Mouse submandibular gland salivary apomucin contains repeated N-glycosylation sites

A cDNA clone encoding mouse submandibular gland salivary mucin apoprotein was isolated and characterized. The mucin cDNA encodes a protein of 273 amino acids with a calculated molecular weight of 29,606. This apomucin is approximately 60% Thr, Ser, and Pro, and has a pI of 11.25. In addition to the signal sequence, the apomucin can be divided into four structural domains. The first of these contains over 30% Thr, Ser, and Pro, but only a few probable O-glycosylation sites. The second domain contains 10 repeats, each 9 or 13 amino acids in length, with Thr representing more than 50% of the amino acids, while Ser accounts for only 2%. Each repeat begins with a putative N-glycosylation site; hence this domain likely contains both N- and O-linked oligosaccharides. The third domain lacks a repeat motif, but is rich in both Thr and Ser, and therefore is potentially highly O-glycosylated. The final domain is composed mainly of basic and non-polar amino acids and does not contain Cys. This mucin shows considerable homology with the rat submandibular salivary mucin, but little overall homology with other mucins. In situ hybridization verifies that the mucin transcript is localized primarily in the acinar cells of the submandibular gland.