A structure for calmodulin-activated cyclic nucleotide phosphodiesterase deduced from proteolysis studies.

In summary, our studies suggest that unproteolyzed PDE, illustrated as a dimer of identical 63,000-D polypeptide chains in Fig. 5, can exist in two equilibrium conformations. In the absence of calmodulin, the predominant conformational form has a nonfunctional catalytic site, whereas the minor conformational form has a fully functional site. Preferential complexation of calmodulin with the minor conformation shifts the population to that form resulting in a marked activation of catalysis. Alternatively, cleavage of a sizable terminal fragment, termed the calmodulin binding domain, facilitates irreversible acquisition of the functional conformation in the absence of calmodulin activation. By contrast, proteolytic cleavage at the opposite terminus does not significantly alter the conformational equilibrium. This model suggests that a single in vivo cytoplasmic cyclic nucleotide PDE can accommodate many of the variations in the polypeptide chain sizes and in the fold activation observed for enzyme preparations in vitro. The model also places this form of cyclic nucleotide PDE activity in vivo under absolute control by intracellular Ca2+ concentration.