NMR structure of the forkhead-associated domain from the Arabidopsis receptor kinase-associated protein phosphatase

Fork nead-associated (FHA) domains are phosphoprotein-binding modules found in diverse signaling proteins that bind partners phosphorylated on threonine or serine. Kinase-associated protein phosphatase from Arabidopsis employs its FHA domain for negative regulation of receptor-like kinase signaling pathways, which are important in plant development. The solution structure of the free state of kinase-interacting FHA domain (KI-FHA) of kinase-associated protein phosphatase has been determined with high precision and accuracy using residual dipolar couplings. KI-FHA is a sandwich of a five-stranded mixed beta-sheet with a six-stranded antiparallel beta-sheet. Despite homology only in the recognition loop;, this fold is shared with FHA domains from checkpoint proteins from yeast and humans, as well as with nonhomologous MH2 domains of Smad tumor suppressors. A shared pattern of hydrophobicity throughout FHA domains and Smad MH2 domains may stabilize the core of the p-sandwich. Evolutionary trace analysis of FHA domains suggests class-specific residues in the recognition loops that could tune their phosphoprotein-binding specificity. This surface agrees with that of KI-FHA in contact with a phosphothreonine peptide ligand. Evolutionary trace analysis also predicts an unexpected swath of class-specific residues on another face of FHA domains. Protein interactions with these faces may affect assembly of transmembrane signaling complexes in plans, and in other FHA domain-containing assemblies.