Cell shape determination in Escherichia coli
Mary J Osborn and Lawrence Rothfield
The rigid cell wall peptidoglycan (murein) is a single giant
macromolecule whose shape determines the shape of the
bacterial cell. Insight into morphogenetic mechanism(s)
responsible for determining the shape of the murein sacculus
itself has begun to emerge only in recent years. The discovery
that MfreB and Mbl are cytoskeletal actin homologues that
form helical structures extending from pole to pole in rodshaped cells has opened an exciting new field of microbial
cell biology. MreB (in Gram-negative rods) and Mbl
(in Gram-positive species) are essential for murein synthesis
along the lateral wall and hence, the rod shape of the cell.
Known members of the morphogenetic system include MreB
(or Mbl), MreC, MreD and PBP2, but Rod A and murein
biosynthetic enzymes involved in peptidoglycan precursor
synthesis and assembly are likely to be recruited to the same
multimolecular apparatus. However, the actual role of
MreB in assembly of the morphogenetic complex is still not
clear and little is known about regulatory mechanisms
controlling the switch from lateral murein elongation to septa1
murein synthesis at the time of cell division.
Address
Department of Molecular, Microbial and Structural Biology, University of
Connecticut Health Center, Farmington, CT 06032, United States
Corresponding author: Osborn, Mary J (osborn@nso2.uchc.edu)
Current Opinion in Microbiology 2007, 10:606–610
This review comes from a themed issue on
Prokaryaotes
Edited by Martin Dworkin
Available online 5th November 2007
1369-5274/$ – see front matter
# 2007 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.mib.2007.09.004
indistinguishable by available analytical methods and
(2) that there must therefore exist some morphogenetic
apparatus responsible for determining the shape of the
sacculus. The prediction was indeed correct, but it has
taken over 30 years to identify major morphogenetic
factors and begin to understand their roles in the complex
interplay that results in synthesis of a murein sacculus of
fixed and reproducible size and shape.
This review focuses primarily on the roles of the cytoskeletal Mre system in determining cell shape in E. coli
and on possible mechanisms for control of cell diameter.
For broader or different perspectives, the reader is
referred to several excellent recent reviews [3–6,7,8].
Peptidoglycan biosynthesis and the cell cycle
Rod-shaped cells face two distinct problems in replicating
their shape over the course of the cell cycle. Cell
elongation requires lateral extension of the murein sacculus by intercalation of new glycan strands and crosslinking of peptide subunits into the pre-existing shell [9].
However, as the cell prepares for cell division, elongation
ceases and the cell switches to specific synthesis of septal
peptidoglycan that will form the new pole of each daughter cell.
A distinguishing characteristic of polar murein is its
stability in vivo. Pulse-chase experiments have shown
rapid turnover of lateral murein. By contrast, polar, that
is, septal murein is metabolically inert, showing essentially no turnover of glycan or peptide subunits in subsequent generations [10]. Interestingly, a second type of
inert peptidoglycan was revealed in experiments in which
septation was prevented. Discrete patches of stable murein were seen in the resulting non-septate filaments at
regular intervals that corresponded to potential division
sites. This inert ‘preseptal murein’ is of particular interest
[11] and is discussed further in a later section.
Introduction
It has been known for almost 50 years that the intact and
purified murein (peptidoglycan) sacculus retains the
shape and size of the cell from which it has come [1]
and that disruption of the structural integrity of the rigid
peptidoglycan leads to the loss of normal cell shape. In
this sense, murein determines the bacterial cell shape.
But what determines the shape of the murein sacculus? As
early as 1971, Schwarz and Leutgeb [2] showed that
restoration of murein synthesis in E. coli spheroplasts gave
rise to osmotically resistant round cells whose isolated
sacculi were also round. They concluded (1) that sacculus
shape was not determined solely by chemical composition
or structure of the peptidoglycan polymer, which was
Current Opinion in Microbiology 2007, 10:606–610
The roles of penicillin binding proteins (PBPs)
in murein synthesis and cell shape
determination
Covalent binding by penicillins defines a menagerie of
proteins (in E. coli no fewer than 12 [12]) that are involved
in peptidoglycan metabolism and establishment/maintenance of normal sacculus morphology. The high molecular weight family – PBPs 1a, 1b, 1c, 2, and 3 – are
enzymes with transglycosylase and/or transpeptidase
activities and catalyze synthesis and insertion of new
polymer strands into the sacculus [12]. As such, these
are obvious candidates for control by the putative
morphogenetic apparatus. The remaining seven, of lower
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Cell shape determination in Escherichia coli Osborn and Rothfield 607
molecular weight (PBP4-7, DacD, AmpC, AmpH), include amidases and peptidases that cleave either polymeric peptidoglycan or disaccharide–pentapeptide
precursor subunits [12].
Mutational studies originally showed that PBP2 and
PBP3 carry out different and complementary functions
in establishment of the rod shape. PBP2 is required for
lateral wall synthesis; loss of function results in cessation
of cell elongation and formation of round cells [13].
Conversely, PBP3 is essential for septal murein synthesis;
inactivation gives rise to long non-septate filaments [13].
Localization studies using fluorescence techniques have
generally re-inforced these functional assignments. Thus,
GFP-PBP2 was found in foci along the lateral wall, as
expected for its role in cell elongation [14]. More surprisingly, the protein was also concentrated at midcell in
septating cells, suggesting a hitherto unrecognized function in cell division. Again consistent with mutational
analysis, PBP3 (FtsI) was primarily localized at midcell
[15] but appeared diffusely along the cylinder in nondividing cells [16]. The results suggest the existence of
specific morphogenetic switches to account for the redistribution of both PBP2 and PBP3 during the cell cycle.
The PBP1 family of bifunctional transglycosylase–transpeptidases is thought to participate in both lateral and
septal murein syntheses. Consistent with this idea, studies on localization of PBP1b showed immunofluorescent
staining both along the length of the cell and at division
sites [17]. Interestingly, PBP1b interacted directly with
PBP3 both in vivo and in vitro, potentially accounting for
its divisomal localization.
Regulation of PBP localization and activity
The Mre system and PBP2 function
Mutations in MreB result in conversion from rod shape to
sphere [18], implying a role for MreB in cell shape
determination. The discovery by Errington and colleagues [19] that MreB and its homologue in B. subtilis,
Mbl, form cytoskeletal structures – extended filamentous
helices that underlie the cell membrane – created an
explosion of interest in bacterial cytoskeletal elements in
general and the function of Mre proteins in determination
of rod shape in particular. The extraordinary similarity of
the MreB crystal structure to that of eucaryotic actin [20]
firmly established it as a true cytoskeletal protein.
Disruption of the pole-to-pole MreB helix by mutational
inactivation or exposure to the specific inhibitor, A22,
results in failure of lateral murein synthesis and formation
of spherical cells [21,22,23], implicating the MreB cytoskeleton in determination of rod shape. The phenotype is
similar to that resulting from inactivation of PBP2,
suggesting a functional relationship between the two
proteins. Disruption in the two genes immediately downwww.sciencedirect.com
stream of MreB, MreC, and MreD also yielded spherical
phenotypes in E. coli and B. subtilis [23,24]. In B. subtilis,
Errington and colleagues showed that synthesis of lateral
murein is governed by the MreB homologue, Mbl, [25]
and that localization of GFP-MreC and GFP-MreD
appeared to be similar to that of Mbl [23]. Kruse et al.
[24] also showed that E. coli MreC directly interacts with
MreB and MreD in a bacterial two hybrid system. In
addition, MreC, a bitopic membrane protein, interacts
with several high molecular weight PBPs, including
PBP2, in affinity chromatography (in C. crescentus [26])
and in a bacterial two hybrid system (in B. subtilis [27]).
These results suggest that lateral murein synthesis is
carried out by a helical array of interacting proteins that
includes MreB (or Mbl), MreC, MreD, and PBP2. The
model predicts that new peptidoglycan is inserted into
the lateral wall in a helical pattern that reflects the helical
pattern of the biosynthetic complex. Indeed, labeling of
nascent peptidoglycan in B. subtilis with fluorescent
derivatives of vancomycin [25,28] or ramoplanin [28]
showed clear helical distribution along the lateral wall,
and it is highly probable that E. coli peptidoglycan is also
inserted helically [10,29].
Three possible mechanisms for the roles of the MreB
(and Mbl) helices in lateral murein synthesis might be
considered: (1) the MreB or Mbl helix might act as a
permanent scaffold protein that recruits and maintains a
multiprotein assemblage responsible for lateral wall synthesis; (2) MreB (or Mbl) might act as a transient scaffold,
analogous to the scaffold proteins required for viral capsid
assembly, which disassembles when head formation is
complete. MreB would then be required for assembly, but
not for maintenance, of the murein biosynthetic apparatus; (3) MreB might not function as the primary scaffold,
but be secondarily recruited to another, pre-existing
helical scaffold for assembly of the murein biosynthetic
complex.
The actual role of the MreB cytoskeleton is still unclear.
The helical distribution of MreC and MreD is apparently
independent of MreB. In Caulobacter, Dye et al. [21]
found that MreC formed spirals that did not colocalize
with MreB helices and persisted in cells treated with the
MreB inhibitor, A22, under conditions in which MreB
helices disassemble. Conversely, MreB helices were
retained in cells depleted of MreC. GFP-PBP2 also
showed a helical distribution in Caulobacter [30]. The
helical distribution depended on MreC, and the helical
pattern partially overlapped with MreC but not with
MreB. The authors reported (Figure 7B in reference
[30]) the PBP2 organization was lost in MreB-depleted
cells, but the present authors believe that these images
show clear evidence of a helical pattern. In the studies of
Kruse et al. [24] in E. coli, the distribution of GFP-MreC
also appeared significantly different from that of MreB
Current Opinion in Microbiology 2007, 10:606–610
608 Prokaryaotes
even though the two proteins interacted directly in the
two-hybrid system.
Overall, the results to date suggest that MreB and MreCD
helices may be independent structures. However, the
question of the relationship between MreB, MreCD,
and PBP2 helices merits re-examination, especially in
round cells of E. coli resulting from depletion of one of
these. It would be interesting to add spheroplasts to the
experimental list to test the possible scaffold function of
peptidoglycan. Carballido-Lopez and Errington [31]
reported that the helical pattern of Mbl was ‘fragmented’
in B. subtilis protoplasts, but immunofluorescence still
showed a pattern of peripheral dots that could be part
of an organized cytoskeletal arrangement in the round
cells. In retrospect, three-dimensional reconstruction
might be revealing.
Even if MreB is not itself the primary scaffold for assembly of the biosynthetic machinery, it might function to
recruit a limited subset of players to sites of biosynthesis.
Indeed, Carballido-Lopez et al. [32] have shown that
MreBH, one of the MreB isomorphs in B. subtilis that
colocalize with MreB, is required for helical organization
of the periplasmic lytic endopeptidase, LytE, and interacts with it in the yeast two hybrid system.
In addition to the helical pattern that extends from pole to
pole, bands of MreB at or near midcell have been
observed consistently in a fraction of the cell population
(see reference [7] for review). Time lapse studies in
Caulobacter [33] demonstrated shifts in MreB distribution
from a pole-to-pole helix during elongation to a midcell
band or ring in predivisional cells, providing clear evidence that the distribution of the protein is dynamic over
the cell cycle. It is tempting to speculate that the MreB
rings may be involved in the formation of preseptal
murein, bands of metabolically inert peptidoglycan at
or near potential division sites in non-septate filaments
of FtsA, FtsQ, or PBP3 (FtsI) mutants of E. coli [10].
Interestingly, the bands were absent from FtsZ-depleted
filaments.
RodA and cell shape: still a mystery
Mutational inactivation of RodA yields an osmotically
resistant, round-cell phenotype indistinguishable from
that resulting from antibiotic or mutational inactivation
of PBP2 (see references [34,35]). De Pedro et al. [34]
have provided convincing evidence in E. coli that, in both
cases, synthesis of new lateral wall ceases and synthesis of
septal murein becomes constitutive. The round cells are,
in effect, composed of two poles with no sidewall murein
in between. Unfortunately, the role of RodA in lateral
murein synthesis and its relationship to PBP2 localization
and/or activity remain unclear. RodA is highly homologous to FtsW, a component of the divisome that is
required to recruit PBP3 to midcell [35] possibly
Current Opinion in Microbiology 2007, 10:606–610
suggesting that RodA may play a similar role in PBP2
localization to lateral wall or midcell or both. To our
knowledge, RodA has not been localized, and the question of its interactions with PBP2 and the Mre family
remains unclear. It has been suggested [36] that RodA
and FtsW may be involved in recruitment of lipid-linked
murein precursors to sites of synthesis.
Control of cell width: another mystery
The cell width of a given E. coli strain is remarkably
constant during steady-state exponential growth. However, changes in growth rate result in significant changes
in cell diameter, such that rapidly growing cells are
relatively fat and slowly growing cells are relatively
thin. The mechanisms regulating cell width remain
almost entirely unknown. Growth rate information must
somehow be translated into the altered size of the
murein sacculus, but how is the rate change sensed
by the machinery for wall growth? And how can that
machinery respond to change the diameter of the rod
cylinder?
It has been proposed [37] that a change in growth rate
may be sensed as a change in cytoplasmic turgor pressure,
hence a change in the stress placed on the cell envelope.
The stress theory of morphogenesis is discussed in detail
by Harold in this issue. It should be emphasized, however, that there are other possible transduction mechanisms, for example, changes in concentrations of second
messengers or other signaling molecules (see reference
[38] for an example) that might be recognized by the
cytoskeleton and murein synthetic machinery.
Whatever the signaling mechanism, the response to a
change in growth rate may involve changes in the formation or maintenance of inert murein. Murein at cell
poles is normally inert, neither undergoing new synthesis
nor autolytic turnover, and nascent septal murein has
been shown to mature to the inert state very rapidly after
synthesis [39]. It is possible that the diameter of the
cylinder is established during cell division when the
geometry of the septum (new cell pole) is determined.
Then how is the final diameter of the new pole established at cell division? Preseptal murein, which is formed
at or adjacent to potential division sites is also metabolically inert and could play a role in establishing the
diameter of the nascent new pole during septation.
The reader is referred to the 2003 review by Young [3]
for an excellent discussion of inert murein.
A question that has apparently not yet been addressed
experimentally asks what happens during the transition to
the new cell diameter as a result of changing growth rate,
or during reversible rod-to-sphere transitions resulting
from inactivation or re-activation of lateral murein synthesis. At early stages of the rod-to-sphere transition,
pear-shaped cells are seen in which the old pole retains
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Cell shape determination in Escherichia coli Osborn and Rothfield 609
its original diameter, while the new pole is greatly
expanded [34]. This strongly suggests that the transition
of new septal murein to its inert form is delayed to allow
prolonged insertion of new murein, thereby increasing
the diameter of the cell at the junction of pole and
cylinder. The delay could result from increased activity
and/or recruitment of autolytic peptidases and enzymes of
septal murein synthesis to the growing septum, possibly
directed by Mre-related cytoskeletal components. In
addition, a potential role for PBP2 as a negative regulator
of septum diameter was suggested by observations that
the diameter of new poles formed shortly after inhibition
of PBP2 by mecillinam was significantly greater than that
of the pre-existing old poles and lateral walls [14].
Similar, less drastic responses might function to increase
the diameter of the new pole and thereby the width of the
rod cylinder as part of the normal response to increased
growth rate. Transitions to smaller septum diameters
might then occur during nutritional shiftdown by reversal
of the above mechanisms. It is not known whether
analogous changes in lateral murein synthesis, directed
by Mre cytoskeletal elements, occur during transitions to
altered cell widths.
Low molecular weight PBPs, in particular the D,D-carboxypeptidase, PBP5, are candidates to be regulators of
septal diameter [40]. Studies of mutants in low molecular
weight PBPs by Young (reviewed in reference [3]) provide strong evidence that PBP5 plays a key role in
removal of ‘excess’ inert murein and maintenance of
normal rod shape. Localization of PBP5 as a function
of transitions in cell diameter would be of considerable
interest in this context.
Outlook
Despite significant recent progress, an understanding of
how rod-shaped bacteria such as E. coli establish, maintain, and modify cell shape remains elusive. We know
that enzymes responsible for synthesis of murein along
the length of the cell cylinder are associated with cytoskeletal elements that are likely to play important roles in
the topology of new cell wall synthesis. We also know that
the Mre cytoskeleton is dynamic, but we understand
little of the how or why. We speculate that the cytoskeleton may act to transduce cellular signals to regulate
synthesis of cylindrical murein and/or to regulate the
dimensions of the cell cylinder, but information remains
lacking.
We are confident that our understanding of cell shape
control will increase rapidly. Major immediate challenges
will be to understand the molecular mechanisms responsible for cell shape changes under different growth conditions, and to define the biological significance of the cell
cycle related shifts in cellular location of cytoskeletal
proteins and murein biosynthetic proteins. It is not unlikely that significant surprises await us.
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References and recommended reading
Papers of particular interest, published within the period of review,
have been highlighted as:
of special interest
of outstanding interest
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