PROJECT SUMMARY The cell envelope of Gram-negative bacteria is characterized by having two lipid bilayers, the inner membrane (IM) and the outer membrane (OM). The OM is not a typical biological membrane because while its inner leaflet contains phospholipids, its outer leaflet is covered with the glycolipid LPS (or lipopolysaccharide). LPS molecules are densely packed at the cell surface, creating a permeability barrier against small hydrophobic molecules that otherwise diffuse across phospholipid bilayers. As a result, Gram-negative bacteria are naturally resistant to many antibiotics. The barrier imposed by LPS is indeed the main reason why very few novel antibiotics effective against Gram-negative pathogens have been developed in recent years. Therefore, studying OM biogenesis is not only important to understand bacterial physiology, but also to devise antimicrobial strategies that can overcome the barrier function of the OM. Our long-term goal is to understand at the molecular level how Gram- negative bacteria build their cell envelope. Here, we will leverage our expertise in genetic and biochemical studies of the cell envelope to investigate two highly conserved systems that are essential for OM biogenesis and growth of the Gram-negative bacterium Escherichia coli. We will investigate how the Lpt system extracts newly synthesized LPS molecules from the IM so that they can be transported across the cell envelope through a protein bridge to be assembled at the cell surface. Our studies will focus on how LPS extraction and transport is powered by the LptB2FGC ATP-binding cassette (ABC) transporter. ABC transporters are ATP-driven machines that all cells use to translocate substrates across cellular compartments. They are powered by an ATPase that transduces the energy derived from binding and hydrolyzing ATP to its transmembrane-domain partners, which translocate the substrate. However, it remains unknown how the actions of the ATPase and cognate transmembrane domains are coupled so that the transporter can function. The LptB2FGC is functionally and structurally unusual: it extracts the glycolipid LPS from the IM to place it onto a protein bridge, and its transmembrane domains LptF/G associate with the transmembrane (TM) helix of another protein, LptC. We propose to investigate the in vivo role of this unprecedented structural feature, and how the function of the LptB2 ATPase is coupled to the action of the transmembrane domains LptF/G during the LPS transport cycle. To do so, we will investigate how LptC’s TM helix downregulates ATPase activity, and how uncharacterized functional domains of LptF/G participate in LPS transport. In addition, we will also study the AsmA-like proteins in E. coli. This family of proteins remain mostly uncharacterized, but we have discovered they perform a function that is essential for growth of E. coli. In this funding period, we will advance our understanding of this protein family by conducting structure-functio...