The activation of six target enzymes by calmodulin phosphorylated on Tyr99 (PCaM) and the binding affinities of their respective calmodulin binding domains were tested. The six enzymes were: myosin light chain kinase (MLCK), 3H -5 H -cyclic nucleotide phosphodiesterase (PDE), plasma membrane (PM) Ca 2+ -ATPase, Ca 2+ -CaM dependent protein phosphatase 2B (calcineurin), neuronal nitric oxide synthase (NOS) and type II Ca 2+ -calmodulin dependent protein kinase (CaM kinase II). In general, tyrosine phosphorylation led to an increase in the activatory properties of calmodulin (CaM). For plasma membrane (PM) Ca 2+ -ATPase, PDE and CaM kinase II, the primary effect was a decrease in the concentration at which half maximal velocity was attained (K act ). In contrast, for calcineurin and NOS phosphorylation of CaM significantly increased the V max . For MLCK, however, neither V max nor K act were affected by tyrosine phosphorylation. Direct determination by fluorescence techniques of the dissociation constants with synthetic peptides corresponding to the CaM-binding domain of the six analysed enzymes revealed that phosphorylation of Tyr99 on CaM generally increased its affinity for the peptides.

Previous work has shown that calmodulin (CaM) is constitutively phosphorylated in rat liver, probably by casein kinase II [Quadroni, M., James, P., and Carafoli, E. (1994) J. Biol. Chem. 269, 16116-16122]. A procedure is now described for the isolation of the phosphorylated forms of calmodulin (PCaM) free from CaM, since in vitro phosphorylation experiments yield a 50:50 mixture of 3-4 times phosphorylated CaM and native CaM. The activation of six target enzymes by PCaM was tested: myosin light chain kinase, 3',5'-cyclic nucleotide phosphodiesterase, plasma membrane Ca2+-ATPase, Ca2+-CaM-dependent protein phosphatase 2B (calcineurin), neuronal nitric oxide synthase, and CaM-kinase II. In general, the phosphorylation of CaM caused a decrease in enzyme binding affinity, increasing the Kact by 2-4-fold for MLCK, PDE, PM Ca2+-ATPase, and calcineurin. The Vmax at saturating concentrations of PCaM was less affected, with the exception of CaM-kinase II, which was only minimally activated by PCaM and NOS whose Vmax was increased 2.6 times by PCaM with respect to CaM. Phosphorylation of calmodulin had very little effect on the binding of calcium to the enzyme despite the fact that Ser 101 which is phosphorylated is located in the third calcium binding loop. CD measurements performed on CaM and PCaM indicated that phosphorylation causes a marked decrease in the alpha-helical content of the protein. Phosphorylated CaM is very prone to dephosphorylation and was thus tested as a substrate for several phosphatases. It was unaffected by calcineurin (PP2B), but was a reasonable substrate for the pleiotropic phosphatases PP1gamma and PP2A.