel digested with 15 µL Trypsin solution
(Sequencing Grade Modified Trypsin V511, Promega, Madison, WI, US; 0.1 µg μL−1 in 40
mM NH4HCO3/9% ACN). After incubation for 5 h at 37 °C, the reaction was stopped by
adding 1 μL of 1% TFA. The peptide solution was finally analyzed using mass spectrometry
(MS).
Protein identification by nano-liquid chromatography-tandem mass spectrometry (nLC-ESIMS/MS) of 2-DE spots
Peptides present in 5 µL of sample were pre-concentrated on line onto a 300 µm i.d. x 5 mm,
5 µm particle size ZORBAX 300SB-C18 trap column (Agilent Technologies, Waldbronn,
Germany), using a 100 μL min-1 flow rate of 3% ACN, 0.1% formic acid, in a nano-HPLC
system 1200 series (Agilent Technologies). Backflow elution of peptides from the trap
column was carried out, and separation done with a 75 µm i.d. x 150 mm, 3.5 µm particle
size ZORBAX 300SB-C18 column (Agilent Technologies), using a 300 nL min-1 nano-flow
rate and a 55 min linear gradient from solution 97% A (0.1% formic acid) to 90% of solution
B (90% ACN, 0.1% formic acid). The nano-HPLC was connected to a HCT Ultra highcapacity ion trap (Bruker Daltoniks, Bremen, Germany) using a PicoTip emitter (50 μm i.d.,
8 μm tip i.d., New Objective, Woburn, MA, USA) and an on line nano-electrospray source.
Capillary voltage was -1.8 kV in positive mode and a dry gas flow rate of 10 L min-1 was
used with a temperature of 180 °C. The scan range used was from 300 to 1500 m/z. The mass
window for precursor ion selection was ±0.2 Da and the rest of parameters were those
recommended by the manufacturer for MS/MS proteomics work. Peak detection,
deconvolution and processing were performed with Data Analysis 3.4 (Bruker Daltoniks).
Protein identification was carried out using the mascot search engine (Matrix Science;
London, UK) and the non-redundant databases NCBInr 20120204 (17172511 sequences;
5891506632 residues) and Plants_EST EST_110 (151512996 sequences; 26700120026
residues). Search parameters were: monoisotopic mass accuracy, peptide mass tolerance ±0.2
Da, fragment mass tolerance ±0.6 Da, one allowed missed cleavage, allowed fixed
modification carbamidomethylation (Cys), and variable modification oxidation (Met).
Positive identification was assigned with Mascot scores above the threshold level (p<0.05), at
least 2 identified peptides with a score above homology, 10% sequence coverage and similar
experimental and theoretical molecular weight and pI values. We used the GO biological
process annotation (http://www.geneontology.org) of the individual identified proteins for
classification.
In Silico analysis of subplastidial localization of identified pea proteins
The UniProt entry and the GO biological process annotation were obtained from the NCBI
entries using the PIR id Mapping tool (pir.georgetown.edu/pirwww/search/idmapping.shtml).
Results and Discussion
Protein profiling of membrane preparations enriched in IE and OE using shotgun
proteomics and 2-DE
The protein profiles of IE and OE membrane preparations from P. sativum were studied using
a shotgun nLC-MS/MS approach. A total of 546 protein species (NCBI entries) occurred in
all four biological replicates of at least one membrane type and were positively identified.
Due to the high sensitivity of the methodology used, all but one of the proteins identified
(545) were detected both in the IE and OE, although the relative abundances in each
membrane fraction were very different in many cases. To determine the relative enrichment
of proteins in the IE and OE fractions, a semi-quantitative analysis was carried out using
peptide signal intensity data to calculate the percentage of abundance (as [signal in IE/(signal
in IE + signal in OE)]). From the 546 identified protein species, 321 showed significant
differences in signal intensity between the IE and OE preparations (ANOVA, p<0.05;
Supplementary Table 2). Two groups of proteins were clearly separated, with 180 and 141 of
them having larger signal intensities in IE and OE preparations, respectively (Figure 1). The
225 proteins not showing significant differences in signal intensity between the IE and OE
preparations are listed in Supplementary Table 3.
The protein profiles of membrane preparations enriched in chloroplast inner (IE) and outer
envelope (OE) from Pisum sativum were also studied using 2-DE (IEF-SDS-PAGE
electrophoresis). Two biological and four technical replicates were analyzed for each
membrane type. Typical real scans of 2-DE gels obtained from IE and OE membrane protein
extracts are shown in Figure 2A,B, respectively. The average number of detected spots (mean
± SD; n=8) in gels obtained from IE and OE membranes was 182 ± 8 and 189 ± 6,
respectively. The number of consistent spots (those present in 100% of the gels of each
membrane type) was 160 and 181 for the IE and OE preparations, respectively. All consistent
spots were analyzed by nLC-MS/MS, and 92 and 59 spots in the IE and OE membrane
preparations were unambiguously identified as protein species. The location of these spots is
shown in the composite averaged maps in Supplementary Figure 2, and details on the protein
identifications are shown in Supplementary Table 4. These proteins corresponded to 50 and
33 NCBI protein species in IE and OE, respectively. The total number of protein species
identified with the 2-DE approach was 69, because 14 of the proteins were found in both IE
and OE.
In summary, a total of 589 protein species (NCBI entries) were identified in chloroplast
envelopes using the two different proteomic approaches (Supplementary Tables 2-4). The
overlaps between the two techniques are shown using Venn diagrams (Figure 3A). From the
546 proteins identified with the shotgun approach 321 show significant changes (59%), and
from these 295 (92%) were found only with this technique, whereas 26 (8%) were also found
in the 2-DE gels (14 in IE, 5 in OE and 7 in both). Regarding the 2-DE approach, 69 proteins
were found, with 43 not found in the shotgun approach: 22 in IE, 14 in OE and 7 in both
membrane preparations.
Proteins were also assigned to Arabidopsis gene identifiers (AGI codes; Supplementary
Tables 2-4), and a total of 417 AGI were found using the combination of proteomic
approaches. The corresponding Venn diagrams are shown in Figure 3B. From the 392 AGI
identified with the shotgun approach 229 show significant changes (58%), and from these
199 (87%) were found only with this technique, whereas 30 (13%) were also found in the 2DE gels (13 in IE, 6 in OE and 11 in both). Regarding the 2-DE approach, 48 proteins were
found, with 18 not found in the shotgun approach: 12 in IE, 5 in OE and 1 in both membrane
preparations (Figure 3B).
The 247 non redundant AGI identified in the two chloroplast envelope membrane
preparations (229 by shotgun and 18 by 2-DE; Supplementary Tables 2, 4) were contrasted
with the localization proposed in the At_CHLORO database. Among them, 71 were
described in the AT_CHLORO database as envelope-enriched proteins, whereas 47 were
assigned to the envelope and others (thylakoid or stroma), 51 were ascribed to other
subplastidial compartments and 78 more were not previously accounted for as chloroplastic
proteins (Figure 4).
Delving into the IE functions using shotgun proteomics and 2-DE
Proteins (NCBI entries) were then classified according to their functional category (Figure 5).
Transport (44 proteins; detected only by shotgun), photosynthesis light harvesting related
processes (45 proteins; 39 by shotgun and 6 by 2-DE), and proteolysis (30 proteins; 27 by
shotgun and three by 2-DE) were the largest functional categories of proteins associated
preferentially to the IE (Figure 5; Supplementary Tables 2, 4). Also, 38 proteins (34 in
shotgun and four detected only in 2-DE) had unknown functions and higher abundances in
the IE. These 38 proteins yielded 27 AGIs, and from these seven were ascribed in the
AT_CHLORO database to the envelope, whereas one was ascribed to thylakoids. The
remaining 19 AGIs were not present in the AT_CHLORO database.
The IE has a major role in transport
IE membrane preparations were significantly enriched in proteins related to transport, with 44
proteins ascribed to this category (in OE, only seven proteins were found in this category).
This is indicative of a major role of IE in active, carrier or pump-mediated transport
processes. Most of them were identified as envelope proteins in the AT_CHLORO database
(Supplementary Table 2). Among identified proteins, transporters for a wide array of
substrates, including lipids, nucleotides, dicarboxylates, SAM, auxin and inorganic ions were
enriched in the IE preparations. Ten proteins, corresponding to seven AGIs, were ABC
transporters, with four of them belonging to the I (ABCI10 -At4g33460-, 13 -At1g65410- and
14 -At1g19800-) and A (ATH -At5g64940-) subclasses, with known functions in lipid
transport (26). For instance, ABCI13 participates in transfer of ER-derived lipids to the
plastid galactoglycerolipid biosynthetic machinery (27). The remaining three ABC proteins
belonged to the B subclass (ABCB 25 -At5g58270-, 26 -At1g70610- and 28 -At4g25450-)
with roles in transport of lipids, pigments and Fe-sulfur clusters. One of them, ABCB25
(STA1), is functionally equivalent to yeast ATM1, required for the export of Fe-sulfur
clusters from the mitochondrial matrix (28). Five proteins, yielding three AGIs, were
phosphate transporters (PHT4-1 -At2g29650-, PHT2-1 -At3g26570- and PHT4-4 At4g00370-) whereas two more (PPT1 -At5g33320- and TPT -At5g46110-) were
PEP/phosphate and triose-phosphate/phosphate translocators, respectively (Supplementary
Table 2). The later one (TPT) is involved in the export of triose-phosphates generated in the
Calvin cycle to the cytosol for sucrose synthesis and has been previously found in envelopes
(4, 16, 29). Two dicarboxylate transporters (DIT1 -At5g12860- and DIT2-1 -At5g64290-)
and one protein from the major facilitator superfamily (At5g59250) involved in carbohydrate
transport were also enriched in IE. Four more proteins were orthologs to two SAM
transporters in Arabidopsis (SAMTL -At2g35800- and SAMT1 -At4g39460-); an impaired
function of SAMT1 leads to a decreased accumulation of prenyl lipids and affects
chlorophyll (Chl) biosynthesis (30). Nine cation transporters were also enriched in IE
preparations, corresponding to five AGIs of the Arabidopsis K (KEA2 -At4g00630-), Mg
(MGT10 -At5g22830-), Fe (TIC21/PIC1 -At2g15290-) and Cu (HMA1 -At4g37270- and
HMA6 -At4g33520-) transporters (Supplementary Table 2). KEA has been involved in pH
and cation homeostasis, PIC1 transports Fe to the chloroplast (31) and the HMAs have a role
in Zn detoxification in addition to Cu transport (32). Two more proteins (yielding one AGI At3g13070-) similar to CorC, an Mg and Co efflux protein of E. coli, were more abundant in
IE. Nucleotide transporters were also enriched in IE preparations. These include six proteins,
two of them with homology to the Arabidopsis AATP1 -At1g80300-, an ATP/ADP
antiporter, and two with homology to ATBT1 -At4g32400-, a nucleotide exporter to the
cytosol. Two more proteins with homology to MSL2 -At5g10490-, an ion channel involved
in plastid division, and RUS2 -At2g31190-, an auxin transporter, and one predicted protein
(B9H046 -At1g31410-) involved in polyamine transport, were enriched in IE.
With regard to the protein import machinery, as expected IE preparations were enriched
(with relative abundances of 60-85%) in translocons TIC 22 -At4g33350-, TIC 40 At5g16620-, TIC 55 -At2g24820-, TIC 55-IV -At4g25650-, TIC 62 -At3g18890- and TIC
110 -At1g06950-, previously described as IE proteins in other plant species including pea (13) (Supplementary Table 2). Two orthologs of the SecA pathway, SecA1 -At4g01800- and
SecA2 -At1g21650-, were also enriched in IE preparations (ca. 85%) suggesting a role of this
pathway in protein import in the IE as it occurs in thylakoids (33). All the identified proteins
by 2-DE in this category were also present in the shotgun results.
The IE has a role in membrane protein homeostasis
Thirty one identified proteins (three of them only present in 2-DE) corresponding to 20 AGIs
were plastid proteases, including members of the well-characterized families FtsH (13) and
Clp (4) (Supplementary Tables 2 and 4). The FtsH (filamentation temperature sensitive)
family of membrane bound proteases was enriched in IE as revealed by the shotgun analysis.
Eighteen identified proteins, yielding eleven AGIs, had large signal intensities in the IE,
including among others: FtsH 2 -At2g30950-, 5 -At5g42270-, 7 -At3g47060-, 9 -At5g58870-,
11 -At5g53170-, 12 -At1g79560- and FtsHi1 -At4g23940- (Supplementary Table 2). In
addition, two AGIs only identified in 2-DE were FtsH2 -At2g30950- and FtsZ2-1 At2g36250- (Supplementary Table 4). Members of this family contain an ATPase -AAA+
type and a Zn(II)-metalloprotease domains, with the exception of the chloroplast specific
FtsHi’s that lacks the Zn motif (34). Excepting FtsH2 and FtsZ2-1 described in thylakoids, all
the other identified members were previously ascribed to the envelope (16, 19). All FtsH
proteases are nuclear encoded and transported to the stroma via TIC and TOC translocons.
FtsH5 is integrated in thylakoids via the Sec pathway in a NTP-dependent way (33).
Orthologs of SecA1 and SecA2 were also enriched in IE (ca. 85%) suggesting that a similar
mechanism could occur in envelopes. These proteases play a role in membrane protein
maintenance, being responsible for removal of misfolded or damaged proteins (34, 35),
therefore suggesting that IE has a role in membrane protein homeostasis.
Seven identified proteins of the Clp/Hsp100 (caseinolytic protease) family, corresponding
to four AGIs, were enriched in IE (Supplementary Table 2). Among them, we found two of
the five catalytic ClpP units of the ClpPR protease complex (1 -AtCg00670- and 4 At5g45390-) and two chloroplastic chaperones, ClpC1 -At5g50920- and ClpD -At5g51070-.
Clp/Hsp100 chaperones act as independent units or with the proteolytic core to degrade
irreversibly damaged proteins (36), and their enrichment in IE further supports a role in
protein homeostasis. The Clp chaperones are also implicated in protein import along with
Hsp70 chaperones, by pulling precursors in as the transit peptide enters the organelle (37).
All of the Clp members found in IE have been previously described in envelopes (4, 16).
Two more identified proteins, a stromal processing peptidase (SPP -At5g42390-)
responsible for cleavage of transit peptides (38) and a chloroplast processing enzyme (SPPA1
-At1g73990-) were also enriched in IE preparations (Supplementary Table 2). None of them
have been previously reported in envelopes. SPP has been allocated in the stroma (38)
whereas SPPA1, with a yet unknown function, has been described in thylakoids (39). A 17.6
kDa heat shock protein was identified only in 2-DE gels of IE membranes (Supplementary
Table 4).
The IE is enriched in proteins related to the thylakoid
IE preparations contained proteins involved in the light-dependent photosynthesis reactions
(Supplementary Table 2). Thirty-nine identified proteins in this class, yielding 31 AGIs, had
relative abundances in IE in the range of 67 to 91%. Nine and 16 of them were ascribed to
envelopes and thylakoids, respectively, whereas six are not present in AT_CHLORO
(Supplementary Table 2). Among identified proteins, we found the subunits A, B, C, D2, F,
G and N of the PSI reaction center (RC) (PSAA, PSAB, PSAC, PSAD2, PSAF, PSAG,
PSAN, respectively), the subunits A, B, C and E of the PSII RC (PSBA, PSBB, PSBC,
PSBE, PSBO2, PSBP1, PSBQ2, respectively), the subunits O2, P1 and Q2 of the oxygen
evolving complex (PSBO1, PSBP1 and PSQ2, respectively), the subunits 1, 3 and 4 of the
light harvesting complex (LHC) I (LHCA1, LHCA3, LHCA4, respectively), the subunits 2, 3
and 4 of the LHC II (LHCB2.1, LHCB3 and LHCB 4.2, respectively), the subunits α, β, γ and
∂ of the CF1 complex (ATPA, ATPB, ATPC1, ATPD, respectively) and the subunits a and b
of the CF0 complex of the chloroplastic ATPase (ATPI and F, respectively). Two more were
identified as Cyt f and plastocyanin (PETA and PETE1, respectively) and two are involved in
the PSII stability (HCF136, CYP38). In addition, six proteins yielding four AGIs related to
thylakoids were identified only in IE 2-DE gels, including subunit 1 of the LHC II (LHCB1),
a ferredoxin-NADP reductase and the subunits α and ε of ATP synthase (Supplementary
Table 4).
In contrast, only two proteins, the subunit D1 of the PSI RC (PSAD1) and the subunit R of
the PSII RC (PSBR) were enriched in OE preparations as revealed in shotgun and none was
detected only in 2-DE. Although these results could indicate that some degree of crosscontamination between thylakoids and IE exist in the samples, we should keep in mind that
thylakoids are generated by invaginations of IE and therefore it is plausible to find
components of the light-dependent reactions in IE.
The IE has a role in chlorophyll and tocopherol biosynthesis
While it has been previously described that the chloroplast envelope contains enzymes
involved in the formation of Chl and tocopherol (12), our results support that these processes
occur preferentially in the IE (Supplementary Table 2). Nine proteins of tetrapyrrole
metabolism, yielding six AGIs, and two of tocopherol biosynthesis were enriched in IE
preparations, with abundances in the range 70-90%. These included two protoporphyrinogen
oxidases (PPOX -At4g01690- and PPO2 -At5g14220-) from the common tetrapyrrol
pathway, pheophorbide A oxygenase (PAO -At3g44880-) of the Chl breakdown pathway and
three proteins of the Chl synthesis pathway: Mg-protoporphyrin IX monomethylester cyclase
(CHL27 -At3g56940-), divinyl protochlorophyllide reductase (PCB2 -At5g18660-), and
SAM Magnesium-protoporphyrin O-methyltransferase (CHLM -At4g25080-). One more, an
uroporphyrinogen decarboxylase (UPD2 -At2g40490-) involved in Chl synthesis, was only
detected in IE 2-DE gels (Supplementary Table 4). The two proteins involved in tocopherol
biosynthesis were tocopherol cyclase (VTE1 -At4g32770-) and methyltransferase (VTE4 At1g64970-). Nine of these proteins had been previously ascribed to envelopes, whereas
CHLM and PCB2 have not been found previously in this compartment and UPD2 was
assigned to the stroma (Supplementary Tables 2 and 4). The 2-DE analysis revealed two
protein species yielding one AGI (glutamate 1-semialdehyde aminotransferase -At3g48730-)
related to the tetrapyrrole metabolism, which was only detected in OE preparations
(Supplementary Table 4).
Envelopes are also involved in carotenoid biosynthesis (12, 16) and three enzymes of this
pathway were found in this study, a phytoene desaturase (PDS3 -At4g14210-) and a carotene
mono-oxygenase (CYP97C1 -At3g53130-), which were more abundant in IE, and a
carotenoid dioxygenase (CCD1 -At3g63520-), which was more abundant in OE
(Supplementary Table 2). Therefore, it is difficult is to ascribe this pathway to one of the
membranes since enzymes involved in their biosynthesis were found in both IE and OE.
The IE has a role in lipid metabolism
Nine proteins ascribed to lipid metabolism, yielding seven AGIs, had abundances in IE
ranging from 62 to 85% (Supplementary Table 2). Identified proteins were part of the early
steps of fatty acid (CAC3 -At2g38040- and AAE16 -At3g23790-), phospholipid (PGP1 At2g39290-) and sulpholipid (SQD2 -At5g01220-) biosynthesis as well other lipid related
processes (CYP74B2 -At4g15440-, CYP97B3 -At4g15110- and LOX6 -At1g67560-). Three
of them (AAE16, CAC3, and CYP97B3) were previously described in envelopes, whereas
the remaining six were not in the AT_CHLORO database.
Delving into the OE composition by shotgun proteomics and 2-DE
While the IE membrane preparations are clearly enriched in proteins belonging to three major
metabolic pathways, proteins preferentially associated to OE participate in a wide range of
metabolic processes. Carbohydrate metabolism (38 proteins; 32 by shotgun and six by 2-DE),
protein transport (19 proteins; 18 by shotgun and one by 2-DE), lipid metabolism (19
proteins; 18 by shotgun and 1 by 2-DE), and protein synthesis (15 proteins detected by
shotgun) were the largest metabolic categories within proteins preferentially associated to
OE. Proteins involved in other metabolic processes such as transcription, signal transduction
and Fe homeostasis were also enriched in OE (Figure 5). Thirteen proteins (11 revealed by
shotgun and two found only in 2-DE), yielding ten AGIs, had unknown functions, and from
these three were ascribed to the envelope in AT_CHLORO, whereas the remaining were not
present in this database. A membrane-associated 30 kDa protein of unknown function and
ascribed to the envelope in AT_CHLORO was detected in IE and OE preparations only in the
2-DE proteomic approach (Supplementary Table 4).
The OE roles in carbohydrate metabolism
Proteins of the light-dependent phase of photosynthesis were not particularly abundant in OE,
but those related to the dark-phase did show a preferential accumulation. Twenty-three
proteins with nine AGIs involved in the three phases of the Calvin cycle were more abundant
in OE (78-99%) (Supplementary Table 2). All enzymes participating in carboxylation and
reduction were found in OE, including both subunits of Rubisco (RBCS1A -At1g67090- and
RBCL -AtCg00490-), phosphoglycerate kinase (At1g56190) and two isoforms of
glyceraldehydo-3-phosphate dehydrogenase (GAPA-2 -At1g12900- and GAPB -At1g42970). Two proteins were involved in the regeneration phase (fructose-bisphosphate aldolase 1 At4g38970- and phosphoribulokinase -At1g32060-) and two members of the chaperonin 60
family (CPN60B1 -At1g55490- and CPN60B3 -At5g56500-) involved in the correct folding
of Rubisco (40) were also enriched in OE. Four of the nine AGIs have been found in
Arabidopsis envelopes whereas the remaining ones were ascribed to the stroma in
AT_CHLORO. All of them, except for RBCL, are nuclear encoded and most have been
annotated as precursors, suggesting that their presence in OE reflects their transport to the
stroma. A preferential allocation of these proteins in OE may indicate that OE is a limiting
step for their transport to the stroma, and might also suggest that they are not functionally
active in envelopes. Six proteins involved in carbohydrate metabolism were only detected in
2-DE gels of OE; these proteins corresponded to three AGIs: RBCL -AtCg00490-, fructosebisphosphate aldolase -At4g38970- and a transketolase -At3g60750- (Supplementary Table
4). Four more proteins yielding two AGIs: RBCL -AtCg00490- and Rubisco activase (RCA At2g39730-) were only detected by 2-DE in both IE and OE, whereas six more yielding four
AGIs (RCA -At2g39730-, CPN60A1 -At2g28000-, CPN60B1 -At1g55490- and PGK1 At3g12780-) were only detected in 2-DE gels of IE (Supplementary Table 4).
Nine more proteins involved in carbohydrate metabolism, yielding six AGIs, were also
more abundant (86-97%) in OE (Supplementary Table 2). These included two hexokinases
(HXK1 -At4g29130- and HXK2 -At2g19860-) involved in glycolysis, a transaldolase of the
pentose phosphate shunt -At1g12230-, an aldehyde dehydrogenase -At1g44170-, a
chloroplastic precursor of the carbonic anhydrase (CA1 -At3g01500-) and a β-glucosidase At3g06510-. Most of them were identified as envelope proteins in At_CHLORO. In contrast,
only two proteins related to carbohydrate metabolism were more abundant in IE: a plastidNAD dependent malate dehydrogenase (At3g47520) and a xyloglucan galactosyltransferase
(KAM1 -At2g20370-).
The OE roles in transport
In contrast with the IE results, only seven proteins related to transport were enriched in OE
(Supplementary Table 2), with three of them yielding the same AGI (ABCG7 -At2g01320-).
Interestingly, the G subclass of ABC transporters contains “reverse orientation” transporters
functioning in lipid export (26), which would imply that the OE is responsible for export
whereas the IE controls the import of lipids into the chloroplast. The remaining four proteins
were the already characterized OE channel pore proteins of 16 -At2g28900-, 21 -At1g76405-,
24 -At1g45170- and 37 -At2g43950- kDa, with functions in aminoacid, phosphorylated
carbohydrate, triose-P, dicarboxylic acid and sugar transport (41-44).
OE preparations were enriched (with relative abundances of 80-99%) in proteins
belonging to the OE translocon, including eighteen proteins with ten AGIs (Supplementary
Table 2). These included the known translocon proteins 34 -At5g05000-, 64-III -At3g17970-,
75-III -At3g46740-, 75-V -At5g19620-, 90 -At5g20300-, 120 -At3g16620-, 132 -At2g16640and 159 -At4g02510- and two members of the HSP70 chaperon family (HSP-70 -At3g12580and HSP70-1 -At5g02500-). One more protein, an ortholog of the Arabidopsis TIC110 At1g06950-, was only identified in 2-DE OE preparations (Supplementary Table 4).
The OE is enriched in ribosomal proteins
Ten identified proteins enriched in OEs, yielding nine AGIs, were constituents of plastid
ribosomal subunits including the RPS4 -AtCg00380-, 8 -AtCg00770- and 17 -At1g79850- of
the 30S subunit and RPL1 -At3g63490-, 2.2 -AtCg01310-, 6 -At1g05190-, 13 -At1g78630-,
14 -AtCg00780-, and 27 -At5g40950- of the 50S subunit. Five more OE-enriched proteins
were related to protein translation, including two elongation factors (SVR3 -At5g13650- and
RABD8 -At4g20360-), one peptidyl-tRNA hydrolase (PTH2 -At5g16870-), and two proteins
involved in ribosomal RNA metabolism, the CSP41B endonuclease -At1g09340- (45) and
RBP31 -At4g24770- (Supplementary Table 2). The genes of the plastidic translational
machinery are distributed between the plastid and the nucleus, with the genes for
processing/modification enzymes, aminoacyl tRNA synthetases, and 60% of the RPs located
in the nuclear genome (46). Six of the identified ribosomal constituents and the PTH2 are
nuclear encoded; this makes tempting to speculate that their enrichment in OE may be a
consequence of their translocation process that could be a bottleneck in their movement into
the plastid. However, the presence of four plastid-encoded RPs in OE opens the possibility
for other explanations.
The 2DE analysis revealed four proteins related to protein synthesis only detected by this
technique. Among these, two (a RPL1 -At3g63490- and a TUFA -At4g20360- elongation
factor) were found in both IE and OE preparations whereas the other two (RPL10 At5g13510- and a tRNA Ile-lysidine synthetase -At3g24560-) were found in OE membranes
(Supplementary Table 4).
The OE roles in lipid metabolism
Eighteen proteins with high abundances in OE (67-97%) were ascribed to lipid metabolism
(Supplementary Table 2). These were represented by nine AGIs involved in fatty acid
synthesis, including the three components of the pyruvate dehydrogenase complex (pyruvate
dehydrogenase E1 -At2g34590-, dihydrolipoamide acetyl transferase -At1g34430- and
lipoamide dehydrogenase -At3g16950-), an acetylCoA carboxylase (CAC2 -At5g35360-), a
dephospho CoA Kinase (ATCOE -At2g27490-), and a long chain fatty acid synthase (LACS9
-At1g77590-), PECT1 -At2g38670- involved in phospholipid synthesis, a lipid associated
protein (PAP -At3g26070-) and a cytochrome b5 reductase (CBR1 -At5g17770-) that
provides electrons for fatty acid desaturases (47). Also, a protein similar to the
dihydrolipoamide acetyl transferase was only found in 2-DE of OE preparations
(Supplementary Table 4). These results indicate that both membranes have an active role in
lipid metabolism.
The OE roles in protein homeostasis
In contrast to the IE results, only seven proteins involved in proteolysis were enriched in OE
preparations (Supplementary Table 2). These corresponded to four AGIs, including ClpP5 At1g02560-, two metallo-endopeptidases (At5g56730 and At1g67690) and UBQ11 At4g05050-. The later two have not been previously described to occur in the chloroplast
envelope.
The OE roles in aminoacid synthesis and redox defense mechanisms
Five proteins yielding four AGIs involved in cystein (ACS1 -At2g43750-), arginine
(At4g24830), threonine (TS1 -At4g29840-) and glutamate biosynthesis (GLU2 -At2g41220-)
had abundances in OE ranging from 71 to 89% (Supplementary Table 2). One more cysteine
synthase (OASA1 -At4g14880-) was identified only by 2-DE in OE preparations
(Supplementary Table 4). All of them, with the exception of GLU2 were ascribed to the
stroma in AT-CHLORO.
Five proteins, yielding four AGIs related to redox metabolism were also more abundant in
OE. These included two peroxiredoxins (2CPB -At5g06290- and PEX22 -At3g21865-),
ascorbate peroxidase (APX3 -At4g35000-) and monodehydroascorbate reductase (MDAR4 At3g27820-) related to redox defense.
The OE roles in signal transduction, transcription and Fe homeostasis
Eight proteins with abundances between 80 and 93% in OE were related to signal
transduction (Supplementary Table 2). Two of them matched phototropin 2 (PHOT2 At5g58140-), a blue light photoreceptor involved in chloroplast movement that has been
recently found in OE (48) and two more matched WAV2 -At5g20520-, a protein involved in
the regulation of external stimuli (49). The remaining four proteins belong to the rab
(RAB8A-like -At5g59840-, RAB1B -At4g17170-) and rho (MIRO1 -At5g27540- and
MIRO2 -At3g63150-) subfamilies of the small GTPase superfamily. The small rab-GTPases
are usually associated with vesicular trafficking, whereas the rho subfamily participates in
cytoskeleton reorganization (50).
Eight proteins yielding eight AGIs and with abundances in OE from 65 to 98% were
related to transcription (Supplementary Table 2). These included two subunits of the RNA
polymerase (RPOC1 -AtCg00180-, RPOA -AtCg00740-), three transcription regulators
(PTAC3 -At3g04260-, 14 -At4g20130- and 16 -At3g46780-) and three histones (HTA3 At1g54690-, HTA12 -At5g02560-, and HTB9 -At3g45980-). A MADS box protein involved
in regulation of transcription was identified only in 2-DE gels of OE preparations
(Supplementary Table 4).
Finally, three proteins with two AGIs (FER2 -At3g11050- and FER4 -At2g40300-) had
higher abundance in OE.
Conclusions and final remarks
In summary, our combined proteomic approach has allowed some progress towards the
characterization of the protein profiles of the inner (IE) and outer (OE) chloroplast envelope
membranes of P. sativum. Envelope preparations necessarily contain intrinsic proteins, but
also include proteins being processed or transported at the time of isolation (since a
proteomic profile is always a snapshot of a dynamic process), as well as a small degree of
contamination from neighboring compartments. In this study we have used the relative
enrichment of each protein in the IE/OE pair of membranes to provide an integrated picture
of the actual composition and dynamics of the system. The IE is clearly enriched in proteins
related to transport, protein metabolism and photosynthesis, whereas proteins preferentially
associated to OE are more heterogeneous and participate in a wide range of metabolic
processes. Relative enrichments clearly indicate that in the envelope the IE is the selective
barrier for the transport of a wide range of metabolites, and also plays a major role in
controlling membrane protein homeostasis, given the high prevalence of metallo-proteases
involved in the removal of damaged proteins. Furthermore, based on our data, metabolic
processes previously described to occur in the envelope, including chlorophyll and tocopherol
biosynthesis, can be ascribed preferentially to IE whereas others such as carotenoid or lipid
biosynthesis occur in both IE and OE. Data also reveal that the IE preparations were
particularly enriched in proteins involved in the light-dependent photosynthetic processes
whereas proteins related to the dark-phase of photosynthesis showed a preferential
accumulation in the OE.
Also, whereas results show a good overlap with the AT_CHLORO database, they add a
new layer of information by allowing further assignation of envelope proteins to the IE and/or
the OE. Furthermore, this combined approach has revealed 78 AGIs (32% of the total number
found) in envelope preparations that had not been previously found in the chloroplast. These
included some protein species involved in chlorophyll and tocopherol biosynthesis, peptide
processing enzymes, lipid metabolism and other categories. Finally, it should be mentioned
that even though shotgun is a technique more powerful than 2-DE for membrane protein
profiling, both are complementary, with many proteins being detected in only one of the two
methodologies.
Acknowledgements
Supported in part by the Spanish Ministry of Economy and Competitivity (MINECO;
projects AGL2010-16515 and AGL2012-31988, co-financed with FEDER), the Aragón
Government (Group A03), and by a Japan Society for the Promotion of Sciences Research
Fellowship for Young Scientists (#247373 to D.T.) and Grants-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Sciences and Technology, Japan
(#22120003 and #24370018 to M.U)..
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Figure legends
Figure 1. Allocation of protein species (NCBI entries) between OE and IE using shotgun
proteomics. Only protein species showing significant differences in allocation between the IE
and OE membrane preparations (p<0.05) are shown.
Figure 2. 2-DE IEF-SDS PAGE protein profile maps of inner (IE) and outer (OE) chloroplast
preparations from pea. Proteins were separated in the first dimension in a linear 3-10 (IE) and
5-8 (OE) gel strips (7 cm) and in the second dimension in 12% acrylamide vertical gels (8 x 8
cm) and stained with Coomassie-blue G-250. Scans of typical gels are shown in A (IE) and
B, (OE).
Figure 3. Venn diagrams showing partial overlaps of identified protein species between the
shotgun and 2-DE techniques, using (A) the NCBI identifiers and (B) the corresponding AGI
blast results.
Figure 4. Localization of identified envelope proteins (using their AGI) compared to the
AT_CHLORO database (16) of Arabidopsis thaliana chloroplastic proteins. A reference nonredundant Arabidopsis proteome was built by carrying out BLAST searches of the identified
(NCBI) envelope proteins of pea in the Arabidopsis genome database (TAIR). Duplicate
identifiers were deleted and only E-values lower that e-30 were considered.
Figure 5. (A) Distribution of identified pea protein species (NCBI) by shotgun proteomics or
only by 2-DE according to their GO:P annotation. (B) Distribution of identified pea protein
species according to their GO:P annotation in OE and IE.
Legends of Supplementary Tables
Supplementary Table 1. List of peptides identified and quantified by shotgun proteomics
and Progenesis LC-MS analyses.
Supplementary Table 2. Protein species showing significant differences in abundance
between IE and OE membrane preparations (ANOVA, p<0.05) using shotgun proteomics.
Grey and white backgrounds indicate proteins identified in IE and OE, respectively. Columns
are: (A) NCBI entry, (B) protein description, (C) plant species, (D) UniProt entry, (E)
percentage of abundance (as [signal in IE/(signal in IE + signal in OE)]), (F) corresponding
AGI, (G) localization using the AT_CHLORO database, (H) AGI name, (I) E blast value, (J)
pI, (K) molecular weight, (L) number of identified peptides, (M) peptides used for
quantitation, (N) confidence score, (O) Anova p, (P) signal mean in IE, (Q) signal SD in IE,
(R) signal mean in OE, (S) signal SD in OE, (T) GO Slim process (U) GO Slim function and
(V) GO Slim component. Positive identification was assigned with at least two unique topranking peptides with scores above the threshold level (p < 0.05). Only proteins identified in
all four biological replicates were considered for this analysis. For all identified proteins,
blast searches were run against the TAIR database and the closest match (using a threshold
value of e-50) was assigned to the protein species. NCBI entries corresponding to AGIs not
present in the AT_CHLORO database are highlighted in red.
Supplementary Table 3. Protein species showing no significant differences in abundance
between IE and OE membrane preparations (ANOVA, p<0.05) using shotgun proteomics.
Analysis details and column description is as in Supplementary Table 2
Supplementary Table 4. Proteins identified in 2-DE IEF-SDS PAGE gels. Grey and white
backgrounds indicate proteins identified in IE and OE, respectively. Columns are: (A) spot
number in the gels, (B) protein description, (C) found in shotgun, (D) UniProt entry, (E)
percentage of abundance (as [signal in IE/(signal in IE + signal in OE)]), (F) corresponding
AGI, (G) localization using the AT_CHLORO database, (H) AGI name, (I) E blast value, (J)
Mascot score/pep/ion, (K) sequence coverage (%), (L) theoretical molecular weight/pI, (M)
experimental molecular weight/pI, (N) GO process, (O) GO:P identifier, (P) inferred
function. Positive identification was retained with Mascot scores above the threshold level
(p≤0.05), at least 2 identified peptides with a score (pep/ion) above homology, at least 10%
sequence coverage and similar experimental and theoretical MW and pI. For all identified
proteins, blast searches were run against the TAIR database and the closest match (using a
threshold value of e-50) was assigned to the protein species. NCBI entries corresponding to
AGIs not present in the AT_CHLORO database are highlighted in red.
Legends of Supplementary Figures
Supplementary Figure 1. One-dimensional gels of the IE and OE (silver-stained). Specific
proteins occurring in the IE and OE preparations were detected using specific antibodies and
used to assess membrane enrichment in the different fractions.
Supplementary Figure 2. Virtual composite images containing all spots present in the real
2-DE IEF-SDS PAGE gels are shown in (A) for IE and (B) for OE. Identified proteins are
marked.