Establishing polar identity in gram-negative rods

Abstract

In rod shaped bacteria, numerous cellular components are targeted to the cell poles, and such localization is often important for optimal function. In particular, recognition of poles is often linked to division site selection, chromosome segregation, chemotactic signaling, and motility. Recent advances in understanding polarity include identification of a Vibrio cholerae protein that mediates polar localization of a chromosome origin and chemotaxis clusters, as well as a downstream protein that contributes solely to localization of chemotaxis proteins. In Caulobacter crescentus, the molecular mechanisms by which polar determinants and effectors are localized, and the key roles for nucleotide-dependent switches, have been defined. Finally, roles for, and interactions between, factors that mediate environmentally determined polarity in Myxococcus xanthus have recently been characterized.

Fig. 1

Development of polar identity in V. cholerae. V. cholerae’s old and new poles are both marked by the transmembrane protein HubP, which interacts either directly or indirectly with 3 ParA family ATPases: ParA, ParC, and FlhG. These partners colocalize with HubP at the old pole in young cells, then adopt a largely bipolar distribution as cells age. Localization of HubP at the division plane probably ensures its presence at the new pole of newborn cells. HubP’s association with ParA1 enables polar localization of the chrI origin-binding protein ParB1, and thereby tethers the chrI ori(s) to the pole(s). HubP-dependent localization of ParC promotes polar localization and reliable inheritance of chemotaxis clusters. ParA1/ParB1 typically arrive at the new pole prior to ParC; however, movement of ParC is not dependent upon ParAB1 (Ringaard). In the absence of HubP-mediated polar localization of FlhG, V. cholerae are prone to production of multiple, rather than single, flagella. Additionally, several HubP clients adopt a diffuse, rather than focal, distribution (shown as small dots).

Fig. 2

Development of polar identity in C. crescentus. In flagellated swarmer cells, PopZ initially tethers ParB and the associated chromosome origin (via parS) to the old cell pole. During pole maturation, ParB is released from the PopZ matrix, the flagellum is ejected, DNA replication is initiated, and PopZ recruits additional stalked pole-specific factors. The replicated chromosome origin and associated ParB migrate across the cell, guided by ParA, until they reach the new pole. TipN, a marker of the new pole and ParA-interacting protein, promotes ParA’s directional movement. Accumulation of ParA at the new pole induces formation of a PopZ matrix at this site, which serves to further tether ParB and the origin. The bipolar distribution of ParB induces bipolar distribution of another ParB-interacting protein, the FtsZ inhibitor MipZ. MipZ displaces FtsZ from the new pole, and FtsZ subsequently polymerizes near the midcell, which is the site of lowest MipZ concentration. TipN is recruited to the midcell in an FtsZ and Tol-Pal dependent fashion prior to cell division. A flagellum is also generated at the new pole prior to division, so that division yields one stalked cell and one flagellated swarmer cell.

Fig. 3

Localization of polarity determinants linked to type IV pilus based motility (“S motility”) in M. xanthus. External stimuli trigger phosphorylation of FrzZ, (via the Frz chemosensory pathway), which localizes to the leading pole. The response regulator RomR, which lies downstream in the signaling pathway, localizes to both poles, but preferentially to the lagging pole. It interacts with MglA and MglB, and the localization of all 3 proteins is interdependent. Pilus based motility is controlled by the motor ATPases PilB and PilT, which are preferentially found at the leading at lagging poles, respectively. Non-specific polar targeting of these proteins requires bactofillin (BacP) and SofG, and sorting of ATPases to the proper poles is mediated by MglAGTP, which is always present at the leading pole. Upon recognition of appropriate stimuli, the distribution of pole-specific proteins is reversed, resulting in a changed direction of movement.