MUC4 IS A MAJOR COMPONENT OF SALIVARY MUCIN MG1 SECRETED BY THE HUMANSUBMANDIBULAR-GLAND

High molecular weight salivary mucin (MG1) is an important component o f saliva, contributing to the lubricative and tissue-protective functi ons of this biological fluid. We have shown previously that the human mucin gene MUC5B is expressed at high levels in sublingual gland and i s a significant constituent of MG1. Since many epithelia express multi ple mucin genes, it seemed likely that MG1 in salivary secretions is a lso a heterogeneous mixture of mucin gene products. The aim of this st udy was to determine whether MUC4, a mucin shown in Northern blotting experiments to be expressed in salivary glands, was a significant prot ein component of MG1 in salivary secretions. Two cDNA clones containin g MUC4 tandem repeats were isolated from a human submandibular gland c DNA library. In addition, recombinant MUC4 produced in a bacterial exp ression system crossreacted with an antibody directed against deglycos ylated MG1. This shows conclusively that human salivary mucin MG1 cont ains both MUC5B and MUC4 gene products suggesting that each mucin may perform distinct functions in the oral cavity. (C) 1998 Academic Press .