Previous work has shown that the transcriptional regulator beta-catenin can translocate to the nuclei when cells are stimulated with the type 1 insulin-like growth factor (IGF-1). We show by immunocoprecipitation and by confocal microscopy that beta-catenin binds to and co-localizes with the insulin receptor substrate-1 (IRS-1), a docking protein for both the insulin and the IGF-1 receptors. IRS-1 is required for IGF-1-mediated nuclear translocation of beta-catenin, resulting in the activation of the beta-catenin target genes. IGF-1-mediated nuclear translocation of beta-catenin is facilitated by the nuclear translocation of IRS-1. Both IRS-1 and beta-catenin are recruited to the cyclin D1 promoter, an established target for beta-catenin, but only IRS-1 is recruited to the ribosomal DNA (rDNA) promoter. UBF proteins (known to interact with both IRS-1 and beta-catenin) are also detectable in the cyclin D1 and rDNA promoters. These results indicate that IRS-1 (activated by the IGF-1 receptor) is one of several proteins that regulate the subcellular localization and activity of beta-catenin. The ability of IRS-1 to localize to both RNA polymerase II (with beta-catenin) and RNA polymerase I-regulated promoters suggest an explanation for the effect of IRS-1 on both cell growth in size and cell proliferation. This possibility is supported by the demonstration that enforced nuclear localization of IRS-1 causes nuclear translocation of beta-catenin and transformation of normal mouse embryo fibroblasts (colony formation in soft agar).