Biosynthesis and shedding of epiglycanin: a mucin-type glycoprotein of the mouse TA3Ha mammary carcinoma cell

Epiglycanin is a mucin-type glycoprotein present at the surface of TA3Ha mouse mammary tumour cells. It is a long rod-like glycoprotein with a molecular mass of 500kDa. Its function has not yet been established but its overexpression can affect cell–cell and cell–matrix adhesion. To understand better the biological function of epiglycanin, we have studied the biochemical structure and biosynthesis of epiglycanin in TA3Ha cells. Pulse–chase labelling experiments with [³H]threonine revealed an early precursor with a molecular mass of approx. 300kDa containing approx. 5–10kDa of N-linked glycans. The precursor was gradually converted into a high-molecular-mass mature form, owing mainly, if not entirely, to O-glycosylation. The mature molecule consists of two major glycoforms that differ in sialylation. Unlike secreted mucins, epiglycanin did not form cysteine-bound multimers, providing further evidence that epiglycanin belongs to the class of membrane-associated mucins. The mature form, but not the precursor form, is shed from the cell surface. The half-life of epiglycanin on the cell surface was found to be approx. 60h. These results provide the first detailed analysis of the biochemical structure and biosynthesis of epiglycanin.