CLONING AND ANALYSIS OF CDNA-ENCODING A MAJOR AIRWAY GLYCOPROTEIN, HUMAN TRACHEOBRONCHIAL MUCIN (MUC5)

Two unique nucleotide probes for human tracheobronchial mucin glycopro tein (TBM) were generated via polymerase chain reaction with degenerat e 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 p olyp cDNA library. Clone NP3a was localized to chromosome 11 and conta ined a 3168 nucleotide open reading frame which encoded three TBM pept ide fragments, thus confirming that clone NP3a partially encodes TBM. TBM also contains five tandem repeats of TTVGP/S and an octapeptide GQ CGTCTN, which is conserved in human intestinal mucin MUC2 and rat inte stinal mucin-like protein (MLP) suggesting that this sequence has a fu nctional significance for secreted mucins. TBM has amino acid similari ty to the cysteine-rich domains at the carboxyl termini of MUC2, rat M LP, bovine and porcine submaxillary mucins, and human von Willebrand f actor. Strikingly, a large percentage of the cysteine residues in the overlaps are highly conserved: 90% in MUC2 and von Willebrand factor, 80% in bovine submaxillary mucin, 70% in porcine submaxillary mucin, a nd 64% in rat MLP, suggesting that conserved cysteines may be importan t for the tertiary structure of secreted glycoproteins. These studies demonstrate that clone NP3a is a candidate for MUC5, making it the onl y human mucin gene reported to date whose gene product has been isolat ed from airway secretions.