Protein-protein and protein-peptidoglycan interactions leading to assembly of the Type II secretion system

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The research of our laboratory is concerned with the structure and function of the two membranes that surround the cells of a major class of bacteria known as the Gram-negatives. We study interactions between the inner and outer membrane as well as the transport of materials across the outer membrane. This transport is a fundamental life process because membranes act as a barrier in protecting all living cells from the environment, but that barrier must be overcome so that nutrients and proteins required for life can cross them. The major focus of our research is the structure and function of the type two secretion system (T2SS) that allows Gram-negative bacteria to secrete degradative enzymes and toxins. In our previous studies we showed that a complex of two membrane proteins, GspAB, is required for the transport and assembly of the secretin (a complex of 12 GspD proteins), which forms the outer membrane channel through which T2SS proteins are secreted. We have demonstrated that in the bacterium Aeromonas hydrophila, GspA binds to peptidoglycan, a major structural component of the cell envelope, and that GspB binds to GspD. In addition we found that in the bacterium Vibrio cholerae a second mechanism involving the protein GspS also functions in the assembly of the secretin. The objectives of our current experiments are to determine how the interactions of the assembly factors GspAB and GspS with both GspD and peptidoglycan result in the formation of the secretin in association with both peptidoglycan and the outer membrane to form the export channel. We hypothesize that ExeAB itself forms a multimeric scaffold in the peptidoglycan and recruits GspD to also bind there. We are using biochemical and genetic approaches to study the molecular structures of the complexes that these proteins form. We are also studying the relationship between the GspAB and GspS secretin assembly pathways in Vibrio cholerae, the only known bacterium in which both of these pathways operate in the assembly of the same secretin. Again biochemical and genetic as well as physiological approaches are being used to determine the interactions between these proteins and where in the bacterial cell the two assembly pathways operate. These experiments form part of our long-term objective of understanding the mechanisms by which bacteria transport proteins out of the cell. This research will provide new information concerning how Gram negative bacteria secrete enzymes and protein toxins, and lead to new strategies for harnessing bacterial protein synthesis and for interfering with toxin secretion. It will also contribute to the broader field of biological sciences through the elucidation of fundamental mechanisms of protein trafficking that take place in all living cells.