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The ER-Golgi v-SNARE Bet1p is required for cross-linking -agglutinin to the cell wall in yeast

Dept of Biological Sciences and the Center for Gene Structure and Function, Hunter College of the City University of New York, 695 Park Ave, New York, NY 10021, USA

Correspondence
Peter Lipke
lipke{at}genectr.hunter.cuny.edu

In Saccharomyces cerevisiae, glycosylphosphatidylinositol (GPI)-anchored cell wall mannoproteins, including -agglutinin, are secreted to the cell surface through vesicular transport pathways. At the cell surface the GPI anchors are cleaved within the glycan, then transglycosylated to form a covalent cross-link to 1,6--glucan. Among mutants that were temperature-sensitive for growth and for ability to cross-link the mannoprotein -agglutinin to the cell wall, one strain was complemented by BET1, which encodes an ER-Golgi v-SNARE. Temperature-sensitive mutations in BET1 caused aberrations in cell wall structure, including excretion of -agglutinin into the medium, sensitivity to lysis with Zymolyase and hypersensitivity to Calcofluor White. At restrictive temperatures, bet1 mutations block secretion of invertase and other proteins, but -agglutinin was excreted into the extracellular medium. In wild-type parental or bet1 cells, secretion of -agglutinin also continued after protein synthesis was blocked with cycloheximide. This secretion was due to continued export of a significant amount of -agglutinin from compartments distal to the BET1-dependent secretion step. Thus, in bet1 cells the ER-Golgi block allowed secretion to continue, but prevented cell wall incorporation of the -agglutinin. Therefore, a mutation early in the secretion pathway caused aberrant cell wall synthesis by preventing localization of key components required in wall cross-links.

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