Monoglucosylated Oligomannosides are Released during the Degradation Process of Newly Synthesized Glycoproteins

Abstract

The Chinese hamster ovary mutant MI8-5 is known to synthesize Man 9GlcNAc2-P-P-dolichol rather than the fully glucosylated lipid intermediate Glc3Man9GlcNAc 2-P-P-dolichol. This nonglucosylated oligosaccharide lipid precursor is used as donor for N-glycosylation. In this paper we demonstrate that a significant part of the glycans bound to the newly synthesized glycoproteins in MI8-5 cells are monoglucosylated. The presence of monoglucosylated glycans on glycoproteins determines their binding to calnexin as part of the quality control machinery. Furthermore, we point out the presence of Glc 1Man5GlcNAc1 in the cytosol of MI8-5 cells. This indicates that part of the monoglucosylated glycoproteins can be directed toward a deglycosylation process that occurs in the cytosol. Besides studies on glycoprotein degradation based on the disappearance of protein moieties, MI8-5 cells can be used as a tool to elucidate the various step leading to glycoprotein degradation by studying the fate of the glycan moieties.