The role of polybasic compounds in determining the tyrosyl phosphorylation of calmodulin by the human insulin receptor

Phosphorylation of calmodulin. Functional implications

Calmodulin (CaM) is phosphorylated in vitro and in vivo by multiple protein-serine/threonine and protein-tyrosine kinases. Casein kinase II and myosin light-chain kinase are two of the well established protein-serine/threonine kinases implicated in this process. On the other hand, within the protein-tyrosine kinases involved in the phosphorylation of CaM are receptors with tyrosine kinase activity, such as the insulin receptor and the epidermal growth factor receptor, and nonreceptor protein-tyrosine kinases, such as several members of the Src family kinases, Janus kinase 2, and p38Syk. The phosphorylation of CaM brings important physiological consequences for the cell as the diverse phosphocalmodulin species have differential actions as compared to nonphosphorylated CaM when acting on different CaM-dependent systems. In this review we will summarize the progress made on this topic as the first report on phosphorylation of CaM was published almost two decades ago. We will emphasize the description of the phosphorylation events mediated by the different protein kinases not only in the test tube but in intact cells, the phosphorylation-mediated changes of CaM activity, its action on CaM-dependent systems, and the functional repercussion of these phosphorylation processes in the physiology of the cell.

Phosphorylation of calmodulin on Tyr99 selectively attenuates the action of calmodulin antagonists on type-I cyclic nucleotide phosphodiesterase activity

Tyr99 phosphorylation of calmodulin appears to induce a distinct conformational change as is evident from the profound attenuation of the Ca(2+)-induced enhancement of calmodulin's mobility seen during SDS/PAGE. The effect of this conformational change appears to be localized, in that both calmodulin and P-Tyr99-calmodulin show identical dose-dependent activation profiles for stimulation of a physiological effector, type-I (Ca2+/calmodulin-stimulated) cyclic nucleotide phosphodiesterase (PDE) activity and their presence engenders similar dose-dependent PDE activation by Ca2+. In marked contrast with this, with P-Tyr99-calmodulin there were 3-4-fold increases in the IC50 values for inhibition of type-I PDE activity by the calmodulin antagonists TFP and W7, together with increased values for Hill coefficients for inhibition. The polybasic compound poly(L-lysine) potently augmented the action of calmodulin as a PDE activator, causing an approx. 7-fold decrease in the EC50 value for activation of PDE. It is suggested (i) that the Tyr99 phosphorylation of calmodulin, which occurs within a high-affinity Ca(2+)-binding domain, induces a localized conformational change in this peptide which can selectively attenuate the action of calmodulin antagonists on type-I PDE activity while leaving unaffected Ca(2+)-dependent activation, and (ii) that polybasic substances on complexing with calmodulin may serve to enhance the sensitivity of type-I PDE to activation by this regulatory peptide.