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

Epiglycanin is a mucin-type glycoprotein present at the surface of TA3Ha mo use mammary tumour cells. It is a long rod-like glycoprotein with a molecul ar mass of 500 kDa. Its function has not vet been established but its overe xpression can affect cell-cell and cell-matrix adhesion. To understand bett er the biological function of epiglycanin, we have studied the biochemical structure and biosynthesis of epiglycanin in TA3Ha cells. Pulse-chase label ling experiments with [H-3]threonine revealed an early precursor with a mol ecular mass of approx. 300 kDa containing approx. 5-10 kDa of N-linked glyc ans. The precursor was gradually converted into a high-molecular-mass matur e form, owing mainly, if not entirely, to O-glycosylation The mature molecu le consists of two major glycoforms that differ in sialylation. Unlike secr eted mucins, epiglycanin did not form cysteine-bound multimers, providing f urther evidence that epiglycanin belongs to the class of membrane-associate d mucins. The mature form, but not the precursor form, is shed from the cel l 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 bio chemical structure and biosynthesis of epiglycanin.