The trans-Golgi network (TGN) and endosomes are major sites for sorting and trafficking of a variety of biosynthetic and endocytic proteins, including factors that are involved in cholesterol homeostasis, virus infection, and cell growth regulation. Protein transport between the TGN and endosomes occurs via vesicles surrounded by coats composed mainly of clathrin and the adaptors AP-1 and Gga. Though these key coat components have been well-studied, our knowledge about accessory factors that are needed to regulate clathrin-mediated transport between the TGN and endosomes is less complete. This study will characterize an evolutionarily conserved putative clathrin accessory protein of Saccharomyces cerevisiae that appears to act specifically in AP-1 mediated transport. Its function and role in protein trafficking will be addressed by examining clathrin-mediated transport in cells lacking the protein and additional accessory factors will be identified in screens for binding partners. These studies will advance our understanding of the mechanisms of protein transport between the TGN and endosomes and help distinguish between the currently unresolved AP-1 and Gga-mediated pathways. [unreadable] [unreadable]