Apomucin is part of mucin, the major glycoprotein synthesized and secreted by mucous cells of the submaxillary gland. Its highly viscous aqueous solutions serve to lubricate the oral cavity and to protect it from the external environment. Intermolecular disulfide bonds could help maintain a multimeric mucin structure. Submaxillary mucosae. Contains tandemly repeated, identical sequences of 81 residues. Extensively O-glycosylated on most but not all Ser and Thr residues of the repeat units. Highest glycosylation appears to occur on Ser residues which have Gly at positions at +2 or -2 from the glycosylation site or, where Gly is the penultimate residue. The presence of proline (usually at position +3 or -3) appears to also enhance glycosylation.