Molecular cloning, sequence, and specificity of expression of the gene encoding the low molecular weight human salivary mucin (MUC7)

Previous biochemical studies have determined that human saliva contains high and low molecular weight mucin glycoproteins (MG1 and MG2, respectively) that are structurally distinct. In this study, we describe the isolation and characterization of overlapping cDNA clones which code for the MG2 protein core. DNA sequencing revealed a translated region of 1131 nucleotides encoding a protein of 377 amino acid residues with a molecular mass of 39 kDa. The first 20 N-terminal residues were very hydrophobic and probably comprise the MG2 leader peptide. The region encoding the secreted protein can be divided into three distinct domains; unique 5′- and 3′-translated regions containing 4 and 1 potential N-glycosylation sites, respectively, and a central region of six almost perfect tandem repeats of 23 amino acid residues with a high number of Thr and Ser. No sequence homology with any other human or animal mucins, and no significant homology to any other proteins was found. MG2 mRNA is about 2.5 kilobases long, and its expression appears to be species-, tissue-, and cell-specific. We propose to name this gene MUC7 in accordance with the mucin genes cloned to date named MUC1-MUC6.