Melatonin and serotonin interactions with calmodulin: NMR, spectroscopic and biochemical studies

It has been reported that the hormone melatonin binds tightly to the ubiquitous calcium-regulatory protein, calmodulin (CaM) with a Kd value around 0.1 nM [Benítez-King et al., Biochim. Biophys. Acta, 1290 (1993) 191-196]. Normally CaM only binds to target proteins and various 20-residue synthetic peptides encompassing the CaM-binding domain of these target proteins with K(d) values ranging between 1.0 microM and 0.1 nM. Here we have studied the interaction of melatonin and several structurally related compounds--serotonin, 5-hydroxytryptophan, and tryptophan--to CaM through gel band shift assays, enzymatic competition assays with calcineurin, fluorescence spectroscopy, far and near UV circular dichroism spectropolarimetry and NMR spectroscopy. Fluorescence spectra show that the binding is calcium dependent. NMR studies with biosynthetically labelled methyl-13C-Met CaM show that melatonin and the other compounds interact with the hydrophobic cleft regions of the protein. Our NMR data show that melatonin binds to both domains of the dumbbell-shaped CaM, while serotonin appears to bind only to the C-terminal domain. This binding mode is further substantiated by fluorescence and gel band shift competition experiments with synthetic peptides from myosin light chain kinase and constitutive nitric oxide synthase. Circular dichroism spectra indicate that the secondary structure of CaM is not altered by addition of melatonin. Our data are internally consistent and reveal Kd values in the mM range for melatonin. Thus the binding of these compounds to CaM is substantially weaker than was previously reported and is unlikely to be of physiological significance.