Study of the mechanism of septum localization during bacterial cell division

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Mid-cell localization of the cell division septum in bacteria such as E. coli is controlled by a set of proteins including MinC, MinD, MinE, and FtsZ. FtsZ is the first structural component of the septum to polymerize on the inner membrane at the mid-cell when the cell starts to divide. FtsZ polymerization is limited to mid-cell by the action of the three Min proteins. MinC is an inhibitor of FtsZ polymerization, but on its own, it does not exhibit specific membrane localization. Instead, it binds to MinD dimers that form on the membrane in the presence of ATP: MinD is an ATP-dependent membrane binding protein. The two proteins generally co-localize on the membrane. MinE also interacts with MinD dimer and can displace MinC from MinD. More importantly, MinE controls MinD ATPase activity and also influences its membrane interaction, and hence its membrane association/dissociation dynamics. In vivo imaging studies have demonstrated the oscillating pattern formation by MinD and MinE proteins, resulting in a concentration minimum of MinD, and hence MinC, at the mid-cell region when averaged over time. This observation explained why FtsZ polymerization is restricted to mid-cell. However, a detailed molecular mechanism of this bio-patterning reaction system is still poorly understood. This project aims to investigate the biochemical and biophysical mechanism of the dynamic aspects of this reaction system by combining a variety of techniques, including the exploitation of a cell-free reaction system we established recently that recapitulates aspects of in vivo system dynamics. Techniques and instruments have been developed to study these dynamic reaction systems in vitro by using a sensitive fluorescence microscope/CCD camera system. By using fluorescence-labeled MinD and MinE proteins, assembly and disassembly of these proteins on a supported lipid bilayer on the slide glass surface are monitored under a variety of reaction conditions. We successfully reconstituted a number of inter-converting modes of self-organized dynamic pattern formation by MinD and MinE proteins in the presence of ATP on the membrane surface. Mechanistic details of the dynamic pattern organization are currently studied.