This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Endocytosis followed by sorting to the lysosome is a key mechanism for downregulating the abundance of cell surface receptors and transporters. In a genome-wide chemical genetic screen we have identified a poorly characterized S. cerevisiae protein that is required for endosomal sorting of many cell surface proteins. Potential homologs in higher species have also been identified. Phenotype of the yeast strain lacking this gene suggests a defect in fusion/docking of endosomal vesicles. The protein localizes to the cytoplasm as well as endosomes. The cytoplasmic pool appears as a homogenous, 650-kDa complex in gel filtration experiments. We hypothesize that the cytosolic complex may be a subcomplex of the endosome-bound form. Thus, understanding the composition of the 650-kD soluble complex may lead to a better understanding of the mechanism of endosome fusion and docking. We propose to study the composition of the TAP-purified complex by mass spectrometry.