The compartmental organization of eukaryotic cells relies on accurate distribution of proteins to different compartments and on retention of resident proteins within each compartment. The ability to sort proteins with different destinations is a key feature of the transport machinery which governs protein distribution and retention. The molecular basis of this selectivity is the focus of our proposal. Studies of yeast strains carrying mutations (chc1) in the clathrin heavy chain subunit revealed that clathrin plays an important role in several selective protein transport steps, including selective retention of Golgi membrane proteins, receptor-mediated endocytosis of the mating peptide alpha-factor, and sorting of proteins from the Golgi apparatus to the lysosome-like vacuole. Based on clathrin's function in Golgi membrane protein retention, a genetic screen was employed to isolate two additional mutants (lam mutants) which are defective in Golgi membrane protein retention. These studies provide the basis for a combined genetic and biochemical approach to investigate three aspects of selective protein transport: 1) the mechanism of Golgi membrane protein retention; 2) the role played by clathrin-associated proteins in each clathrin-dependent transport process; 3) the mechanism of clathrin- mediated alpha-factor endocytosis. Clathrin's interaction with Golgi membrane proteins will be investigated biochemically by fractionation of clathrin-coated vesicles, and genetically by determining the localization of mutant Golgi membrane proteins lacking cytoplasmic retention signals in chc1 mutants. Additional lam mutants defective in Golgi membrane protein retention will be identified and used to clone wild-type versions of LAM genes. Antibodies raised against LAM gene products will be used to characterize the Lam proteins. To further define the function of clathrin coats in selective transport, genes encoding clathrin-associated proteins (APs) will be cloned and used to generate gene disruptions. Clathrin-dependent transport pathways will be monitored in mutant strains. Different ap and chc1 mutations will be combined to explore genetic interrelationships. Biochemical fractionations will be used to assess physical interactions between different APs and between APs and clathrin. Finally, an in vitro assay for clathrin-dependent selective transport will be developed by reconstituting alpha-factor endocytosis in permeabilized cells. The functional components of the in vitro reaction will be identified and characterized by complementation of permeabilized mutant cells or fractionation of cytosol. Together, these studies will identify previously unrecognized components of the selective protein transport machinery and address the specific roles played by each identified participant.