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 ezpressed from the APC gene and include one form that contains an exon upstream of exon 1, designated BS, but this transcript does not include exon 1. This transcript recently has been shown to be enriched in non-dividing, terminally-differentiated cells. To determine if the BS-containing transcript encoded an alternate APC protein isoform, we generated and affinity-purified a polyclonal antibody directed to protein sequence predicted by exon BS. The BS antibody labeled a band of ~300 kD on immunoblots of cerebral and cerebellar tissue from adult human, baboon, rat and mouse. These same tissue lysates also contained prominent BS-reactive proteins of 290 kD, 200 kD and 150 kD. Lysates from mitotically active cells did not contain these APC isoforms. To verify that BS-reactive proteins were APC isoforms, BS-immunoprecipitates were blotted and labeled with commercially available APC antibodies. All four high molecular weight BS-antibody-precipitated proteins were recognized by antibodies directed against epitopes encoded by APC exons 2 and 15. BS isoforms were not, however, labeled with antibodies to an epitope encoded by APC exon 1, consistent with the prediction that BS-APC isoforms lack the domain encoded by these sequences. Like conventional APC, at least one of the four BS-APC protein isoforms also interacts with β-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 β-catenin. Such isoforms are likely to be functionally distinct from the conventional APC protein.