The GPI transamidase complex of Saccharomyces cerevisiae contains Gaa1p, Gpi8p, and Gpi16p.
Gpi8p and Gaa1p are essential components of the GPI transamidase that adds glycosylphosphatidylinositols (GPIs) to newly synthesized proteins. After solubilization in 1.5% digitonin and separation by blue native PAGE, Gpi8p is found in 430-650-kDa protein complexes. These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). Gpi16p is an essential N-glycosylated transmembrane glycoprotein. Its bulk resides on the lumenal side of the ER, and it has a single C-terminal transmembrane domain and a small C-terminal, cytosolic extension with an ER retrieval motif. Depletion of Gpi16p results in the accumulation of the complete GPI lipid CP2 and of unprocessed GPI precursor proteins. Gpi8p and Gpi16p are unstable if either of them is removed by depletion. Similarly, when Gpi8p is overexpressed, it largely remains outside the 430-650-kDa transamidase complex and is unstable. Overexpression of Gpi8p cannot compensate for the lack of Gpi16p. Homologues of Gpi16p are found in all eucaryotes. The transamidase complex is not associated with the Sec61p complex and oligosaccharyltransferase complex required for ER insertion and N-glycosylation of GPI proteins, respectively. When GPI precursor proteins or GPI lipids are depleted, the transamidase complex remains intact.
Fraering
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Institute of Biochemistry, University of Fribourg, Chemin du Musée 5, CH-1700 Fribourg, Switzerland.
Imhof
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Meyer
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Strub
J M
JM
van Dorsselaer
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A
Vionnet
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Conzelmann
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A
D000217:Acyltransferases / Q000737:chemistry*; D000595:Amino Acid Sequence; D019881:Aminoacyltransferases; D015815:Cell Adhesion Molecules / Q000737:chemistry*; D004721:Endoplasmic Reticulum / Q000737:chemistry*; D006602:Hexosyltransferases; D008055:Lipids / Q000737:chemistry; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins / Q000737:chemistry*; D008565:Membrane Proteins / Q000737:chemistry; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D011498:Protein Precursors / Q000737:chemistry; D000069816:SEC Translocation Channels; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry*; D014166:Transferases / Q000737:chemistry
D015815:Cell Adhesion Molecules; C486254:GAA1 protein, S cerevisiae; C444433:Gpi16 protein, S cerevisiae; D008055:Lipids; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D011498:Protein Precursors; D000069816:SEC Translocation Channels; C499383:SEC61 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D014166:Transferases; D000217:Acyltransferases; D019881:Aminoacyltransferases; C081007:COOH-terminal signal transamidase; C103133:GPI8 protein, S cerevisiae; D006602:Hexosyltransferases; C025620:dolichyl-diphosphooligosaccharide - protein glycotransferase