Protein subcellular localization in bacteria

Abstract

Like their eukaryotic counterparts, bacterial cells have a highly organized internal architecture. Here, we address the question of how proteins localize to particular sites in the cell and how they do so in a dynamic manner. We consider the underlying mechanisms that govern the positioning of proteins and protein complexes in the examples of the divisome, polar assemblies, cytoplasmic clusters, cytoskeletal elements, and organelles. We argue that geometric cues, self-assembly, and restricted sites of assembly are all exploited by the cell to specifically localize particular proteins that we refer to as anchor proteins. These anchor proteins in turn govern the localization of a whole host of additional proteins. Looking ahead, we speculate on the existence of additional mechanisms that contribute to the organization of bacterial cells, such as the nucleoid, membrane microdomains enriched in specific lipids, and RNAs with positional information.

Figure 1.

MipZ localization in C. crescentus. MipZ is anchored adjacent to the cell pole through its interaction with ParB (bound to the origin of replication) and appears to form a nucleoprotein gradient. The site of lowest concentration of the FtsZ inhibitor dictates future division site. Images show MipZ-YFP in a predivisional cell and GFP-ParB in a cell at a similar stage. Images kindly provided by M. Thanbichler.

Figure 2.

Polarly localized DivIVA in Streptomyces coelicolor A3(2). S. coelicolor grows as a branching hyphae similar to filamentous fungi. Growth occurs at the hyphal tips in a manner dependent on DivIVA (Flardh 2003). Images show a DivIVA-GFP fusion. Lower panel: DivIVA-GFP foci. Upper panel: a merged image of DivIVA-GFP and phase contrast. Scale bar indicates 4 µm. Image kindly provided by K. Flardh.

Figure 3.

Chemoreceptors in E. coli show an exponential distribution of cluster sizes with the largest clusters at the poles. The image shows the Tar receptor fused to mEos visualized by super-high resolution fluorescence microscopy (photoactivated localization microscopy or PALM). Blue foci are from a PALM image in TIR-illumination. Red foci are from a PALM image in epi-fluourescence illumination (taken after Tar-mEos protein in the TIR region were bleached). Scale bar indicates 1 µm. Reprinted, with permission, from Greenfield et al. 2009.

Figure 4.

Carboxysomes in S. elongatus are found evenly spaced along the length of the cell. The image shows the carboxysome-localized enzyme RuBisCO fused to the green fluorescent protein. The native chlorophyll fluorescence (red) labels the peripheral membranes. Image kindly provided by D. Savage and P. Silver.