Poles apart: prokaryotic polar organelles and their spatial regulation

Abstract

While polar organelles hold the key to understanding the fundamentals of cell polarity and cell biological principles in general, they have served in the past merely for taxonomical purposes. Here, we highlight recent efforts in unraveling the molecular basis of polar organelle positioning in bacterial cells. Specifically, we detail the role of members of the Ras-like GTPase superfamily and coiled-coil-rich scaffolding proteins in modulating bacterial cell polarity and in recruiting effector proteins to polar sites. Such roles are well established for eukaryotic cells, but not for bacterial cells that are generally considered diffusion-limited. Studies on spatial regulation of protein positioning in bacterial cells, though still in their infancy, will undoubtedly experience a surge of interest, as comprehensive localization screens have yielded an extensive list of (polarly) localized proteins, potentially reflecting subcellular sites of functional specialization predicted for organelles.

Figure 1.

Transmission electron micrograph (taken by Jeff Skerker) of a Caulobacter crescentus swarmer cell showing the polar pili (empty arrowheads), the polar flagellum with the flagellar filament (filled arrowheads), and the hook (white arrow) (see Fig. 2A).

Figure 2.

Spatial regulation of the polar flagellum (A) by the FlhF GTPase in Vibrio cholerae (B) and by the TipF c-di-GMP receptor protein, the TipN birth scar protein, and the PflI positioning factor in Caulobacter crescentus (C). In Panel A, the outer membrane, peptidoglycan layer, and inner membrane are abbreviated by OM, PG, and IM, respectively. In Panels B and C, the wavy and straight lines denote the flagellum and the pili, respectively.

Figure 3.

Polarity switching by the Ras-like GTPase MglA (green oval) and its GTPase-activating protein (GAP) MglB (red oval) in gliding Myxococcus xanthus cells. (A) A time series of gliding M. xanthus cells expressing MglA-YFP and MglB-mCherry that were imaged every 20 seconds by fluorescence microscopy (by Yong Zhang and Tâm Mignot). The yellow coloring indicates the colocalized MglA and MglB. The white arrow indicates the direction of movement that reverses during the course of the experiment. (B) A schematic of the cells and polarly localized proteins from panel A. The straight lines denote the pili that confer S-motility.

Figure 4.

Alignment of conserved residues of deviant Walker-A boxes found in polarly localized members of the ParA/MinD family of ATPases. Shown are Escherichia coli MinD and BcsQ, Caulobacter crescentus ParA, MipZ and CpaE, Rhodobacter sphaeroides PpfA, Vibrio cholerae FlhG, Agrobacterium tumefaciens VirC1, Aggregatibacter actinomycetemcomitans TadZ, and Pseudomonas fluorescens WssJ. Residues that are not identical, but similar, to the consensus sequence in red are shown in blue. Black color codes for variable residues.

Figure 5.

Discovery of stalked protein X (StpX) of Caulobacter crescentus from a genome-wide localization screen (Hughes et al. 2010). (A) A schematic of the C. crescentus cell in panel B with a fluorescent polar stalk. (B) Fluorescence micrograph from cells expressing StpX-GFP and a red-fluorescent membrane stain (picture taken by P. Viollier). (C) A mutant with a mispositioned stalk containing StpX-GFP. The left image is a DIC (differential interference contrast) micrograph, and the image on the right represents the corresponding (green) fluorescent image.