The cytoskeleton is a key regulator of cell morphogenesis. Crescentin, a bacterial intermediate filament-like protein, is required for the curved shape of Caulobacter crescentus and localizes to the inner cell curvature. The domain organization of crescentin shares a striking resemblance to eukaryotic intermediate filament proteins. Here, I use deletion analysis to show that several intermediate filament-like domains in crescentin closely resemble eukaryotic intermediate filament proteins with respect to their relative importance for structure and function. This analysis is supported by random mutagenesis of crescentin, which reveals functionally important residues. Collectively, I use this data to distinguish regions of the protein that are important for different steps in crescentin assembly and function. Next, I show that crescentin forms a single filamentous structure within the cell that collapses into a helix when detached from the cell envelope, suggesting that it is normally maintained in a stretched linear configuration.

Association of the crescentin structure with the cell envelope is critical for function, as mutations in crescentin that abolish this association also abrogate curvature. Deletion of a gene (wbqL) involved in the lipopolysaccharide (LPS) biosynthesis pathway also abolishes cell curvature by apparently interfering with the ability of the crescentin structure to associate with the cell envelope. These data highlight the delicate harmony among unrelated cellular systems. Using the wbqL mutant, I also show that the normal assembly and growth properties of the crescentin structure are independent of its association with the cell envelope. However, this envelope association is important for facilitating the local disruption of the stable crescentin structure at the division site during cytokinesis.

Crescentin causes an elongation rate gradient around the circumference of the sidewall, creating a longitudinal cell length differential and hence curvature. Such curvature can be produced by physical force alone, as demonstrated by growing cells in circular microchambers. Additionally, exogenous production of crescentin in Escherichia coli is sufficient to generate cell curvature in this evolutionarily distant organism. My data argue for a model in which compressive force produced by a stretched crescentin structure anisotropically alters the kinetics of cell wall insertion to produce curved growth. This study suggests that bacteria may use the cytoskeleton for mechanical control of growth to alter morphology.