Dissection of the Pathway of Molecular Recognition by Calmodulin

Amide hydrogen exchange has been used to examine the structural dynamics and energetics of the interaction of a peptide corresponding to the calmodulin-binding domain of smooth muscle myosin light chain kinase (smMLCKp) with calcium-saturated calmodulin. Heteronuclear NMR 15N-1H correlation spectroscopy was used to quantify amide proton exchange rates of the uniformly 15N-labeled domain bound to calmodulin. A key feature of a proposed model for molecular recognition by calmodulin [Ehrhardt et al. (1995) Biochemistry 34, 2731−2738] is tested by examination of the dependence of amide hydrogen exchange on applied hydrostatic pressure. Hydrogen exchange rates and corresponding protection factors (1/K op) for individual amide protons of the bound smMLCKp domain span 5 orders of magnitude at ambient pressure. Individual protection factors decrease significantly in a linear fashion with increasing hydrostatic pressure. A common pressure dependence is revealed by a constant large negative volume change across the residues comprising the core of the bound helical domain. The pattern of protection factors and their response to hydrostatic pressure is consistent with a structural reorganization that results in the concerted disruption of ion pairs between calmodulin and the bound domain. These observations reinforce a model for the molecular recognition pathway where formation of the initial encounter complex is followed by helix−coil transitions in the bound state and subsequent concerted formation of the extensive ion pair network defining the intermolecular contact surface between CaM and the target domain in the final, compact complex structure.