FRAT1, a Substrate-specific Regulator of Glycogen Synthase Kinase-3 Activity, Is a Cellular Substrate of Protein Kinase A

• Published in: The Journal of biological chemistry 2006-11, Vol.281 (46), p.35021-35029
• Main Authors: Hagen, Thilo, Cross, Darren A.E., Culbert, Ainsley A., West, Andrew, Frame, Sheelagh, Morrice, Nick, Reith, Alastair D.
• Format: Article
• Language: English
• Subjects: Adaptor Proteins, Signal Transducing; beta Catenin - metabolism; Cell Line; Cyclic AMP-Dependent Protein Kinases - metabolism; Freshwater; Glycogen Synthase Kinase 3 - metabolism; Humans; Intracellular Signaling Peptides and Proteins - metabolism; Phosphorylation; Protein Kinase C - metabolism; Proto-Oncogene Proteins - metabolism; Substrate Specificity; Xenopus
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M607003200

FRAT1, like its Xenopus homolog glycogen synthase kinase-3 (GSK-3)-binding protein, is known to inhibit GSK-3-mediated phosphorylation of β-catenin. It is currently unknown how FRAT-GSK-3-binding protein activity toward GSK-3 is regulated. FRAT1 has recently been shown to be a phosphoprotein in vivo; however, the responsible kinase(s) have not been determined. In this study, we identified Ser188 as a phosphorylated residue in FRAT1. The identity of the kinase that catalyzes Ser188 phosphorylation and the significance of this phosphorylation to FRAT1 function were investigated. Protein kinase A (PKA) was found to phosphorylate Ser188in vitro as well as in intact cells. Importantly, activation of endogenous cAMP-coupled β-adrenergic receptors with norepinephrine stimulated the phosphorylation of FRAT1 at Ser188. GSK-3 was also able to phosphorylate FRAT1 at Ser188 and other residues in vitro or when overexpressed in intact cells. In contrast, endogenous GSK-3 did not lead to significant FRAT1 phosphorylation in cells, suggesting that GSK-3 is not a major FRAT1 kinase in vivo. Phosphorylation of Ser188 by PKA inhibited the ability of FRAT1 to activate β-catenin-dependent transcription. In conclusion, PKA phosphorylates FRAT1 in vitro as well as in intact cells and may play a role in regulating the inhibitory activity of FRAT1 toward GSK-3.