PHOSPHORYLATION OF CALMODULIN ON TYR(99) SELECTIVELY ATTENUATES THE ACTION OF CALMODULIN ANTAGONISTS ON TYPE-I CYCLIC-NUCLEOTIDE PHOSPHODIESTERASE ACTIVITY

Tyr(99) phosphorylation of calmodulin appears to induce a distinct conformational change as is evident from the profound attenuation of the Ca2+-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-Tyr(99)-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-Tyr(99)-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 EC(50) value for activation of PDE. It is suggested (i) that the Tyr(99) phosphorylation of calmodulin, which occurs within a high-affinity Ca2+-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 Ca2+-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.