FRAT-2 Preferentially Increases Glycogen Synthase Kinase 3β-mediated Phosphorylation of Primed Sites, Which Results in Enhanced Tau Phosphorylation

Tau is a microtubule-associated protein found primarily in neurons, and its function is regulated by site-specific phosphorylation. Although it is well established that tau is phosphorylated at both primed and unprimed epitopes by glycogen synthase kinase 3β (GSK3β), how specific proteins that interact with GSK3β regulate tau phosphorylation has not been thoroughly examined. Members of the FRAT ( f requently r earranged in a dvanced T -cell lymphoma) protein family have been shown to interact with GSK3β, and FRAT-1 has been shown to modulate the activity of GSK3β toward tau and other substrates. However, the effects of FRAT-2 on GSK3β activity and tau phosphorylation have not been examined. Therefore in this study the effects of FRAT-2 on GSK3β activity and tau phosphorylation were examined. In situ , FRAT-2 significantly increased GSK3β-mediated phosphorylation of tau at a primed epitope while not significantly affecting the phosphorylation of unprimed sites. Co-immunoprecipitation studies revealed that association of FRAT-2 with GSK3β resulted in a significant increase in phosphorylation of a primed substrate but did not alter phosphorylation of an unprimed substrate. Further, in vitro assays using recombinant proteins directly demonstrated that FRAT-2 enhances GSK3β-mediated phosphorylation of a primed substrate to a greater extent than an unprimed substrate. In addition, FRAT-2 is phosphorylated by GSK3β. This is the first demonstration of a protein differentially regulating the activity of GSK3β toward primed and unprimed epitopes.