Immunochemical identification of novel high-molecular-weight protein isoforms of the adenomatous polyposis coli (APC) gene

Mapping analyses of monoclonal antibodies (MAbs) directed against the amino-terminus of the adenomatous polyposis coli (APC) gene product revealed that epitopes recognized by the MAbs FE9, CF11 and AC4 constitute different peptide sequences encoded by the APC exons 1, 2 and 3, respectively. The protein pattern detected with these specificity-defined immunoreagents, however, differed depending on the particular antibody used on Western blots of cellular urea extracts. APC exon 15-positive "classic" p300apc polypeptide chains were identified by the MAb FE9, MAb CF11 and the C-terminus-specific MAb IE1, but only weak signals were obtained with the AC4 MAb, which defines an exon 3-encoded epitope. In contrast with this immunoreactivity, 2 novel high m.w. products of approx. 150/160 and 200 kDa were exclusively recognized by the AC4 MAb, which was shown to bind to the APC exon 3-encoded peptide sequence SRESTGYL. A molecular form of some 400 kDa was identified to represent a disulfide-bound oligomer of the p150/160apc molecules. The novel APC-related molecules did not contain exon 1- and exon 15-encoded epitopes, as confirmed with the help of the FE9 and IE1 MAbs, respectively. This observation was corroborated by the fact that these novel proteins were not truncated in a collection of familial adenomatous polyposis patients with stop mutations in exon 15. We conclude, that APC MAb AC4-reactive p150/160 and p200 polypeptide chains represent novel genuine products of the APC gene devoid of exon 1- and exon 15-encoded protein interaction domains.