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Genome-wide analysis of endocytic recycling in S. cerevisiae Burston, Helen Elizabeth

Abstract

The process of endocytic recycling, in which cell surface proteins are internalized and re-delivered to the plasma membrane, is essential in all eukaryotes for maintaining plasma membrane composition and regulating the surface levels of signaling receptors. The applicability of Saccharomyces cerevisiae as a model to study endocytic recycling is a subject of debate, as there appears to be critical differences between yeast and mammalian cells. For example, while clathrin and its adaptors are critical for uptake in mammals, they do not seem to be essential in yeast. Endocytic recycling has not been comprehensively studied on a genetic level in yeast, and only limited cargo have been considered, making it difficult to accurately assess the similarity between the two systems. Furthermore, the transport of SNARE proteins is poorly understood, but appears to involve specialized mechanisms. This study uses a genome-wide screening approach to systematically and quantitatively identify genes required for the endocytic recycling of the yeast SNARE protein Snc1, homologous to the mammalian VAMP2/synaptobrevin. Endocytic defects for mutants of many yeast homologs of mammalian endocytosis genes were identified, for the first time. Significantly, a cargo-selective and partially-redundant role for clathrin and its adaptors yAP1801 and yAP1802 was identified. The lipid phosphatase Inp52 was found to mediate AP180 release from endocytic vesicles. Additionally, the previously uncharacterized protein Ldb17, homologous to the mammalian endocytic protein SPIN90, was identified as a new component of the endocytic machinery, and regulates both coat and actin dynamics at endocytic sites. Factors regulating Snc1 recycling were also identified, including the variant clathrin adaptor AP-1R. This is the first reported function for this complex. The previously uncharacterized protein Ima1 was found to be a putative enzyme that specifically binds to AP-1R, and may have activity related to AP-1R function. Overall, this study demonstrates that endocytic recycling in yeast and mammals is more similar than previously appreciated, and identifies new factors in this process. Furthermore, it raises awareness of the degree of cargo-selectivity underlying this pathway, and demonstrates quantitative methods that can be further applied to future studies in both systems.

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