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[摘要]：In yeast, there are at least two vesicle populations upon ER (endoplasmic reticulum) exit, one containing Gap 1 p (general amino-acid permease) and a glycosylated alpha-factor, gp alpha F (glycosylated pro alpha-factor), and the other containing GPI (glycosylphospha-tidylinositol)-anchored proteins, Gas 1 p (glycophospholipid-anchored surface protein) and Yps 1 p. We attempted to identify sorting determinants for this protein sorting event in the ER. We found that mutant Gas 1 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 Gap 1 p-containing vesicles. Furthermore, a mutant glycosylated alpha-factor, gp alpha GPI, which contains both the GPI anchor and S/T region from Gas 1 p, still entered Gap 1 p-containing vesicles, demonstrating that these conserved characteristics do not prevent proteins from entering Gap 1 p-containing vesicles. gp alpha F showed severly reduced budding efficiency in the absence of its ER exit receptor Erv29p, and this residual budding product no longer Gap 1 p-containing vesicles. These results suggest that the interaction of gp alpha F with Erv29p is essential for sorting into Gap 1 p-containing vesicles. We compared the detergent solubility of Gas 1 p and the gp alpha GPI 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, gp alpha GPI was no longer detergent-insoluble, in contrast with the persistent detergent insolubility of Gas 1 p in the ER-derived vesicles. We present different explanations for the different behaviours of GPI-anchored proteins in distinct ER-derived vesicle populations. |
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