Two independent spiral structures control cell shape in Caulobacter

The actin homolog MreB contributes to bacterial cell shape. Here, we explore the role of the coexpressed MreC protein in Caulobacter and show that it forms a periplasmic spiral that is out of phase with the cytoplasmic MreB spiral. Both mreB and mreC are essential, and depletion of either protein results in a similar cell shape defect. MreB forms dynamic spirals in MreC-depleted cells, and MreC localizes helically in the presence of the MreB-inhibitor A22, indicating that each protein can form a spiral independently of the other. We show that the peptidoglycan transpeptidase Pbp2 also forms a helical pattern that partially colocalizes with MreC but not MreB. Perturbing either MreB (with A22) or MreC (with depletion) causes GFP-Pbp2 to mislocalize to the division plane, indicating that each is necessary but not sufficient to generate a helical Pbp2 pattern. We show that it is the division process that draws Pbp2 to midcell in the absence of MreB's regulation, because cells depleted of the tubulin homolog FtsZ maintain a helical Pbp2 localization in the presence of A22. By developing and employing a previously uncharacterized computational method for quantitating shape variance, we find that a FtsZ depletion can also partially rescue the A22-induced shape deformation. We conclude that MreB and MreC form spatially distinct and independently localized spirals and propose that MreB inhibits division plane localization of Pbp2, whereas MreC promotes lengthwise localization of Pbp2; together these two mechanism ensure a helical localization of Pbp2 and, thereby, the maintenance of proper cell morphology in Caulobacter.