BIOSYNTHESIS OF HUMAN COLONIC MUCIN - MUC2 IS THE PROMINENT SECRETORYMUCIN

Background/Aims: Human colonic epithelium produces large amounts of mu cin. The aim of this study was to examine mucin biosynthesis in the hu man colon. Methods: Human colonic mucin was isolated using CsCl densit y gradients, and polyclonal antiserum was raised. Biosynthesis of colo nic mucins was studied by labeling colonic explants with S-35-labeled amino acids or [S-35]sulfate and subsequent immunoprecipitation and so dium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Re sults: The polyclonal antiserum specifically recognized colonic mucin, primarily reacting with peptide epitopes. Biosynthetic pulse/chase ex periments showed a S-35-amino acid-labeled mucin precursor of about 60 0 kilodaltons, which was converted into a mature, glycosylated, and su lfated mucin and subsequently secreted into the medium. This mature mu cin comigrated with isolated colonic mucin with an apparent molecular weight of 550 kilodaltons on SDS-PAGE, whereas gel filtration indicate d that the molecular weight is actually much larger. Independent immun oprecipitation with an anti-Muc2 antiserum showed cross-reactivity wit h the 600-kilodalton precursor. Conclusions: These results show the bi osynthesis of a secretory colonic mucin for the first time. This mucin is synthesized as a precursor protein of approximately 600 kilodalton s, which, after glycosylation, is secreted as a glycoprotein with an a pparent molecular weight of 550 kilodaltons on SDS-PAGE. It is very li kely that this mucin is Muc2.