Chemical modification studies on the Ca2+-dependent protein modulator: the role of methionine residues in the activation of cyclic nucleotide phosphodiesterase.

Methionine residues have been implicated in the activation of cyclic nucleotide phosphodiesterase by the Ca2+-dependent protein modulator [Walsh, M., & Stevens, F.C. (1977) Biochemistry 16,2742-2749]. Treatment of the modulator with N-chlorosuccinimide in the presence of Ca2+ resulted in selective oxidation of methionine residues at positions 71,72, 76, and, possibly, 109 in the modulator sequence. These residues lie on the surface of the molecule exposed to solvent. This modification has several effects on the modulator protein: (1) the Ca2+-binding properties of the oxidized modulator are changed with apparent loss of high-affinity binding sites, (2) the oxidized protein no longer interacts with phosphodiesterase, and (3) troponin C like activities, viz., Ca2+-dependent change in mobility on urea-polyacrylamide gel electrophoresis and formation of a urea-stable complex with troponin I, are lost upon oxidation of the modulator. The phosphodiesterase binding domain of the modulator protein appears to be located between the second and third Ca2+-binding loops, a region of the molecule known from previous partial proteolysis studies [Walsh, M., Stevens, F.C., Kuznicki, J., & Drabikowski, W.(1977), J. Biol. Chem. 252, 7440-7443] to be exposed in the presence of Ca2+.