title: Activity of recycling Golgi mannosyltransferases in the yeast endoplasmic reticulum.
abstract: In yeast primary N- and O-glycans are attached to proteins in the endoplasmic reticulum (ER), and they are elongated in the Golgi. Thus, glycan extension by Golgi enzymes has been taken as evidence for arrival of a protein in the Golgi. Two alpha 1,6-mannosyltransferase activity-containing multiprotein complexes have been reported to recycle between the Golgi and the ER, but since resident ER proteins are not Golgi-modified, Golgi enzymes were not thought to function in the ER. Here we show that when protein exit from the ER was blocked in COPII-defective yeast mutants, the N-glycans of vacuolar carboxypeptidase Y and a set of unidentified glycoproteins were decorated with an alpha 1,6-mannose residue, normally added in the Golgi by Och1p. Immunofluorescent staining demonstrated that Och1p accumulated in the ER under these conditions. Concomitantly, primary O-glycans of a secretory protein were extended, apparently by the medial Golgi transferase Mnt1p. Similar O-glycan extension occurred in wild-type cells when an HDEL-tagged protein was allowed to encounter glycosyltransferases in the Golgi during recycling between ER and Golgi. Golgi-specific glycosylation in the ER was reduced when Golgi-to-ER traffic was blocked, confirming that glycan extension in the ER was mainly due to recycling, rather than newly synthesized transferases.
authors.affiliation: Program in Cellular Biotechnology, Institute of Biotechnology, University of Helsinki, Viikinkaari 9, 00710 Helsinki, Finland.
authors.forename: Leena
authors.initials: L
authors.lastname: Karhinen
authors.forename: Marja
authors.initials: M
authors.lastname: Makarow
mesh_terms: D022181:COP-Coated Vesicles / Q000378:metabolism; D043402:Cathepsin A / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006023:Glycoproteins / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism*; D008358:Mannose / Q000378:metabolism; D008364:Mannosyltransferases / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism; D009154:Mutation; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism
chemical_list: D006023:Glycoproteins; D008562:Membrane Glycoproteins; C075636:OCH1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D008364:Mannosyltransferases; D043402:Cathepsin A; D008358:Mannose