Multiple isoforms of beta-TrCP display differential activities in the regulation of Wnt signaling

The F-box proteins beta-TrCP1 and 2 (beta-transducin repeat containing protein) have 2 and 3 isoforms, respectively, due to alternative splicing of exons encoding the N-terminal region. We identified an extra exon in between the previously known exons 1 and 2 of beta-TrCP1 and beta-TrCP2. Interestingly, sequence analysis suggested that many more isoforms are produced than previously identified, via the alternative splicing of all possible combination of exons II to V of beta-TrCP1 and exons II to IV of beta-TrCP2. Different mouse tissues show specific expression patterns of the isoforms, and the level of expression of the isoform that has been used in most published papers was very low. Yeast two-hybrid assays show that beta-TrCP1 isoforms containing exon III, which are the most highly expressed isoforms in most tissues, do not interact with Skp1. Indirect immunofluorescence analysis of transiently expressed beta-TrCP1 isoforms suggests that the presence of exon III causes beta-TrCP1 to localize in nuclei. Consistent with the above findings, isoforms including exon III showed a reduced ability to block ectopic embryonic axes induced via injection of Wnt8 or beta-catenin in Xenopus embryos. Overall, our data suggest that isoforms of beta-TrCPs generated by alternative splicing may have different biological roles.