The long term goal of this research is to understand the molecular basis of receptor trafficking in mammalian cells. The goal of this proposal is to study how one type of receptor is collected into a transport vesicle and delivered to another subcellular compartment. Specifically, we seek to understand the mechanism by which a newly discovered protein, TIP47, facilitates the collection of mannose 6-phosphate receptors into transport vesicles that bud from endosomes and carry the receptors to the trans Golgi network. TIP47 recognizes a phenylalanine/tryptophan signal in the cytoplasmic domain of the cation-dependent MPR that is essential for its retrieval from endosomes and delivery to the Golgi. We describe here experiments designed to investigate why mannose 6- phosphate receptors are recognized by a specific trafficking machinery depending upon their intracellular localization. We also propose to continue our analysis of the Rab9 GTPase in terms of how it regulates MPR trafficking. Rab GTPases are believed to function in vesicle docking. We have discovered a novel Rab9 effector, p40, which binds preferentially to the active, GTP-bound form of Rab9. p40 is a highly active transport factor. We propose experiments designed to determine if p40 functions in vesicle docking. These studies have important implications for our understanding of growth control and antigen processing, and will provide fundamental information regarding the mechanism of receptor trafficking in mammalian cells.