The mechanisms controlling the proper temporal and spatial regulation of cell division are a fundamental issue in cell biology. Cell division in most bacteria is initiated with the localization of the protein FtsZ in the correct place. FtsZ will form a polymer (called the Z-ring) around the membrane and will then serve as a scaffold for many other proteins that will be responsible for the formation of the septum that separates the two daughter cells. In bacteria, the minCD system and nucleoid occlusion factors are the most studied proteins regulating the positioning of the septa that divides the cell, since they are present in two of the most studied model organisms, Escherichia coli and Bacillus subtilis. However, many other new systems have been discovered in different bacteria in the past years, illustrating the diversity present, even for such an essential mechanism as cell division positioning. Thus, it has become clear that in order to properly understand how a particular mechanism works, especially deciphering cell division in bacteria is key to find new targets for the development of antimicrobials, we need to broaden our study to non-classical organisms.
In Caulobacter crescentus, an alpha-proteobacteria, the MipZ protein alone is able to localize FtsZ, the core protein of the divisome, both in time and space. MipZ inhibits FtsZ polymerization and localizes at the poles of the cell forming a gradient towards the cell center, leaving the latter as the only space where FtsZ can form the Z-ring. In order to create its characteristic gradient localization, MipZ needs to interact with the protein ParB at the poles and with the chromosomal DNA in the cell. ParB is a component of the DNA segregation machinery and recognizes a cluster of sites (parS) in the origin-proximal region of the chromosome. Since the molecular mechanisms supporting the interaction of MipZ with these elements are unknown, the overall objective in the MIPZ project has been the characterization in detail of the MipZ functioning, especially its relationship with ParB and FtsZ.
Regarding the relationship of MipZ with ParB, we could map the interaction interface to the C-terminal region of MipZ, and identify the exact residues involved. The binding region is composed mainly of positively charged amino acids, suggesting that it might bind to a negatively charged region on ParB through electrostatic interactions.
In addition, we characterized the MipZ-FtsZ binding interface and studied in vitro the molecular mechanism underlying the regulation of FtsZ polymerization by MipZ. Based on all the experimental data obtained, we have been able to create a model of the inhibitory activity of MipZ, in which it acts as a minus-end capper and severer. Although its affinity for FtsZ is not very high, a MipZ dimer can cap two FtsZ monomers/polymers and prevent them to incorporate into the forming filament. MipZ can produce a conformational change in FtsZ, which could stimulate the depolymerization process, by binding in the C-terminal region of the core of the protein, close to the T7-loop, which is necessary for FtsZ polymerization.
