Effects of phosphorylation and mutation R145G on human cardiac troponin I function

We have studied functional consequences of the mutations R145G, S22A, and S23A of human cardiac troponin I (cTnI) and of phosphorylation of two adjacent N-terminal serine residues in the wildtype cTnI and the mutated proteins. The mutation R145G has been linked to the development of familial hypertrophic cardiomyopathy. Cardiac troponin was reconstituted from recombinant human subunits including either wild-type or mutant cTnI and was used for reconstitution of thin filaments with skeletal muscle actin and tropomyosin. The Ca2+-dependent thin filament-activated myosin subfragment I ATPase (actoS1-ATPase) activity and the in vitro motility of these filaments driven by myosin were measured as a function of the cTnI phosphorylation state. Bisphosphorylation of wild-type cTnI decreases the Ca2+ sensitivity of the actoS1 -ATPase activity and the in vitro thin filament motility by about 0.15-0.21 pCa unit. The nonconservative replacement R145G in cTnI enhances the Ca2+ sensitivity of the actoS1 -ATPase activity by about 0.6 pCa unit independent of the phosphorylation state of cTnI. Furthermore, it min-des a strong suppressing effect on both the maximum actoS1-ATPase activity and the maximum in vitro filament sliding velocity which has been observed upon bisphosphorylation of wild-type cTnI. Bisphosphorylation of the mutant cTnI-R145G itself had no such suppressing effects anymore. Differential analysis of the effect of phosphorylation of each of the two serines, Ser23 in cTnI-S22A and Ser22 in cTnI-S23A, indicates that phosphorylation of Ser23 may already be sufficient for causing the reduction of maximum actoS1-ATPase activity and thin filament sliding velocity seen upon phosphorylation of both of these serines.