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

The interaction of serine/threonine-phosphorylated calmodulin with syntheti c peptides corresponding to the calmodulin-binding domains of six enzymes h as been studied by fluorescence spectroscopy. For five peptides, the dissoc iation constant of the calmodulin-peptide complex (K-d) increased when calm odulin was phosphorylated. An increase of more than one order of magnitude was observed with peptides derived from smooth-muscle myosin light-chain ki nase and cAMP phosphodiesterase. In contrast, only a slight increase in K-d was noted with two peptides derived from the plasma membrane Ca2+-ATPase a nd for the peptide derived from nitric oxide synthase, No significant chang e in affinity was detected with the peptide derived from calcineurin. In co ntrast, a decrease in the dissociation constant was observed with the pepti de derived from the Ca2+-calmodulin dependent kinase II. Phosphorylation al so affected the peptide calmodulin binding stoichiometry: a decrease from t wo to one binding sites was observed with the peptides derived from myosin light-chain kinase and phosphodiesterase.