The epithelial sialomucin, episialin, is sialylated during recycling

Episialin, the sialomucin encoded by the MUC1 gene is abundantly present at the surface of various epithelial cells. It is a transmembrane molecule that matures through several intermediate forms generated by proteolysis and sequential addition and processing of numerous O-linked glycans. Here we demonstrate that one of the biosynthetic intermediates, the premature form, which is incompletely glycosylated appears at the cell surface. By neuraminidase protection assays we show that cell surface-associated forms of episialin are constitutively internalized (0.9% of surface fraction/min) and recycled, with the intracellular residence time and total cycle time being approximately 66 and 143 min, respectively. During recycling, sialic acid residues are added to the premature form of episialin resulting in complete maturation of the molecule. Complete sialylation of episialin requires several rounds of recycling which continues after sialylation is apparently completed. The recycling of episialin may serve to achieve and maintain a high degree of sialylation.