PROJECT SUMMARY The outer membrane in Gram-negative bacteria is a dynamic interface that mediates bacterial interaction with the host by displaying proteins on its surface. The recently discovered surface-exposed lipoproteins (SLPs) play critical roles in pathogenesis, including iron acquisition, adhesion, immune evasion and serve as valuable vaccine targets. Despite their biomedical importance, the mechanism underlying lipoprotein localization to the cell surface is the least understood aspect of bacterial envelope biogenesis. The long- term goal of research in my laboratory is to define the mechanism of lipoprotein targeting and export to the bacterial cell surface. We expect that groups of lipoproteins with similar topologies and/or structural features share dedicated assembly pathways. Improving our understanding of molecular determinants for export will enable the development of predictive computational models for lipoprotein localization and genomic identification of SLPs. One SLP subfamily includes lipoproteins that depend on a partner β- barrel outer membrane protein (OMP) for surface exposure. We discovered the RcsF lipoprotein in Escherichia coli as the first example of this type. We further discovered that the highly conserved and essential β-barrel assembly machinery (Bam) complex plays a critical role in the biogenesis of RcsF, uncovering the novel function of the Bam complex in lipoprotein biogenesis. Here, we propose to use a combination of genetics, biochemistry, and mass spectrometry approaches to identify the molecular mechanism by which the Bam complex displays lipoproteins on the cell surface. We are specifically interested in how the Bam complex recognizes lipoproteins and coordinates lipoprotein surface exposure with OMP assembly. The completion of the proposed studies will substantially expand our understating of biogenesis of SLPs and the Gram-negative cell envelope. The knowledge gained from the proposed studies will enable formulation of computational models for identification of novel SLPs and much-needed vaccines targets.