Biosynthesis and modification of Golgi mannosidase II in HeLa and 3T3 cells

Abstract

The biosynthesis and post-translational modification of mannosidase II, an enzyme required in the maturation of asparagine-linked oligosaccharides in the Golgi complex, has been investigated. Antibody raised against this enzyme purified from rat liver Golgi membranes was used to immunoprecipitate mannosidase II from rat liver, 3T3 cells, or HeLa cells. Mannosidase II immunoprecipitated from rat liver Golgi membranes, when analyzed by polyacrylamide gel electrophoresis, migrated with an apparent molecular weight of approximately 124,000. In contrast, the enzyme purified from rat liver Golgi membranes was shown to contain both the 124,000-dalton component and a 110,000-dalton polypeptide believed to result from degradation of intact mannosidase II during purification. Mannosidase II from 3T3 and HeLa cells migrated on polyacrylamide gels with apparent molecular weights of approximately 124,000 and 134,000-136,000, respectively. When immunoprecipitated from radiolabeled cultures, mannosidase II from both cell types was similar in the following respects: (a) the initial synthesis product had an apparent molecular weight of approximately 124,000; (b) in cultures treated with tunicamycin the initial synthesis product had an apparent molecular weight of approximately 117,000; (c) endoglycosidase H digestion of the initial synthesis product gave an apparent molecular weight similar to the tunicamycin-induced polypeptide; (d) the mature enzyme was mostly (HeLa) or entirely (3T3) resistant to digestion by endoglycosidase H. Loss of [35S]methionine from intracellular mannosidase II occurred with a half-life of approximately 20 h; there was no appreciable accumulation of labeled immuno-reactive material in the medium. HeLa mannosidase II, but not the 3T3 enzyme, was additionally modified 1-3 h after synthesis, the initial synthesis product being converted to a doublet with an apparent molecular weight of approximately 134,000-136,000. Evidence is presented that this mobility shift may result from O-glycosylation. Mannosidase II from both cell types could be labeled with [32P]phosphate or [35S]sulfate. The latter is apparently attached to oligosaccharide as indicated by inhibition of labeling by tunicamycin; the former was shown with the HeLa enzyme to be present as serine phosphate moieties. In addition, [3H]palmitate could be incorporated into the enzyme in 3T3 cells.(ABSTRACT TRUNCATED AT 400 WORDS)