Functional relationship between bovine brain phosphodiesterase activator protein and bovine heart troponin C

Phosphodiesterase activator protein has been purified from bovine brain and its properties compared with that of bovine heart troponin C. While both proteins activate ‘activator depleted phosphodiesterase’ in the presence of Ca²⁺, a 200-fold greater concentration of troponin C was necessary and the maximal effect was less than that with the activator protein. The activator protein formed a Ca²⁺ -dependent complex with bovine heart troponin I during electrophoresis in 6 M urea-polyacrylamide gel. However, the mobility of this complex was different from that of troponin C · troponin I complex and the affinity between troponin C and troponin I was much stronger than that between the activator protein and troponin I. Ca²⁺ induced changes in the electrophoretic mobility of activator protein and the pattern of its elution during gel filtration which were similar to the Ca²⁺-dependent conformational changes observed with troponin C. Bovine heart troponin I reduced basal, troponin C and the activator protein stimulation of phosphodiesterase activity. These results are compatible with the concept that phosphodiesterase activator protein and troponin C might have a functional relationship.