Ca2+-regulatory function of the inhibitory peptide region of cardiac troponin I is aided by the C-terminus of cardiac troponin T: Effects of familial hypertrophic cardiomyopathy mutations cTnI R145G and cTnT R278C, alone and in combination, on filament sl

• FHC mutants TnI R145G and TnT R278C were tested alone and together in motility assays.
• Inhibition of smax, but not left-shift of pCa50, by TnI R145G is rescued by TnT R278C.
• Sliding at low Ca2+ (diastolic dysfunction) with TnI R145G is worsened by TnT R278C.
• Protein sequence analyses suggest a common evolutionary origin for TnI and TnT.
• Function and structure analyses suggest TnT C-terminus aids TnI inhibitory peptide.

Investigations of cardiomyopathy mutations in Ca2+ regulatory proteins troponin and tropomyosin provide crucial information about cardiac disease mechanisms, and also provide insights into functional domains in the affected polypeptides. Hypertrophic cardiomyopathy-associated mutations TnI R145G, located within the inhibitory peptide (Ip) of human cardiac troponin I (hcTnI), and TnT R278C, located immediately C-terminal to the IT arm in human cardiac troponin T (hcTnT), share some remarkable features: structurally, biochemically, and pathologically. Using bioinformatics, we find compelling evidence that TnI and TnT, and more specifically the affected regions of hcTnI and hcTnT, may be related not just structurally but also evolutionarily. To test for functional interactions of these mutations on Ca2+-regulation, we generated and characterized Tn complexes containing either mutation alone, or both mutations simultaneously. The most important results from in vitro motility assays (varying [Ca2+], temperature or HMM density) show that the TnT mutant “rescued” some deleterious effects of the TnI mutant at high Ca2+, but exacerbated the loss of function, i.e., switching off the actomyosin interaction, at low Ca2+. Taken together, our experimental results suggest that the C-terminus of cTnT aids Ca2+-regulatory function of cTnI Ip within the troponin complex.

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