PROJECT SUMMARY/ABSTRACT My group seeks to understand, in molecular detail, the steps taken by each of the major classes of membrane proteins to achieve their final assembled state. About one-quarter of all genes code for membrane proteins that are first inserted into the plasma membrane of prokaryotes or the endoplasmic reticulum (ER) of eukaryotes. These proteins perform many essential functions as receptors, channels, enzymes, anchors and transporters. Biosynthesis of membrane proteins is an inherently inefficient process, and numerous human diseases are linked to defective folding of membrane proteins. Thus, understanding how membrane proteins are made is a fundamental question in cell biology with important implications for the treatment of human diseases. Of the ~5,000 membrane proteins coded in the human genome, the majority have more than one transmembrane domain. Yet our understanding of how these “multi-pass” proteins are inserted, folded and assembled into functional entities is at an early stage. Work in my group over the past several years led us to discover a novel ~390 kDa translocon in the ER that is involved in the biogenesis of most multi-pass membrane proteins in human cells. We are now focused on defining the molecular mechanisms underlying this process, using an interdisciplinary set of biochemical, structural, cell biological, genetic and bioinformatic approaches. These studies promise new insight into the fundamental biological challenge of membrane protein biogenesis.