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Glycobiology, 2002, Vol. 12, No. 11 749-762
© 2002 Oxford University Press

The Saccharomyces cerevisiae alg12{Delta} mutant reveals a role for the middle-arm {alpha}1,2Man- and upper-arm {alpha}1,2Man{alpha}1,6Man- residues of Glc3Man9GlcNAc2-PP-Dol in regulating glycoprotein glycan processing in the endoplasmic reticulum and Golgi apparatus

John F. Cipollo1,3 and Robert B. Trimble2,3,4

3 Department of Biomedical Sciences, State University of New York at Albany School of Public Health, Albany, NY 12201, USA and 4 Wadsworth Center, New York State Department of Health, P.O. Box 509, Albany, NY 12201, USA

N-glycosylation in nearly all eukaryotes proceeds in the endoplasmic reticulum (ER) by transfer of the precursor Glc3Man9GlcNAc2 from dolichyl pyrophosphate (PP-Dol) to consensus Asn residues in nascent proteins. The Saccharomyces cerevisiae alg (asparagine-linked glycosylation) mutants fail to synthesize oligosaccharide lipid properly, and the alg12 mutant accumulates a Man7GlcNAc2-PP-Dol intermediate. We show that the Man7GlcNAc2 released from alg12{Delta}-secreted invertase is Man{alpha}1,2Man{alpha}1,2Man{alpha}1,3(Man{alpha}1,2Man{alpha}1,3Man{alpha}1,6)-Manß1,4-GlcNAcß1-4GlcNAc{alpha}/ß, confirming that the Man7GlcNAc2 is the product of the middle-arm terminal {alpha}1,2-mannoslytransferase encoded by the ALG9 gene. Although the ER glucose addition and trimming events are similar in alg12{Delta} and wild-type cells, the central-arm {alpha}1,2-linked Man residue normally removed in the ER by Mns1p persists in the alg12{Delta} background. This confirms in vivo earlier in vitro experiments showing that the upper-arm Man{alpha}1,2Man{alpha}1,6-disaccharide moiety, missing in alg12{Delta} Man7GlcNAc2, is recognized and required by Mns1p for optimum mannosidase activity. The presence of this Man influences downstream glycan processing by reducing the efficiency of Ochlp, the cis-Golgi {alpha}1,6-mannosyltransferase responsible for initiating outer-chain mannan synthesis, leading to hypoglycosylation of external invertase and vacuolar protease A.

1 Present address: Boston University Goldman School of Dental Medicine, Boston, MA 02118–2392, USA

2 To whom correspondence should be addressed; E-mail: trimble@wadsworth.org


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E. S. Trombetta
The contribution of N-glycans and their processing in the endoplasmic reticulum to glycoprotein biosynthesis
Glycobiology, September 1, 2003; 13(9): 77R - 91R.
[Abstract] [Full Text] [PDF]


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