How do bacteria localize proteins to the cell pole?

Abstract

It is now well appreciated that bacterial cells are highly organized, which is far from the initial concept that they are merely bags of randomly distributed macromolecules and chemicals. Central to their spatial organization is the precise positioning of certain proteins in subcellular domains of the cell. In particular, the cell poles - the ends of rod-shaped cells - constitute important platforms for cellular regulation that underlie processes as essential as cell cycle progression, cellular differentiation, virulence, chemotaxis and growth of appendages. Thus, understanding how the polar localization of specific proteins is achieved and regulated is a crucial question in bacterial cell biology. Often, polarly localized proteins are recruited to the poles through their interaction with other proteins or protein complexes that were already located there, in a so-called diffusion-and-capture mechanism. Bacteria are also starting to reveal their secrets on how the initial pole 'recognition' can occur and how this event can be regulated to generate dynamic, reproducible patterns in time (for example, during the cell cycle) and space (for example, at a specific cell pole). Here, we review the major mechanisms that have been described in the literature, with an emphasis on the self-organizing principles. We also present regulation strategies adopted by bacterial cells to obtain complex spatiotemporal patterns of protein localization.

Keywords: Bacterial cell cycle; Polar localization; Spatial organization.

Fig. 1.

Localization of polar proteins through the recognition of polar features. (A) A protein (e.g. ParA1 in V. cholerae) diffusing in the cytoplasm (as indicated by single arrows) is trapped at the poles transiently (double arrows) through an affinity for a polar protein (e.g. HubP in V. cholerae) that is already localized at the cell poles. (B) Higher-order protein assemblies are favored in membrane regions of stronger curvature. Left: representation of a rod-shaped cell showing the radius of curvature (R) and the stronger negative curvature (C, curved arrows) at the cell poles (blue areas) compared with the sides of the cylinder (gray area), as described previously (Huang and Ramamurthi, 2010). Middle and right: formation of a higher-order protein assembly occurring preferentially in membrane regions of stronger negative curvature (e.g. DivIVA in B. subtilis). Arrows indicate the free diffusion of oligomers. (C) Formation of large protein assemblies such as higher-order structures (e.g. PopZ in C. crescentus) or protein aggregates (e.g. misfolded proteins in E. coli) is energetically favored outside the nucleoid region. (D) Differences in composition of the cytoplasmic membrane and peptidoglycan between cell poles and the rest of the cell envelope can serve as cues for localization of polar proteins. The particular case of a protein (e.g. ProP in E. coli) that preferentially binds anionic phospholipids enriched at the poles, such as cardiolipin, is depicted. E, extracellular space; OM, outer membrane; P, periplasmic space; CM, cytoplasmic membrane; C, cytoplasm. Note that the schematics in all figures are not to scale and do not reflect the structure of the illustrated proteins.

Fig. 2.

Localization of polar proteins resulting from cell growth and division. (A) A protein associated with the peptidoglycan is progressively directed towards the poles as the cell proceeds through cycles of growth and division, as new peptidoglycan is inserted laterally and becomes more and more inert during cell aging (e.g. ActA in L. monocytogenes). (B) Proteins stably localized at the midcell before cell division remain associated with the newly formed cell poles in the progeny (e.g. TipN in C. crescentus).

Fig. 3.

Pole-to-pole oscillation through nucleotide switch and membrane binding. In E. coli, MinE binds MinD-ATP dimers at the edge of a membrane-bound MinD-ATP dimer zone and triggers ATP hydrolysis, which releases MinD-ADP monomers in the cytosol (Step 1, thin arrows). MinE then binds to the next MinD-ATP. Iterations of this process lead to a progressive retraction of the MinD-ATP dimer zone at the membrane (Step 2, thick arrow). Following ADP/ATP nucleotide exchange, MinD-ATP redimerizes and dimers collectively reassociate with the membrane far from the MinE ring, i.e. at the opposite pole (Steps 2 and 3). After ‘running out’ of membrane-associated MinD-ATP dimers at one pole, MinE is released into the cytoplasm, diffuses until it associates with the edge of the new MinD-ATP dimer zone at the membrane (Steps 3 and 4).

Fig. 4.

Possible strategies for spatial and temporal regulation of polar localization. (A) Asymmetric polar patterns can be naturally produced by a cell division event. Left: bipolar to old-pole localization (e.g. PopZ in C. crescentus). Right: propagation of an old-pole accumulation (e.g. protein aggregates in E. coli). In the case of protein aggregates, misfolded proteins produced in the progeny accumulate onto the existing polar aggregate. Eventually, de novo polar accretions can appear in progeny that did not acquire a polar focus (top cell), for example after new protein synthesis. (B) The ability of some proteins to self-assemble and thereby to localize at the poles (e.g. Wag31, the DivIVA homolog in M. tuberculosis) could be influenced by modifications such as phosphorylation upon a specific signal (e.g. expression of the kinases of Wag31 in exponential phase in M. tuberculosis may increase Wag31 phosphorylation). The question mark indicates a hypothetical step. (C) The concentration of a self-assembling protein or oligomer (e.g. PopZ oligomer in C. crescentus), and thereby its propensity to multimerize, can be modified locally through protein–protein interaction with a partner whose subcellular distribution is asymmetric (e.g. ParA during DNA segregation in C. crescentus). In Step 1, the protein (blue) has an asymmetric distribution inherent to a cell cycle event. Concentration of the diffusing protein oligomer (red) increases locally owing to interaction with the asymmetric protein. In Step 2, the self-assembly of a protein or oligomer leads to the formation of a large structure at the pole. This provides spatial and temporal regulation to a multimerization-dependent polar localization.