NMR and molecular dynamics studies of the interaction of melatonin with calmodulin

Pineal hormone melatonin (N-acetyl-5-methoxytryptamine) is thought to modulate the calcium/calmodulin signaling pathway either by changing intracellular Ca(2+) concentration via activation of its G-protein-coupled membrane receptors, or through a direct interaction with calmodulin (CaM). The present work studies the direct interaction of melatonin with intact calcium-saturated CaM both experimentally, by fluorescence and nuclear magnetic resonance spectroscopies, and theoretically, by molecular dynamics simulations. The analysis of the experimental data shows that the interaction is calcium-dependent. The affinity, as obtained from monitoring (15)N and (1)H chemical shift changes for a melatonin titration, is weak (in the millimolar range) and comparable for the N- and C-terminal domains. Partial replacement of diamagnetic Ca(2+) by paramagnetic Tb(3+) allowed the measurement of interdomain NMR pseudocontact shifts and residual dipolar couplings, indicating that each domain movement in the complex is not correlated with the other one. Molecular dynamics simulations allow us to follow the dynamics of melatonin in the binding pocket of CaM. Overall, this study provides an example of how a combination of experimental and theoretical approaches can shed light on a weakly interacting system of biological and pharmacological significance.