NOVEL PROTEIN ISOFORMS OF THE APC TUMOR-SUPPRESSOR IN NEURAL TISSUE

The conventional protein isoform of the APC tumor suppressor is 310 kD and is encoded by exons 1-15 of the APC gene, Other RNAs are expresse d from the APC gene and include one form that contains an exon upstrea m of exon 1, designated BS, but this transcript does not include exon 1, This transcript recently has been shown to be enriched in non-divid ing, terminally-differentiated cells (Santoro and Groden, 1997). To de termine if the BS-containing transcript encoded an alternate APC prote in isoform, we generated and affinity-purified a polyclonal antibody d irected to protein sequence predicted by exon BS, The BS antibody labe led a band of similar to 300 kD on immunoblots of cerebral and cerebel lar tissue from adult human, baboon, rat and mouse, These same tissue lysates also contained prominent BS-reactive proteins of 290 kD, 200 k D and 150 kD, Lysates from mitotically active cells did not contain th ese APC isoforms, To verify that BS-reactive proteins were APC isoform s, BS-immunoprecipitates were blotted and labeled with commercially av ailable APC antibodies, All four high molecular weight BS-antibody-pre cipitated proteins were recognized by antibodies directed against epit opes encoded by APC exons 2 and 15. BS isoforms were not, however, lab eled with antibodies to an epitope encoded by APC exon 1, consistent w ith the prediction that BS-APC isoforms lack the domain encoded by the se sequences, Like conventional APC, at least one of the four BS-APC p rotein isoforms also interacts with beta-catenin. BS-APC isoforms that lack exon 1-encoded sequences are incapable of dimerization with the conventional form of APC, yet retain the ability to bind beta-catenin, Such isoforms are likely to be functionally distinct from the convent ional APC protein.