Reciprocal relationship between α1,2 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))Man(5)GlcNAc(2) oligosaccharides are transferred onto nascent proteins. Pulse-chase experiments indicate that these newly synthesized glycoproteins are retained in intracellular compartments and converted to Man(4)GlcNAc(2) species. In this paper, we demonstrate that the alpha 1,2 mannosidase, which is involved in the processing of Man(5)GlcNAc(2) into Man(4)GlcNAc(2), 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, Man(4)GlcNAc(2) species were produced at the expense of Glc(1)Man(5)GlcNAc(2) species. Thus, the trimming of Man(4)GlcNAc(2) to Man(4)GlcNAc(2), which is catalyzed by this mannosidase, could be involved in the control of the glucose-dependent folding pathway.