Bacterial polarity

Abstract

Many recent studies have revealed exquisite subcellular localization of proteins, DNA, and other molecules within bacterial cells, giving credence to the concept of prokaryotic anatomy. Common sites for localized components are the poles of rod-shaped cells, which are dynamically modified in composition and function in order to control cellular physiology. An impressively diverse array of mechanisms underlies bacterial polarity, including oscillatory systems, phospho-signaling pathways, the sensing of membrane curvature, and the integration of cell cycle regulators with polar maturation.

Figure 1

The Min system and oscillating polar asymmetry in E. coli. The dynamically unstable MinCD helix interacts with MinE, resulting in disassembly of the structure (upper panels). Subsequently, MinCD re-assembles in an area of low MinE concentration at the opposite pole of the cell (lower panel).

Figure 2

Establishing polarity in Bacillus subtilis. (a) FtsZ is re-organized into two symmetrically positioned rings upon entry into sporulation (upper panels), and after one ring is chosen for completion (arrow), asymmetry is reinforced by a feedback signaling pathway that controls compartment-specific gene expression (lower panel). (b) The genetic asymmetry model for spore fate determination. At the early stages of sporulation, only the origin-proximal third of the chromosome is present in the forespore. Origin-distal gene expression is limited to the mother cell while origin-proximal gene expression is concentrated in the forespore. (c) SpoIIE concentration could determine spore fate. SpoIIE is initially targeted to the division plane, and if it is equally shared between mother cell and forespore compartments, the relative concentration of SpoIIE will be higher in the smaller forespore compartment following septation. SpoIIE may also have specific affinity for the forespore side of the division plane.

Figure 3

Convex and concave membrane curvatures in a bacterial cell. Some membrane-associated proteins have specific affinity for convex or concave membrane surfaces.

Figure 4

Polar maturation in Caulobacter crescentus. (a) Cell poles ‘age’ over the course of cell division. (b) In C. crescentus, polar aging is coupled with the timing of chromosome replication, transitions in polar protein composition, and cell cycle progression. Important polar transition periods are numbered (1: regulatory proteins are switched at swarmer to stalked pole transition; 2: assembly of cell cycle regulators at new pole; 3: compartmentalization and formation of new poles at cell division). Different sets of polar regulatory proteins promote (blue shading) or inhibit (green shading) chromosome replication by controlling the activity of a master regulator, CtrA.