Modulator binding protein. Bovine brain protein exhibiting the Ca²⁺ dependent association with the protein modulator of cyclic nucleotide phosphodiesterase

A bovine brain protein recently discovered as an inhibitor of cyclic nucleotide phosphodiesterase has been partially purified and characterized. This inhibitor protein specifically counteracts the activation of phosphodiesterase by the Ca²⁺-dependent protein modulator. It shows no inhibitory activity against the Ca²⁺-independent form of phosphodiesterase or the basal activity of the Ca²⁺-activatable enzyme. If the Ca²⁺-activatable phosphodiesterase is desensitized of its Ca²⁺ activatability by trypsin treatment, the enzyme also loses its response to the inhibitor protein. The effect of the inhibitor protein on the phosphodiesterase is reversible; both the enzyme and the modulator activities can be completely recovered from the inhibited reaction. Kinetic studies of the mutual effects of the protein modulator and the inhibitor protein in the phosphodiesterase reaction have suggested the existence of a stoichiometric interaction between the two protein effectors. Gel filtration analyses with the use of a Sephadex G-200 column have revealed that the protein modulator and the inhibitor protein can associate to form a protein complex in the presence of Ca²⁺. No interaction between the Ca²⁺-activatable phosphodiesterase and the protein inhibitor has been observed. The results suggest that the inhibitor protein inhibits the Ca²⁺-activatable phosphodiesterase by forming a complex with the protein modulator. Since the inhibitor protein is similar to the known modulator-regulated enzymes, Ca²⁺-activatable phosphodiesterase and adenylate cyclase, in undergoing Ca²⁺-dependent association with the protein modulator, it may be another modulator-regulated enzyme (or protein). We propose to designate this protein the modulator binding protein until its positive identification can be established. The molecular weights of the free modulator binding protein and the complex of the protein modulator and the modulator binding protein have been estimated on a calibrated Sephadex G-200 column to be approx. 95,000 and 160,000, resp.