CARDIAC TROPONIN-I MUTANTS - PHOSPHORYLATION BY PROTEIN-KINASE-C AND PROTEIN-KINASE-A AND REGULATION OF CA2+-STIMULATED MGATPASE OF RECONSTITUTED ACTOMYOSIN S-1

The significance of site-specific phosphorylation of cardiac troponin I (TnI) by protein kinase C and protein kinase A in the regulation of Ca2+-stimulated MgATPase of reconstituted actomyosin S-1 was investigated. The TnI mutants used were T144A, S43A/S45A, and S43A/S45A/T144A (in which the identified protein kinase C phosphorylation sites, Thr-144 and Ser-43/ Ser-45, were, respectively, substituted by Ala) and S23A/S24A and N32 (in which the protein kinase A phosphorylation sites Ser-23/Ser-24 were either substituted by Ala or deleted). The mutations caused subtle changes in the kinetics of phosphorylation by protein kinase C, and all mutants were maximally phosphorylated to various extents (1.3-2.7 mol of phosphate/mol of protein). Protein kinase C could cross-phosphorylate protein kinase A sites but the reverse essentially could not occur. Compared to wild-type TnI and T144A, unphosphorylated S43A/S45A, S43A/S45A/T144, S23A/ S24A, and N32 caused a decreased Ca2+ sensitivity of Ca2+-stimulated MgATPase of reconstituted actomyosin S-1. Phosphorylation by protein kinase C of wildtype and all mutants except S43A/S45A and S43A/S45A/ T144A caused marked reductions in both the maximal activity of Ca2+-stimulated MgATPase and apparent affinity of myosin S-1 for reconstituted (regulated) actin. It was further noted that protein kinase C acted in an additive manner with protein kinase A by phosphorylating Ser-23/Ser-24 to bring about a decreased Ca2+ sensitivity of the myofilament. It is suggested that Ser-43/Ser-45 and Ser-23/Ser-24 in cardiac TnI are important for normal Ca2+ sensitivity of the myofilament, and that phosphorylation of Ser-43/ Ser-45 and Ser-23/Ser-24 is primarily involved in the protein kinase C regulation of the activity and Ca(2+)sensitivity, respectively, of actomyosin S-1 MgATPase.