Re-evaluating the role of Frat in Wnt-signal transduction.

Frat proteins are potent activators of canonical Wnt-signal transduction. By binding to GSK3, Frat prevents the phosphorylation and concomitant degradation of beta-catenin and allows the activation of downstream target genes by beta-catenin/TCF complexes. The identification of the Xenopus Frat homologue GBP as an essential component of the maternal Wnt-pathway during embryonic axis formation suggested that Frat might fulfill a similar role in higher vertebrates. As a result most, if not all, studies addressing Frat function have focused on its ability to bind GSK3 and induce signaling through beta-catenin/TCF. Consequently, Frat has been advocated as the "missing link" that bridged signaling from Dishevelled to GSK3 in the canonical Wnt-pathway. Recent mouse-knockout studies however, call for a reevaluation of the physiological role of Frat. Mice that lack all Frat-family members appear to be normal and display no obvious defects in beta-catenin/TCF signaling. This observation reopens the question as to how GSK3 activity is controlled in vertebrate canonical Wnt-signal transduction in view of the apparent dispensability of Frat. Here we will review the studies that have been conducted on Frat proteins to date, with a specific focus on those that implicate a role for Frat in Wnt-signal transduction. In addition, we will discuss potential alternatives for the endogenous function of Frat.