N-glycosylation is required for the surface localization of MUC17 mucin

The nucleic acid sequence of the human gene, MUC17, indicates that this mucin contains an SEA domain, a transmembrane domain, and putative N-glycosylation sites in the carboxyl terminus. Mucins that possess an SEA domain are usually proteolytically cleaved within that domain to yield two subunits, the smaller of which is associated with the Surface membrane. Homogenates of ASPC-1 pancreatic cancer cells showed three main bands of immunoreactivity with alpha-SEA (a polyclonal antibody directed against a site downstream of the postulated cleavage site) after SDS-PAGE and Western blotting (38, 45, and 49 kDa). Experiments utilizing N-glycan specific hydrolases showed that the 38 kDa band contained high mannose glycans whereas the 45 and 49 kDa bands contained complex-type glycans. Only two smaller alpha-SEA reactive bands (30 and 32 kDa) were present after cells had been treated with the N-glycosylation inhibitor tunicamycin. Surface biotinylation studies showed that only the forms possessing complex-type N-glycans were localized to the cell surface. Both tunicamycin and brefeldin A, an inhibitor of protein transport, reduced surface localization. In summary, our results indicate that the surface localization of the smaller Subunit of MUC17 is dependent on its N-glycosylation status.