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 re vealed that epitopes recognized by the MAbs FE9, CF11 and AC4 constitu te different peptide sequences encoded by the APC exons 1, 2 and 3, re spectively. The protein pattern detected with these specificity-define d immunoreagents, however, differed depending on the particular antibo dy used on Western blots of cellular urea extracts. APC exon 15-positi ve ''classic'' p300(apc) polypeptide chains were identified by the MAb FE9, MAb CF11 and the C-terminus-specific MAb IE1, but only weak sign als were obtained with the AC4 MAb, which defines an exon 3-encoded ep itope. In contrast with this immunoreactivity, 2 novel high m.w. produ cts 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 seq uence SRESTGYL. A molecular form of some 400 kDa was identified to rep resent a disulfide-bound oligomer of the p150/160(apc) molecules. The novel APC-related molecules did not contain exon 1- and exon 15-encode d epitopes, as confirmed with the help of the FE9 and IE1 MAbs, respec tively. This observation was corroborated by the fact that these novel proteins were not truncated in a collection of familial adenomatous p olyposis patients with stop mutations in exon 15. We conclude, that AP C MAb AC4-reactive p150/160 and p200 polypeptide chains represent nove l genuine products of the APC gene devoid of exon 1- and exon IE1-enco ded protein interaction domains. (C) 1996 Wiley-Liss, Inc.