Bacterial cell division (cytokinesis) is a complex subcellular differentiation event that is often achieved with remarkable rapidity. About twenty minutes after dividing, an exponentially growing Escherichia coli cell has doubled its length and begins to divide again. In order to ensure equipartitioning of the newly replicated genome into daughter cells, a division septum forms at the center of the elongated rod-shaped cell by coordinated ingrowth of the cytoplasmic membrane, the rigid murein (peptidoglycan) layer, and the outer membrane of the cell envelope. The invagination process is complete within a few minutes and, after the membranes have sealed, the daughter cells with their discrete genomes separate and continue to grow.
Formation of the division septum must be temporally and topologically coordinated with other cellular events such as chromosome replication and nucleoid separation. Temporal regulation is essential to ensure that septum formation does not occur before chromosome segregation is completed, while topological regulation ensures that the septum is formed at midcell to permit equipartitioning of cytosolic components into daughter cells. The existence of efficient mechanisms for coordinating septation is evidenced by the remarkable fidelity with which the division septum is placed at midcell at the correct time in the cell cycle -- there is minimal production of chromosomeless minicells resulting from an off-center division event, and nucleoids are rarely guillotined by septal closure across an incompletely segregated chromosome.
In E. coli, topological regulation of cell division is accomplished by the coordinated action of three proteins, MinC, MinD, and MinE. MinC and MinD act in concert to form a nonspecific division inhibitor that is capable of blocking septation at all potential division sites. In E. coli, MinC and MinD oscillate from one cell pole to the other, thereby causing a periodic block of the polar division sites. MinE protects the central division site by localizing to an annular structure at or very close to midcell. MinCD action is antagonised in the vicinity of this ring due the ability of MinE to dissociate the MinCD complex.
Despite significant recent advances in our understanding of the cellular localization of the Min proteins, we still do not understand their mechanism of action. A key to understanding topological regulation in these bacteria will be determination of the three-dimensional structures of these proteins and elucidation of the molecular details of their interactions with one another; these are the immediate goals of this research program. A variety of methods (NMR, X-ray crystallography, circular dichroism) are being used to solve the structures of these proteins and structure-directed mutagenesis is being used to elucidate key functional epitopes. In vitro interaction assays and protease mapping in combination with structure determination techniques are being used to probe the molecular details of the interactions between these proteins.
Recently, we solved the structure of the MinE topological specificity domain (see figure above) and used structure-directed mutagenesis to identify a spatially restricted site on the surface of the protein that is critical for its topological specificity func tion.
Relevant papers
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