Kinase interaction domain of kinase-associated protein phosphatase, a phosphoprotein-binding domain

Kinase-associated protein phosphatase interacts specifically with plant rec eptor-like protein kinases. This interaction is thought to be a key step in signal perception and transduction. The minimal kinase interaction (KI) do main of kinase-associated protein phosphatase was mapped to a 119-aa segmen t spanning residues 180 to 298. A forkhead-associated (FHA) homology region resides in this minimal KI domain. Site-directed mutagenesis of four highl y conserved sites in this FHA homology region abolishes the KI domain's int eraction with receptor-like protein kinases, indicating that the FHA region is essential for binding. Serial deletion analysis indicates that 30 aa on each side of the FHA region are also needed for binding; this minimal func tional unit is designated as the KI domain. Kinetic studies using surface p lasmon resonance indicate that the binding between the KI domain and recept or-like protein kinases has a dissociation constant (K-D) of about 25-100 n M, which is similar to the binding affinity of two other well characterized phosphorylation-dependent protein-binding domains (14-3-3 and Src homology 2) and their high-affinity phosphopeptide ligands.