Yeast Exocytic v-SNAREs Confer Endocytosis

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Abstract
In yeast, homologues of the synaptobrevin/VAMP family of v-SNAREs (Snc1 and Snc2) confer the docking and fusion of secretory vesicles at the cell surface. As no v-SNARE has been shown to confer endocytosis, we examined whether yeast lacking the SNC genes, or possessing a temperature-sensitive allele of SNC1 (SNC1(ala43)), are deficient in the endocytic uptake of components from the cell surface. We found that both SNC and temperature-shifted SNC1(ala43) yeast are deficient in their ability to deliver the soluble dye FM4-64 to the vacuole. Under conditions in which vesicles accumulate, FM4-64 stained primarily the cytoplasm as well as fragmented vacuoles. In addition, alpha-factor-stimulated endocytosis of the alpha-factor receptor, Ste2, was fully blocked, as evidenced using a Ste2-green fluorescent protein fusion protein as well as metabolic labeling studies. This suggests a direct role for Snc v-SNAREs in the retrieval of membrane proteins from the cell surface. Moreover, this idea is supported by genetic and physical data that demonstrate functional interactions with t-SNAREs that confer endosomal transport (e.g., Tlg1,2). Notably, Snc1(ala43) was found to be nonfunctional in cells lacking Tlg1 or Tlg2. Thus, we propose that synaptobrevin/VAMP family members are engaged in anterograde and retrograde protein sorting steps between the Golgi and the plasma membrane.