Cloning and analysis of cDNA encoding a major airway glycoprotein, human tracheobronchial mucin (MUC5)

Abstract

Two unique nucleotide probes for human tracheobronchial mucin glycoprotein (TBM) were generated via polymerase chain reaction with degenerate primers deduced from the TBM:TR-3A tryptic peptide sequence and were used to isolate a 3.6 kilobase cDNA, clone NP3a, from a human nasal polyp cDNA library. Clone NP3a was localized to chromosome 11 and contained a 3168 nucleotide open reading frame which encoded three TBM peptide fragments, thus confirming that clone NP3a partially encodes TBM. TBM also contains five tandem repeats of TTVGP/S and an octapeptide GQCGTCTN, which is conserved in human intestinal mucin MUC2 and rat intestinal mucin-like protein (MLP) suggesting that this sequence has a functional significance for secreted mucins. TBM has amino acid similarity to the cysteine-rich domains at the carboxyl termini of MUC2, rat MLP, bovine and porcine submaxillary mucins, and human von Willebrand factor. Strikingly, a large percentage of the cysteine residues in the overlaps are highly conserved: 90% in MUC2 and von Willebrand factor, 80% of bovine submaxillary mucin, 70% in porcine submaxillary mucin, and 64% in rat MLP, suggesting that conserved cysteines may be important for the tertiary structure of secreted glycoproteins. These studies demonstrate that clone NP3a is a candidate for MUC5, making it the only human mucin gene reported to date whose gene product has been isolated from airway secretions.