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 500 kDa. 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 [(3)H]threonine revealed an early precursor with a molecular mass of approx. 300 kDa containing approx. 5-10 kDa 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. 60 h. These results provide the first detailed analysis of the biochemical structure and biosynthesis of epiglycanin.