Serine/threonine phosphorylation of calmodulin modulates its interaction with the binding domains of target enzymes

The interaction of serine/threonine-phosphorylated calmodulin with synthetic peptides corresponding to the calmodulin-binding domains of six enzymes has been studied by fluorescence spectroscopy. For five peptides, the dissociation constant of the calmodulin-peptide complex (K-d) increased when calmodulin was phosphorylated. An increase of more than one order of magnitude was observed with peptides derived from smooth-muscle myosin light-chain kinase and cAMP phosphodiesterase. In contrast, only a slight increase in K-d was noted with two peptides derived from the plasma membrane Ca2+-ATPase and for the peptide derived from nitric oxide synthase, No significant change in affinity was detected with the peptide derived from calcineurin. In contrast, a decrease in the dissociation constant was observed with the peptide derived from the Ca2+-calmodulin dependent kinase II. Phosphorylation also affected the peptide calmodulin binding stoichiometry: a decrease from two to one binding sites was observed with the peptides derived from myosin light-chain kinase and phosphodiesterase.