Determination of Cell Shape in Caulobacter

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

In many organisms a major determinant of cellular morphology is the shape of the cell wall. In bacteria, the cell wall, or peptidoglycan layer, serves as a scaffold that dictates both cell shape and reinforces the cell against internal osmotic pressure. A fundamental problem in bacterial cell biology is understanding the molecular mechanisms that coordinate peptidoglycan assembly to generate the diverse forms exhibited by bacteria. In rod shaped bacteria the proteins encoded by the highly conserved rod shape-determining genes probably play a critical role in directing peptidoglycan assembly. One of these, MreB has been recently shown to be the bacterial actin homolog, suggesting that a cytoskeletal-like structure is responsible for determining rod shape. The overall objective of this project is to genetically and biochemically define the molecular mechanisms that result in generating the rod-like morphology of the bacterium Caulobacter crescentus. The ease at which synchronized populations can be obtained, plus the intrinsic asymmetry of the predivisional cell make C. crescentus an ideal organism for tracking the subcellular location of the proteins that orchestrate cell wall synthesis during the course of the cell cycle. MreB is required for the maintenance of a rod-shaped cell and forms spirals that traverse along the longitudinal axis of Caulobacter cells which switch to a midcell location at a time coinciding with the initiation of cell division. The penicillin-binding proteins (PBPs) also form a banding pattern that resembles, and depends on, MreB localization. MreB forms a complex with the PBPs, suggesting that MreB filaments function as a cytoskeleton that serves as an organizer or tracking device for the PBP peptidoglycan biosynthesis complex both during cell elongation and septum formation. The project will systematically examine the role of MreB in coordinating peptidoglycan assembly. The research consists of three major goals: (a) determining the role of genes within the Caulobacter rod-shape operon (mreC, mreD, rodA, pbpA, CC1548) in coordinating cell wall morphogenesis, (b) defining the molecular requirements for MreB/PBP complex formation by determining the composition of the MreB/PBP complex, determining whether the periplasmic protein, MreC can also form a complex with PBPs, and defining the role of an N-acetyl transferase encoded by the mreB operon in controlling cell shape, and (c) determining the mechanism of the cell cycle-dependent recruitment of MreB to the midcell.