BIOSYNTHESIS OF A LOW-MOLECULAR-MASS RAT SUBMANDIBULAR-GLAND MUCIN GLYCOPROTEIN IN COS7 CELLS

We have examined the biosynthesis of a low-molecular-mass mucin from r at submandibular gland (RSMG) expressed recombinantly in COS7 tissue c ulture cells, focusing primarily on the addition of carbohydrate to th e protein core of the mucin. We find evidence for N-linked glycosylati on, but this modification is not required for secretion of the mucin. Similarly, although the recombinant RSMG mucin, like its native counte rpart, contains large amounts of O-linked carbohydrate, chain extensio n beyond the initial O-linked GalNAc moiety is not required for secret ion. We have identified partially glycosylated mucin by a combination of metabolic pulse-chase and lectin precipitations of the biosynthetic intermediates. Our results suggest that the addition of GalNAc to thr eonine and serine in the RSMG mucin does not occur simultaneously, as has been described for other O-glycosylated proteins.