Why and how bacteria localize proteins

Abstract

Despite their small size, bacteria have a remarkably intricate internal organization. Bacteria deploy proteins and protein complexes to particular locations and do so in a dynamic manner in lockstep with the organized deployment of their chromosome. The dynamic subcellular localization of protein complexes is an integral feature of regulatory processes of bacterial cells.

Fig. 1.

Localization patterns. (A) An anterior-posterior cellular localization axis is exhibited by the Caulobacter histidine kinases PleC (red) and DivJ (green) that dynamically and selectively localize to specific cell poles. The ZapA cell division protein (blue) localizes to the FtsZ ring. (B) In these same cells, a dorsal-ventral localization axis is exhibited by crescentin (cres) intermediate filaments that localize along the inner concave side of the crescent-shaped cell and are responsible for this distinctive cell shape (10); in contrast, the chemotaxis sensor array localizes at the convex outer side of the crescent near the cell pole (5). DAPI, 4′,6-diamidino-2-phenylindole.

Fig. 2.

Chromosome attachment to the cell pole. (A) The Caulobacter parS centromere bound to the ParB partition protein is attached to the cell pole by interaction with the polar PopZ polymeric network (14, 15). The initiation of replication triggers the assembly of PopZ at the opposite pole, where it captures the duplicated copy of parS/ParB. The diagram shows PopZ (red), ParB (green), PopZ + ParB (yellow), and chromosomes depicted as rings (dark green). (B) Sporulating cells of B. subtilis anchor chromosomes to the cell poles via the sporulation protein RacA, which binds to sites near the replication origin and to DivIVA at the cell poles (16, 17). The images show RacA tagged with green fluorescent protein (green), the nucleoids (blue), and the cell membrane (red).

Fig. 3.

Cell topology and inhibitor gradients control place and time of cell division. Repression of FtsZ polymerization by polar localized proteins that exhibit a minimum of the repressor at midcell restricts the site of division ring assembly in both E. coli and Caulobacter. However, E. coli’s strategy depends on oscillation of the MinCD repressor from pole to pole (A), whereas Caulobacter establishes a gradient of the MipZ repressor with the highest concentration at the cell poles (B). In S. pombe, the Pom1 repressor is localized to the poles (C). In small cells, the gradient of Pom1 extending from the poles overlaps at Cdr2 located at midcell and represses its activity. Consequently, the Cdk1 pathway is blocked, preventing entry into mitosis. As the cell grows, the midcell repressor concentration diminishes until Cdr2 (and thus the Cdk1 pathway) is no longer repressed and entry into mitosis is facilitated.

Fig. 4.

Geometric cues for protein localization. (A) Schematic depiction of the hemispherical poles of the cell. The more extreme negative curvature at the inside surface of the poles (relative to the inside surface of the lateral walls of the cell) could be a geometric cue for proteins, such as the cell division protein DivIVA, that localize to the poles. (B) The process of engulfment during spore formation in B. subtilis in which the membrane of the large mother cell (on the left) migrates around to surround and eventually pinch off the nascent spore. The positive curvature of the surface of the engulfment membrane from within the mother cell is a geometric cue for the sporulation protein SpoVM. The green color indicates the localization of DivIVA in (A) and SpoVM in (B) to regions of negative and positive curvature, respectively.