Studies on the "insoluble" glycoprotein complex from human colon - Identification of reduction-insensitive MUC2 oligomers and C-terminal cleavage

The "insoluble" glycoprotein complex was isolated from human colonic tissue and mucin subunits were prepared following reduction, Antibodies raised ag ainst peptide sequences within MUC2 revealed that virtually all of this muc in occurs in the insoluble glycoprotein complex. In addition, reduction rel eased a 120-kDa C-terminal MUC2 fragment, showing that proteolytic cleavage in this domain may occur and leave the fragment attached to the complex vi a disulfide bonds. The variable number tandem repeat region and the irregul ar repeat domain were isolated after trypsin digestion and shown to have mo lecular weights of 930,000 and 180,000, respectively, suggesting a molecula r weight for the entire MUC2 monomer of approximately 1.5 million. Gel chro matography and agarose gel electrophoresis revealed several populations of MUGS subunits, and analytical ultracentrifugation showed that these have mo lecular weights on the order of 2 million, 4 million, and 5 million, corres ponding to monomers, dimers, and trimers, respectively, Agarose gel electro phoresis of subunits from individuals expressing both a "long" and a "short " MUC2 allele revealed a larger number of populations, consistent with the presence of short and long monomers and oligomers arising from permutations of the two types of monomers. In addition to disulfide bonds, MUGS monomer s are apparently joined by a "novel," reduction-insensitive bond.