Structural and Functional Characterization of the Multipass Translocon

All cells contain hydrophobic lipid bilayers, or membranes, which act as a barrier to the outside of the cell. Additionally, eukaryotic cells contain additional internal membrane- bound compartments known as organelles. Within these membranes exist a diverse set of proteins that play important roles for cellular function and comprise about a third of the proteome. Because these membrane proteins reside and function within a hydrophobic membrane, the cell must complete the challenging task of trafficking them from the cytosol and inserting them into the correct cellular membrane. This process of membrane protein targeting and insertion is known as membrane protein biogenesis. In eukaryotes, most membrane proteins are inserted co-translationally, where they are first targeted to the endoplasmic reticulum and dock to the protein conducting channel Sec61 for their biogenesis. However, it has recently become more evident that the cell contains additional membrane protein biogenesis factors that can facilitate the insertion of more complicated membrane proteins. Here, I present research focusing on the structure and function of a translocon specializing in multipass membrane protein biogenesis. More specifically, this translocon is termed the "Multipass Translocon" and is comprised of three subcomplexes that assemble to Sec61 and the ribosome during multipass membrane protein biosynthesis. These studies reveal the mechanisms by which substrate directs the co-translational assembly of the Multipass Translocon and highlights the importance of the Multipass Translocon for multipass client protein biosynthesis. Additionally, I describe my ongoing efforts to capture structures of multipass folding intermediates within the Multipass Translocon. Overall, this work characterizes the translocon as a dynamic assembly that co-translationally adapts its subunit composition to meet the various biosynthetic requirements of its diverse client proteins.