Molecular characterization of a major high molecular weight mucin from human sublingual gland

Human submandibular/sublingual gland secretions contain a multimeric high molecular weight mucin (MG1) and a monomeric low molecular weight mucin (MG2). MG2 is the product of the MUC7 gene, whereas the gene for MG1 has not been identified. Previously, we isolated a clone (pSM2-1) from a human sublingual gland cDNA expression library using an antibody against deglycosylated MG1 (Troxler et al., Biochem. Biophys. Res Commun., 217, 1112-1119, 1995). In order to identify the mucin gene from which pPM2-1 was derived, Northern blots of human submandibular and sublingual gland RNA were hybridized with a series of probes for tandem repeats found in the high molecular weight secreted mucins MUC2, MUC3, MUC4, MUC5AC, MUC5B, and MUC6. The only known mucin expressed at high levels in sublingual gland was MUC5B, and no known mucin was expressed at high levels in submandibular gland. A series of overlapping clones was obtained by rescreening the sublingual gland cDNA library and by reverse transcriptase-polymerase chain reaction. The resulting clones connected pSM2-1 to a series of MUC5B tandem repeats at the 3' end of the repeat domain and provided the complete nucleotide and deduced amino sequence of the carboxyl terminal region of MG1. This region is enriched with respect to cysteine (approximately 10 mol %) and contained a D domain and a carboxyl terminal domain that could be aligned with the corresponding domains in human intestinal MUC2, human tracheobronchial MUC5AC, and human von Willebrand factor. The limited expression of known mucin genes, together with the considerable mucin synthesizing capacity of submandibular gland, suggests that a novel (previously not described) mucin gene is expressed in this gland and constitutes a portion of MG1 in salivary secretions.