Sorting signals can direct receptor-mediated export of soluble proteins into COPII vesicles.
Soluble secretory proteins are first translocated across endoplasmic reticulum (ER) membranes and folded in a specialized ER luminal environment. Fully folded and assembled secretory cargo are then segregated from ER-resident proteins into COPII-derived vesicles or tubular elements for anterograde transport. Mechanisms of bulk-flow, ER-retention and receptor-mediated export have been suggested to operate during this transport step, although these mechanisms are poorly understood. In yeast, there is evidence to suggest that Erv29p functions as a transmembrane receptor for the export of certain soluble cargo proteins including glycopro-alpha-factor (gpalphaf), the precursor of alpha-factor mating pheromone. Here we identify a hydrophobic signal within the pro-region of gpalphaf that is necessary for efficient packaging into COPII vesicles and for binding to Erv29p. When fused to Kar2p, an ER-resident protein, the pro-region sorting signal was sufficient to direct Erv29p-dependent export of the fusion protein into COPII vesicles. These findings indicate that specific motifs within soluble secretory proteins function in receptor-mediated export from the ER. Moreover, positive sorting signals seem to predominate over potential ER-retention mechanisms that may operate in localizing ER-resident proteins such as Kar2p.
Otte
Stefan
S
Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755, USA.
Barlowe
Charles
C
D022181:COP-Coated Vesicles / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005656:Fungal Proteins / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D000072235:Mating Factor; D008565:Membrane Proteins / Q000378:metabolism; D010455:Peptides / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D011498:Protein Precursors / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D011506:Proteins / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D012995:Solubility; D033921:Vesicular Transport Proteins
C432391:Erv29 protein, S cerevisiae; D005656:Fungal Proteins; D018840:HSP70 Heat-Shock Proteins; C060153:KAR2 protein, yeast; D008565:Membrane Proteins; D010455:Peptides; D011498:Protein Precursors; D011506:Proteins; D011956:Receptors, Cell Surface; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; D000072235:Mating Factor