Regulation of cell polarity in bacteria

Abstract

Bacteria are polarized cells with many asymmetrically localized proteins that are regulated temporally and spatially. This spatiotemporal dynamics is critical for several fundamental cellular processes including growth, division, cell cycle regulation, chromosome segregation, differentiation, and motility. Therefore, understanding how proteins find their correct location at the right time is crucial for elucidating bacterial cell function. Despite the diversity of proteins displaying spatiotemporal dynamics, general principles for the dynamic regulation of protein localization to the cell poles and the midcell are emerging. These principles include diffusion-capture, self-assembling polymer-forming landmark proteins, nonpolymer forming landmark proteins, matrix-dependent self-organizing ParA/MinD ATPases, and small Ras-like GTPases.

Figure 1.

Curvature as a geometrical cue for protein localization. DivIVA of B. subtilis localizes at sites with negative curvature and SpoVM of B. subtilis recognizes the positive curvature of the endospore membrane.

Figure 2.

Polymer-forming landmark proteins. (A) The domain structure of DivIVA is indicated with the lipid-targeting domain in gray and coiled-coil regions in red. Bottom: localization of DivIVA and associated proteins during a cell cycle. (B) The domain structure of DivIVA of S. coelicolor is indicated as in A. Bottom: left diagram illustrates the hyphal growth pattern and the right diagram illustrates the localization of DivIVA, Scy, and FilP at a hyphal tip. (C) The domain structure of BacP with the bactofilin domain in red. Bottom: localization of BacP and associated proteins. GTP hydrolysis by SofG is indicated. Note that SofG is only associated with the BacP landmark at one pole. (D) The domain structures of BacA and BacB with the bactofilin domains in red. Bottom: localization of BacA and BacB and associated protein during the cell cycle and with the old and new poles marked. Flagellum and pili are indicated at the nonstalked pole. (E) The domain structure of PopZ is indicated with α-helical regions in red and a proline-rich domain in gray. Bottom: localization of PopZ and associated proteins during the cell cycle including the ParABS-dependent chromosome segregation process and with the old and new poles marked. Flagellum and pili are indicated at the nonstalked pole. Black ovals represent the chromosome.

Figure 3.

Nonpolymer-forming landmark proteins. (A) The domain structure of TipN is indicated with transmembrane domains in black and a coiled-coil region in red. Bottom: localization of TipN and associated proteins during the cell cycle including the ParABS-dependent chromosome segregation process and with the old and new poles marked. c-di-GMP levels in different cell types are indicated as high (upward arrow) and low (downward arrow). Flagellum and pili are indicated at the nonstalked pole. (B) The domain structure of PodJ is indicated with coiled-coil regions in red, a transmembrane domain in black, three TPR domains in gray, and the peptidoglycan binding muramidase domain in yellow. Bottom: localization of PodJ and associated proteins during a cell cycle. Flagellum and pili are indicated at the nonstalked pole. (C) The domain structure of HubP is indicated with the LysM peptidoglycan-binding domain in yellow, a transmembrane domain in black, and the repeat rich region in red. Bottom: localization of HubP and associated proteins during a cell cycle including the chromosome segregation process, the flagellum, and the array of chemotaxis proteins.

Figure 4.

Protein localization by matrix-dependent, self-organizing ParA/MinD ATPases. (A) PpfA localization in R. sphaeroides and its association with the chromosome (dark gray) and the cluster of chemotaxis proteins. TlpT-induced ATP hydrolysis by PpfA is indicated. (B) Localization of the MinCDE proteins in E. coli. MinE-induced ATP hydrolysis by MinD is indicated. The level of MinC is lowest at midcell where the Z-ring is formed. Chromosomes are indicated in dark gray. (C) Localization of MipZ in C. crescentus. A MipZ gradient is formed over the chromosome (dark gray) and with the lowest MipZ concentration at midcell where the Z-ring is formed. (D) PomZ localization in M. xanthus. PomZ localizes at midcell before and independently of FtsZ. Chromosomes are indicated in dark gray. The PomZ monomer is shown to not bind to the chromosome; however, this has not been experimentally verified.

Figure 5.

Protein localization by a small GTPase. Localization of MglA, MglB, and RomR in M. xanthus together with the T4P ATPases PilB and PilT before and after a Frz-induced cellular reversal. MglB-induced GTP hydrolysis by MglA at the lagging pole is indicated by the bent arrow.