title: The presence of an ER exit signal determines the protein sorting upon ER exit in yeast.
abstract: In yeast, there are at least two vesicle populations upon ER (endoplasmic reticulum) exit, one containing Gap1p (general aminoacid permease) and a glycosylated alpha-factor, gpalphaF (glycosylated proalpha-factor), and the other containing GPI (glycosylphosphatidylinositol)-anchored proteins, Gas1p (glycophospholipid-anchored surface protein) and Yps1p. We attempted to identify sorting determinants for this protein sorting event in the ER. We found that mutant Gas1 proteins that lack a GPI anchor and/or S/T region (serine- and threonine-rich region), two common characteristic features conserved among yeast GPI-anchored proteins, were still sorted away from Gap1p-containing vesicles. Furthermore, a mutant glycosylated alpha-factor, gpalphaGPI, which contains both the GPI anchor and S/T region from Gas1p, still entered Gap1p-containing vesicles, demonstrating that these conserved characteristics do not prevent proteins from entering Gap1p-containing vesicles. gpalphaF showed severely reduced budding efficiency in the absence of its ER exit receptor Erv29p, and this residual budding product no longer entered Gap1p-containing vesicles. These results suggest that the interaction of gpalphaF with Erv29p is essential for sorting into Gap1p-containing vesicles. We compared the detergent solubility of Gas1p and the gpalphaGPI in the ER with that in ER-derived vesicles. Both GPI-anchored proteins similarly partitioned into the DRM (detergent-resistant membrane) in the ER. Based on the fact that they entered different ER-derived vesicles, we conclude that DRM partitioning of GPI-anchored proteins is not the dominant determinant of protein sorting upon ER exit. Interestingly, upon incorporation into the ER-derived vesicles, gpalphaGPI was no longer detergent-insoluble, in contrast with the persistent detergent insolubility of Gas1p in the ER-derived vesicles. We present different explanations for the different behaviours of GPI-anchored proteins in distinct ER-derived vesicle populations.
authors.affiliation: Department of Biochemistry, University of Geneva, Sciences II, 30 quai Ernest Ansermet, CH-1211 Geneva, Switzerland.
authors.forename: Reika
authors.initials: R
authors.lastname: Watanabe
authors.forename: Guillaume A
authors.initials: GA
authors.lastname: Castillon
authors.forename: Anja
authors.initials: A
authors.lastname: Meury
authors.forename: Howard
authors.initials: H
authors.lastname: Riezman
mesh_terms: D026905:Amino Acid Transport Systems / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D047468:Immunoprecipitation; D016366:Open Reading Frames; D011485:Protein Binding; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D026905:Amino Acid Transport Systems; C491145:GAP1 protein, S cerevisiae; C048632:MF(ALPHA)1 protein, S cerevisiae; D011498:Protein Precursors; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins