Interactions in a multi-protein complex required for bacterial cell division

In Escherichia coli , the process of cell division is mediated by a complex of proteins, called the divisome. Currently, 23 proteins have been identified as components of the divisome in E. coli , ten of which are essential for division. Assembly of the divisomal proteins at midcell occurs by a mostly linear hierarchy. While this suggests a stepwise pathway for assembly, it appears that additional interactions amongst the divisomal proteins are necessary for assembly and stability of the divisome. In this work, I focused on a subcomplex of essential division proteins composed of FtsQ, FtsL and FtsB. In addition to participating in this subcomplex, FtsB and FtsL can interact independently of FtsQ, and are capable, on their own, of recruiting to midcell the cell division proteins FtsW and FtsI. Specifically, my dissertation work focused on studying how FtsB and FtsL interact with each other and with other cell division proteins. We identified domains of FtsB and FtsL that are involved in interaction with each other, with FtsQ and for the recruitment of division proteins by characterizing truncations of these proteins. The C-terminal domains of both FtsB and FtsL are required for interaction with FtsQ but are dispensable for other interactions. In combination with studies that identified the C-terminus of FtsQ as being important for interaction with FtsB and FtsL, it appears that assembly of the FtsQ/FtsL/FtsB occurs via their C-terminal domains. The interaction between FtsB and FtsL requires their transmembrane domains and periplasmic coiled-coil motifs. Finally, the cytoplasmic domain of FtsL plays a role in recruiting FtsW to division sites. Our identification of interaction domains within FtsB and FtsL, in combination with their small size and lack of significant sequence conservations, suggests that these proteins act as scaffolds for assembly of the divisome. We also examined the presence or absence of FtsB, FtsL and FtsQ homologs in bacteria. We identified homologs of these three proteins in a majority of bacteria examined, suggesting that this subcomplex is important for cell division in a diverse set of bacteria, perhaps by serving as a scaffold for assembly of division proteins.