Cardiac Troponin I Directly Binds and Inhibits Mitochondrial ATP Synthase: a Noncanonical Role in the Post-Ischemic Heart

Cardiac troponin I (cTnI) is a sarcomeric protein critical to myocyte contraction. Unexpectedly, we found that some cTnI localized to the mitochondrial matrix in the heart, inhibited mitochondrial functions when stably expressed in non-cardiac cells and increased opening of the mitochondrial permeability transition pore under oxidative stress. Direct, specific, and saturable binding of cTnI to ATP synthase was demonstrated in vitro, using immune-captured ATP synthase, and in cells using proximity ligation assay. cTnI binding doubled F1F0 ATPase activity, whereas skeletal troponin I and several human mutant cTnI variants associated with familial hypertrophic cardiomyopathy did not. A rationally-designed ten amino acid peptide, P888, inhibited cTnI binding to ATP synthase and cTnI-induced increase in ATPase activity in vitro and reduced cardiac injury following transient ischemia in vivo. We therefore suggest that mitochondria-associated cTnI may inhibit cardiac ATP synthase under basal conditions; pharmacological agents that release this inactivating effect of cTnI and thus preventing ATP hydrolysis during cardiac ischemia may increase the reservoir of functional mitochondria to reduce cardiac injury. ### Competing Interest Statement The authors have declared no competing interest.