About this Research Topic
Bacterial cells often position protein machineries with exquisite precision. A paradigm of such positioning is the cytokinetic apparatus. The division machinery (divisome) in most bacteria relies on correct positioning of the bacterial tubulin-homologue FtsZ. FtsZ self assembles into a ring-like structure and subsequently recruits other division proteins. In most rod-shaped bacteria FtsZ is regulated by components of the Min system. The Min system is composed of an FtsZ-assembly inhibitor, MinC and topological factors that ensure correct MinC localization. MinC is recruited to the membrane by MinD. In its ATP-bound form MinD is membrane-associated and shuttles between the membrane and cytosol for reloading with nucleotide, after ATP hydrolysis. ATPase activity is triggered in E. coli and other Gram-negative bacteria by MinE. Based on affinity of MinE to the membrane and MinD, a robust pole-to-pole oscillation of the MinCD complex is generated. Over time the exact center of the cell has a minimal concentration of MinC, and hence allows for FtsZ-ring assembly. In Bacillus subtilis the Min system is composed of different topological factors that determine MinCD positioning. DivIVA is a curvature sensitive protein that recruits MinCD via an adapter protein, MinJ, to the cell poles. The classical view is that this system is stable, leading to a concentration gradient of MinCD from the pole to the cell center. However, recent data suggest that the Min system in B. subtilis is not static. There is accumulating evidence that the complex is recruited to the mature divisome, and hence, likely preventing reassembly of the division machinery at the same site. This ensures that division only occurs once per cell cycle. Mathematical modeling and in vitro reconstruction of the Min system allows an unrivaled insight into the molecular mechanisms.
Many bacteria have evolved systems that protect the nucleoid from being trapped under a division ring. This effect, called nucleoid occlusion, is mediated by DNA-bound proteins. In B. subtilis Noc binds to DNA sequences that are dispersed around the chromosome, but are absent from the terminus region. This leads to a Noc-free zone in the cell center after segregation of the replicated origins. In E. coli an analogous protein, SlmA, has evolved. In coccoid cells, such as Staphylococcus aureus, Noc is the so far only known system to determine the correct positioning of the divisome.
Very recently, other division site selection mechanisms have been identified. In Caulobacter crescentus a ParA-homologue, MipZ, is responsible for septum placement and the molecular mechanism of MipZ-gradient formation has been elucidated. In other rod-shaped bacteria lacking a MinCD system, such as actinobacteria, chromosome-positioning plays an essential role in division site selection.
Interestingly, two positive regulatory systems have been described. In Myxococcus xanthus a ParA-homologue, PomZ, marks the site of division before FtsZ assembles and in Streptomyces the SsgB protein seems to position FtsZ in aerial hyphae.
In summary, the field currently experiences a very fruitful phase and offers many rewarding new insights into the molecular mechanisms of protein complex positioning. Interesting new twists are observed and will stimulate future research.
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