Target Enzyme Recognition by Calmodulin: 2.4 Å Structure of a Calmodulin-Peptide Complex

The crystal structure of calcium-bound calmodulin (Ca 2+ -CaM) bound to a peptide analog of the CaM-binding region of chicken smooth muscle myosin light chain kinase has been determined and refined to a resolution of 2.4 angstroms (Å). The structure is compact and has the shape of an ellipsoid (axial ratio ∼2:1). The bound CaM forms a tunnel diagonal to its long axis that engulfs the helical peptide, with the hydrophobic regions of CaM melded into a single area that closely covers the hydrophobic side of the peptide. There is a remarkably high pseudo-twofold symmetry between the closely associated domains. The central helix of the native CaM is unwound and expanded into a bend between residues 73 and 77. About 185 contacts (<4 Å) are formed between CaM and the peptide, with van der Waals contacts comprising ∼80% of this total.