Golgi apparatus immunolocalization of endomannosidase suggests post-endoplasmic reticulum glucose trimming: Implications for quality control

Trimming of N-linked oligosaccharides by endoplasmic reticulum (ER) glucosi dase II is implicated in quality control of protein folding. An alternate g lucosidase II-independent deglucosylation pathway exists, in which endo-ar- mannosidase cleaves internally the glucose-substituted mannose residue of o ligosaccharides. By immunogold labeling, we detected most endomannosidase i n cis/medial Golgi cisternae (83.8% of immunogold labeling) and less in the intermediate compartment (15.1%), but none in the trans-Golgi apparatus an d ER, including its transitional elements. This dual localization became mo re pronounced under 15 degreesC conditions indicative of two endomannosidas e locations. Under experimental conditions when the intermediate compartmen t marker p58 was retained in peripheral sites, endomannosidase was redistri buted to the Golgi apparatus. Double immunogold labeling established a mutu ally exclusive distribution of endomannosidase and glucosidase II, whereas calreticulin was observed in endomannosidase-reactive sites (17.3% in inter mediate compartment, 5.7% in Golgi apparatus) in addition to the ER (77%). Our results demonstrate that glucose trimming of N-linked oligosaccharides is not Limited to the ER and that protein deglucosylation by endomannosidas e in the Golgi apparatus and intermediate compartment additionally ensures that processing to mature oligosaccharides can continue. Thus, endomannosid ase localization suggests that a quality control of N-glycosylation exists in the Golgi apparatus.