title: FRAT1, a substrate-specific regulator of glycogen synthase kinase-3 activity, is a cellular substrate of protein kinase A.
abstract: FRAT1, like its Xenopus homolog glycogen synthase kinase-3 (GSK-3)-binding protein, is known to inhibit GSK-3-mediated phosphorylation of beta-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 Ser188 in vitro as well as in intact cells. Importantly, activation of endogenous cAMP-coupled beta-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 beta-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.
authors.affiliation: Discovery Research Biology, Neurology Centre of Excellence in Drug Discovery, Computational, Analytical, and Structural Sciences, GlaxoSmithKline Pharmaceuticals, Harlow, Essex CM19 5AD, United Kingdom. Thilo.Hagen@nottingham.ac.uk
authors.forename: Thilo
authors.initials: T
authors.lastname: Hagen
authors.forename: Darren A E
authors.initials: DA
authors.lastname: Cross
authors.forename: Ainsley A
authors.initials: AA
authors.lastname: Culbert
authors.forename: Andrew
authors.initials: A
authors.lastname: West
authors.forename: Sheelagh
authors.initials: S
authors.lastname: Frame
authors.forename: Nick
authors.initials: N
authors.lastname: Morrice
authors.forename: Alastair D
authors.initials: AD
authors.lastname: Reith
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D002460:Cell Line; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D013379:Substrate Specificity; D051176:beta Catenin / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C491819:FRAT1 protein, human; D047908:Intracellular Signaling Peptides and Proteins; D011518:Proto-Oncogene Proteins; D051176:beta Catenin; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D038362:Glycogen Synthase Kinase 3