Intracellular Compartments
and Protein Sorting
Haixu Tang
School of Informatics
The major intracellular
compartments of an animal cell
Relative Volumes Occupied by the Major
Intracellular Compartments
INTRACELLULAR COMPARTMENT
PERCENTAGE OF TOTAL
CELL VOLUME
Cytosol
54
Mitochondria
22
Rough ER cisternae
9
Smooth ER cisternae plus Golgi
cisternae
6
Nucleus
6
Peroxisomes
1
Lysosomes
1
Endosomes
1
An electron micrograph
Development of plastids
Hypothetical schemes for the
evolutionary origins of some organelles
Four distinct families
1) the nucleus and the cytosol, which communicate
through nuclear pore complexes and are thus
topologically continuous (although functionally
distinct);
2) all organelles that function in the secretory and
endocytic pathways, including the ER, Golgi
apparatus, endosomes, lysosomes, the numerous
classes of transport intermediates such as transport
vesicles, and possibly peroxisomes;
3) the mitochondria;
4) the plastids (in plants only).
Secretory vs. endocytic
pathways
Protein traffic
Protein traffic
• Gated transport
• Transmembrane transport
• Vesicular transport
– membrane-enclosed transport
intermediates
Sorting sequences
Some sorting sequences
Prediction of protein sorting
• Psort web server: http://psort.nibb.ac.jp/
– prediction of protein localization sites in cells
from their primary amino acid sequence
Construction of Membrane-enclosed Organelles
Require Information in the Organelle Itself
• The information required to construct a membraneenclosed organelle does not reside exclusively in the
DNA that specifies the organelle's proteins. Epigenetic
information in the form of at least one distinct protein that
preexists in the organelle membrane is also required,
and this information is passed from parent cell to
progeny cell in the form of the organelle itself.
Presumably, such information is essential for the
propagation of the cell's compartmental organization,
just as the information in DNA is essential for the
propagation of the cell's nucleotide and amino acid
sequences.
Nuclear pore complexes
Nuclear Envelope
Nuclear lamina
• Consists of "intermediate filaments", 30-100 nm thick.
• These intermediate filaments are polymers of lamin,
ranging from 60-75 kD.
• A-type lamins are inside, next to nucleoplasm; B-type
lamins are near the nuclear membrane (inner). They
may bind to integral proteins inside that membrane.
• The lamins may be involved in the functional
organization of the nucleus.
Nuclear localization signals
(NLSs)
Protein import through nuclear pores
Possible paths for free diffusion through
the nuclear pore complex
Nuclear Import / Export Receptors
The control of nuclear import
during T-cell activation
The breakdown and re-formation of
the nuclear envelope during mitosis
The subcompartments of
mitochondria and chloroplasts
A signal sequence for
mitochondrial protein import
Protein translocators in the
mitochondrial membra
Protein translocation depends on
the temperature
Protein import by mitochondria
Energy required
Two plausible models of how mitochondrial
hsp70 could drive protein import
Protein import from the cytosol into the
inner mitochondrial membrane or
intermembrane space
Translocation
of a precursor
protein into
the thylakoid
space of
chloroplasts
The Endoplasmic Reticulum
Free and membrane-bound
ribosomes
The signal hypothesis
The signal-recognition particle
(SRP)
SRP direct ribosomes to the ER
membrane
Protein translocation
Single-pass transmembrane
protein
Multipass membrane protein
rhodopsin
Protein glycosylation in the
rough ER
The export and degradation of
misfolded ER proteins
The unfolded protein response in yeast
Phospholipid exchange proteins