Abstract
The Frat1 proto-oncogene was first identified as a gene contributing to tumor progression in T-cell lymphomas induced by retroviral insertional mutagenesis with the Moloney murine leukemia virus. The biological function of Frat remained elusive until its Xenopus homologue GBP was isolated as a glycogen synthase kinase 3 (GSK3)-binding protein and was shown to be an essential component of the maternal Wnt-signaling pathway. To date two Frat homologues have been described in the mouse, Frat1 and Frat3. The proteins encoded by these two genes are 84% identical. Here we describe the cloning and characterization of a third murine Frat homologue, Frat2, which is the mouse ortholog of human FRAT2. Frat1 and Frat2 are juxtaposed on chromosome 19 in a chromosomal organization conserved between man and mouse. We show that Frat1 and Frat2 are phosphorylated, which is the first evidence that these proteins are subject to posttranslational modification. Like Frat1, Frat2 is able to bind to GSK3beta. However, a side-by-side comparison of the murine Frat proteins for their capacity to induce signaling through beta-catenin/T-cell factor reveals that Frat2 is a less potent activator of the canonical Wnt pathway. Frat2 protein accumulates to higher levels upon transfection into 293T cells than either Frat1 or Frat3. Thus, whereas Frat1 may be a core component of canonical Wnt-signaling, Frat2 might very well be part of a divergent intracellular GSK3beta pathway.