O-glycosylation is essential for cell surface expression of the transcobalamin receptor CD320

CD320 is a cell surface receptor that mediates vitamin B₁₂ uptake in most tissues. To date, the mechanisms that regulate CD320 expression on the cell surface are not fully understood. In this work, we studied CD320 expression in transfected human embryonic kidney (HEK) 293 and hepatoma HepG2 cells. By glycosidase and trypsin digestion, monensin and brefeldin treatment, western blotting, flow cytometry, and lectin binding, we found that CD320 underwent N- and O-glycosylation and sialylation, resulting in a ∼70-kDa band that formed a high-molecular-weight complex on the cell surface. Site-directed mutagenesis altering Asn126, Asn195, and Asn213 residues, individually or together, abolished N-glycosylation in CD320 but did not block its intracellular trafficking and expression on the cell surface in HEK293 and HepG2 cells. In contrast, treatment of the cells with Ben-gal, a structural analog of GalNAc-α-1-O-Ser/Thr, inhibited O-glycosylation and cell surface expression of CD320 and decreased vitamin B₁₂ uptake. Analysis of CD320 deletion mutants indicated that O-glycosylation sites in a Ser/Thr-rich region near the transmembrane domain were important for CD320 expression on the cell surface. These results reveal an important role of O-glycans, but not N-glycans, in the intracellular trafficking and cell surface expression of CD320, providing new insights into the cellular mechanisms in regulating CD320 function and vitamin B₁₂ metabolism.