Monoglucosylated oligomannosides are released during the degradation process of newly synthesized glycoproteins

The Chinese hamster ovary mutant MI8-5 is known to synthesize Man(9)GlcNAc( 2)-P-P-dolichol rather than the fully glucosylated lipid intermediate Glc(3 )Man(9)GlcNAc(2)-P-P-dolichol. This nonglucosylated oligosaccharide lipid p recursor is used as donor for N-glycosylation. In this paper we demonstrate that a significant part of the glycans bound to the newly synthesized glyc oproteins in MI8-5 cells are monoglucosylated, The presence of monoglucosyl ated glycans on glycoproteins determines their binding to calnexin as part of the quality control machinery. Furthermore, we point out the presence of Glc(1)Man(5)GlcNAc(1) in the cytosol of MI8-5 cells. This indicates that p art of the monoglucosylated glycoproteins can be directed toward a deglycos ylation process that occurs in the cytosol, Besides studies on glycoprotein degradation based on the disappearance of protein moieties, MI8-5 cells ca n be used as a tool to elucidate the various step leading to glycoprotein d egradation by studying the fate of the glycan moieties.