Bend into shape

Bacteria come in a variety of shapes, as most species elaborate on the ‘default’ sphere to resemble ovoids, rods, bend rods, spirals, branched filaments or other more complicated forms. How cells that are under considerable turgor pressure maintain a nonspherical shape is unclear, though it is known to depend on structural elements on either side of the cytoplasmic membrane: the murein (peptidoglycan) sacculus on the outside, and forerunners of the eukaryotic cytoskeleton on the inside. In this issue, the results by Cabeen et al argue for one attractive mechanism whereby these elements cooperate to drive cellular morphogenesis. The murein sacculus is one large mesh-like molecule of linear glycan strands that are covalently linked by short peptides. It surrounds the entire cytoplasmic membrane where turgor pressure stretches it considerably, primarily at the peptide cross-links. It is critical in maintaining cell shape and integrity, and its destruction (e.g. by lysozyme) causes cells to quickly convert to fragile spheres that will burst under common (hypotonic) conditions. Purified sacculi typically retain the particular shape of the bacterium it was isolated from, implying that it is somehow welded into the murein meshwork. This is likely accomplished, at least partly, by controlling where and when new murein strands are incorporated as cells elongate and divide. New strands are incorporated by the combined actions of murein hydrolases and synthases, many of which are direct targets of b-lactam antibiotics. The hydrolases break bonds in the sacculus, whereas the synthases assemble and incorporate fresh glycan strands into the ‘gap’ left by the hydrolases. Several, perhaps all, murein hydrolases and synthases are part of larger transmembrane murein holoenzymes. This likely ensures tight coordination between their activities, which is needed to prevent cell rupture (Cabeen and Jacobs-Wagner, 2007; den Blaauwen et al, 2008; Vollmer and Bertsche, 2008). Much evidence indicates that cytoskeletal filaments on the cytoplasmic face of the membrane exert spatio-temporal control on growth and shape of the sacculus by serving as tracks for the murein (holo) enzymes. The best conserved track is laid by FtsZ, which orchestrates cytokinesis (cell fission, septation, constriction) in almost all bacteria. This forerunner of tubulin forms a ring at the prospective constriction site and then attracts and guides the murein enzymes that produce and process septal murein during the constriction process. Most nonspherical bacteria also produce one or more forms of bacterial actin (MreB), which is required to maintain nonspherical shape and usually assembles in spiral-like configurations along the long axis of the cell. Similar to the FtsZ-ring during cell constriction, MreB spirals are thought to act as tracks for murein enzymes that incorporate new murein in a spiral-like fashion during cell elongation (Cabeen and Jacobs-Wagner, 2007; den Blaauwen et al, 2008; Vollmer and Bertsche, 2008). Cabeen et al now make the case for an entirely different mechanism whereby a third cytoskeletal element helps to control cell shape in Caulobacter crescentus whose name reflects its curved-rod morphology (Cabeen et al, 2009). Earlier, the group of Jacobs-Wagner identified a straight-rod mutant of C. crescentus that was defective in crescentin (CreS) (Ausmees et al, 2003). This 50-kD protein resembles metazoan intermediate filaments (IFs), and readily polymerizes in vitro. In vivo, CreS forms a lateral filamentous structure that invariably lines the concave side (inner curvature) of the cell, suggesting that it helps to induce cell curvature quite directly (Ausmees et al, 2003). Additional IF-like proteins have since been identified in other bacteria (Bagchi et al, 2008), suggesting that, like tubulin and actin, IFs were a prokaryotic invention as well. Four laboratories joined forces to elucidate how CreS causes rod-shaped cells to curve (Cabeen et al, 2009). Remarkably, treatment of C. crescentus cells with the b-lactam mecillinam induced a gradual detachment of CreS from the cell periphery to yield a single filamentous structure that, once free in the cytoplasm, coiled-up with a pronounced lefthanded twist. This result indicates that (1) the cellular CreS filament likely consists of some stable super arrangement (e.g. bundles) of the B10-nm wide ‘proto-filaments’ that are seen in vitro (Ausmees et al, 2003), (2) association of the filament with the membrane is somehow dependent on the integrity of the murein sacculus and (3) the attached filament is normally in a stretched conformation. The latter raised the possibility that a tensed CreS filament bends a cell simply by exerting a sufficiently large compressive force to ‘scrunch-up’ the murein mesh-work on its side of the cylinder. However, this is inconsistent with the fact that de-proteinized C. crescentus sacculi retain the curved appearance of cells (Poindexter and Hagenzieker, 1982), and the authors formally show that curved cells require an extended period of growth (i.e. new murein synthesis) to straighten-out in the absence of a functional CreS structure. Rather, insertion of The EMBO Journal (2009) 28, 1193–1194 | & 2009 European Molecular Biology Organization | Some Rights Reserved 0261-4189/09 www.embojournal.org