Mutagenesis of cardiac troponin I. Role of the unique NH sub(2)-terminal peptide in myofilament activation

Phosphorylation of Ser residues in the NH sub(2)-terminal extension unique to cardiac troponin I (cTnI) is known to occur through protein kinase A and to alter myofilament Ca super(2+) activation. Yet, how the NH sub(2)-terminal extension may itself affect thin filament Ca super(2+) signaling is unknown. To approach this question we have used molecular cloning, mutagenesis, and bacterial synthesis of a full-length cTnI and a truncated mutant (cTnI/NH sub(2)) missing the 32 amino acids. Using reconstituted preparations we could show no differences between cTnI and cTnI/NH sub(2) either in inhibition of actomyosin ATPase activity, in Ca super(2+)-reversible inhibitory activity, or in the relation between pCa and Ca super(2+) binding to the regulatory site of cTnC at either pH 7.0 or 6.5. There were also no significant differences at either pH in the pCa-MgATPase activity relation of myofibrils into which the various species of TnI had been exchanged. Our results indicate: 1) that phosphorylation most likely induces a new state of TnI activity rather than altering an intrinsic effect of the NH sub(2)-terminal peptide on Ca super(2+) activation; and 2) that domains outside the NH sub(2)-terminal extension are important with regard to differences in effects of acidic pH on Ca super(2+) activation on cardiac and skeletal myofilaments.