Defective mucin-type glycosylation on α-dystroglycan in COG-deficient cells increases its susceptibility to bacterial proteases

Deficiency in subunits of the conserved oligomeric Golgi (COG) complex results in pleiotropic defects in glycosylation and cause congenital disorders in humans. Insight regarding the functional consequences of this defective glycosylation and the identity of specific glycoproteins affected are lacking. A chemical glycobiology strategy was adopted to identify surface glycoproteins most sensitive to altered glycosylation in COG-deficient CHO cells. Following metabolic labeling, an unexpected increase in GalNAz incorporation into several glycoproteins, including alpha-dystroglycan (α-DG), was noted in cog1-deficient ldlB cells. Western blot analysis showed a significantly lower molecular weight for α-DG in ldlB cells compared to WT CHO cells. The underglycosylated α-DG molecules on ldlB cells are highly vulnerable to bacterial proteases that co-purify with V. cholerae neuraminidase, leading to rapid removal of the protein from the cell surface. The purified bacterial mucinase StcE can cleave both WT and ldlB α-DG but did not cause rapid degradation of the fragments, implicating other V. cholerae proteases in the final proteolysis of the fragments. Extending terminal glycosylation on the existing mucin-type glycans of ldlB α-DG stabilized the resulting fragments, indicating that fragment stability, but not the initial fragmentation of the protein, is influenced by the glycosylation status of the cell. This discovery highlights a functional importance for mucin-type O-glycans found on α-DG and reinforces a growing role for these glycans as regulators of extracellular proteolysis and protein stability.