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 Caz+-activatable phosphodiesterase is desensitized of its Ca”+ 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 effecters. Gel filtration analyses with the use of a Sephadex G-ZOO 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 Ca”+-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 approximately 95,000 and 160,000, respectively.