The presence of an ER exit signal determines the protein sorting upon ER exit in yeast

Biochem J. 2008 Sep 1;414(2):237-45. doi: 10.1042/BJ20080715.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Transport Systems / chemistry
  • Amino Acid Transport Systems / genetics
  • Amino Acid Transport Systems / metabolism
  • Endoplasmic Reticulum / metabolism*
  • Golgi Apparatus / metabolism
  • Immunoprecipitation
  • Open Reading Frames
  • Protein Binding
  • Protein Precursors / genetics
  • Protein Precursors / metabolism
  • Protein Transport
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / metabolism*
  • Saccharomyces cerevisiae Proteins / chemistry
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism*
  • Vesicular Transport Proteins / chemistry
  • Vesicular Transport Proteins / genetics
  • Vesicular Transport Proteins / metabolism

Substances

  • Amino Acid Transport Systems
  • GAP1 protein, S cerevisiae
  • MF(ALPHA)1 protein, S cerevisiae
  • Protein Precursors
  • Saccharomyces cerevisiae Proteins
  • Vesicular Transport Proteins