Activity of recycling Golgi mannosyltransferases in the yeast endoplasmic reticulum

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 alpha1,6-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 alpha1,6-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.