Protein Kinase C Phosphorylation of Cardiac Troponin I and Troponin T Inhibits Ca2+-Stimulated MgATPase Activity in Reconstituted Actomyosin and Isolated Myofibrils, and Decreases Actin-Myosin Interactions

The inhibitory effects of the phosphorylation of bovine cardiac troponin I (TnI) and troponin T (TnT) by protein kinase C (PKC) on the activity of Ca2+-stimulated MgATPase of reconstituted actomyosin complex, as a function of the concentration of myosin or myosin subfragment 1 (S-1), were investigated. Phosphorylation of TnI and/or TnT invariably decreased the Ca2+-stimulated enzyme activity of reconstituted actomyosin or actomyosin S-1, regardless of the concentration of whole myosin or S-1. The inhibition due to phosphorylated TnI was partially overcome as the concentration of myosin or S-1 increased, suggesting simple competition of phosphorylated TnI with myosin or S-1 for actin binding sites. Inhibition due to phosphorylated TnT, however, remained constant at all concentrations of myosin or S-1, suggesting that phosphorylated TnT may inhibit full Ca2+-activation of the thin filament. Both phosphorylated TnI and TnT inhibited the Ca2+-stimulated binding of S-1 ADP to regulated actin, consistent with the notion that the effects of phosphorylation of TnI and TnT affected interactions of the thin filament with the thick filament. Effects of PKC phosphorylation of the contractile components in adult rat cardiac myofibrils were also investigated. PKC phosphorylation of TnI and TnT, as well as other proteins in the contractile complex, resulted in the inhibition of Ca2+-stimulated MgATPase activity with little change in the Ca2+-sensitivity. Thus, the negative inotropic effects attributable to activation of PKC by phorbol esters, as reported by others, could be explained in part through PKC mediated phosphorylation of components of the contractile apparatus.