TRANSPORT OF AXL2P DEPENDS ON ERV14P, AN ER-VESICLE PROTEIN RELATED TO THE DROSOPHILA CORNICHON GENE-PRODUCT

COPII-coated ER-derived transport vesicles from Saccharomyces cerevisi ae contain a distinct set of membrane-bound polypeptides. One of these polypeptides, termed Erv14p (ER-vesicle protein of 14 kD), correspond s to an open reading frame on yeast chromosome VII that is predicted t o encode an integral membrane protein and shares sequence identity wit h the Drosophila cornichon gene product. Experiments with an epitope-t agged version of Erv14p indicate that this protein localizes to the ER and is selectively packaged into COPII-coated vesicles. Haploid cells that lack Erv14p are viable but display a modest defect in bud site s election because a transmembrane secretory protein, Ax12p, is not effi ciently delivered to the cell surface. Ax12p is required for selection of axial growth sites and normally localizes to nascent bud tips or t he mother bud neck. In erv14 Delta strains, Ax12p accumulates in the E R while other secretory proteins are transported at wild-type rates. W e propose that Erv14p is required for the export of specific secretory cargo from the ER, The polarity defect of erv14 Delta yeast cells is reminiscent of cornichon mutants, in which egg chambers fail to establ ish proper asymmetry during early stages of oogenesis. These results s uggest an unforeseen conservation in mechanisms producing cell polarit y shared between yeast and Drosophila.