A novel target recognition revealed by calmodulin in complex with Ca²⁺-calmodulin-dependent kinase kinase

The structure of calcium-bound calmodulin (Ca²⁺/CaM) complexed with a 26-residue peptide, corresponding to the CaM-binding domain of rat Ca²⁺/CaM-dependent protein kinase kinase (CaMKK), has been determined by NMR spectroscopy. In this complex, the CaMKK peptide forms a fold comprising an α-helix and a hairpin-like loop whose C-terminus folds back on itself. The binding orientation of this CaMKK peptide by the two CaM domains is opposite to that observed in all other CaM-target complexes determined so far. The N- and C-terminal hydrophobic pockets of Ca²⁺/CaM anchor Trp 444 and Phe 459 of the CaMKK peptide, respectively. This 14-residue separation between two key hydrophobic groups is also unique among previously determined CaM complexes. The present structure represents a new and distinct class of Ca²⁺/CaM target recognition that may be shared by other Ca²⁺/CaM- stimulated proteins.