Until recently it seemed that bacteria lacked a cytoskeleton and that the cell wall was the main determinant of bacterial shape. However, proteins with remarkable structural similarity to actin — despite very little amino acid sequence homology — were shown to assemble into filaments and help to determine cell shape in Bacillus subtilis and Escherichia coli. Now, research published in Cell has uncovered another bacterial cytoskeletal protein. The characteristic crescent shape of Caulobacter crescentus is determined in part by a protein that has features of intermediate filaments (IF), another component of the eukaryotic cytoskeleton.

Ausmees et al. used transposon mutagenesis coupled with scrutiny using light microscopy to isolate C. crescentus mutants that were rod-shaped. They identified a protein, called crescentin (CreS), that is responsible for the crescent (vibrioid) shape of the cell. To function in determining cell curvature, crescentin forms helical filaments that seem to interact with the cell membrane. Crescentin filaments localize asymmetrically to the concave side of the cell, which contributes to the vibrioid cell shape. IF proteins and crescentin both have extensive coiled-coil motifs arranged in a similar fashion and both can form filaments in vivo. Furthermore, since crescentin can form IF-like filaments in vitro under conditions similar to those used for filament formation by animal IF proteins, it seems that crescentin is indeed a bacterial IF protein.

It is likely that a combination of actin, IF homologues and peptidoglycan, the major cell-wall component, determine the shape of bacteria. Now that bacterial counterparts of actin, tubulin and IF have been found, it seems that bacteria can be good models for cell-shape determination after all.