Project Summary The objective of this proposal is to gain mechanistic and pathological understanding of protein glycosylation and trafficking. During the previous funding cycle, we have made important contributions to this process. We have determined the structure of the transmembrane domain insertase EMC complex, revealing an elongated cavity in the transmembrane region of the structure that can accommodate a weakly hydrophobic transmembrane helix. We have solved the structures of the protein N-glycosyltransferase (OST) and the protein O-mannosyltransferase Pmt1-Pmt2, revealing the evolutionarily conserved GT-C folds of their catalytic subunits. Protein trafficking requires lipid vesicle formation, a process that is dependent on lipid flippase activity to establish compositional asymmetry between the two leaflets of the bilayer. In this regard, we have determined the structures of all three classes of yeast lipid flippases. This renewal proposal continues our overarching goal to understand protein glycosylation and trafficking. We propose to address two specific knowledge gaps: the structure and mechanism of two protein mannosyltransferases, and how the recently discovered ternary protein complex Arl1-Gea2-Drs2 couples lipid flipping activity with membrane curvature formation, thereby facilitating the downstream vesicle budding process for protein and membrane trafficking. Drs2 is a phosphatidylserine flippase required for the formation of AP-1/clathrin-coated vesicles that travel back and forth between the trans-Golgi network (TGN) and early endosomes. The small GTPase Arl1 is a member of the ADP-ribosylation factor (Arf) family that is activated by the Arf guanine nucleotide exchange factor Gea2. Arl1 operates exclusively in the TGN and is the least well-understood member of the Arf family. We will reconstitute the ternary complex in vitro and perform a comprehensive structure-function study. Protein glycosylation and trafficking is intimately linked to tumorigenesis and cancer progression. Our mechanistic studies will fill these fundamental knowledge gaps, and our derived structures may facilitate the development of small molecules for cancer treatment.