Reciprocal relationship between alpha1,2 mannosidase processing and reglucosylation in the rough endoplasmic reticulum of Man-P-Dol deficient cells.

The study of the glycosylation pathway of a mannosylphosphoryldolichol-deficient CHO mutant cell line (B3F7) reveals that truncated Glc(0-3)Man5GlcNAc2 oligosaccharides are transferred onto nascent proteins. Pulse-chase experiments indicate that these newly synthesized glycoproteins are retained in intracellular compartments and converted to Man4GlcNAc2 species. In this paper, we demonstrate that the alpha1,2 mannosidase, which is involved in the processing of Man5GlcNAc2 into Man4GlcNAc2, is located in the rough endoplasmic reticulum. The enzyme was shown to be inhibited by kifunensine and deoxymannojirimycin, indicating that it is a class I mannosidase. In addition, Man4GlcNAc2 species were produced at the expense of Glc1Man5GlcNAc2 species. Thus, the trimming of Man5GlcNAc2 to Man4GlcNAc2, which is catalyzed by this mannosidase, could be involved in the control of the glucose-dependent folding pathway.