bacterially expressed and purified these human cTnI mutants and examined their functional and structural consequences. Inserting the human cTnI into skinned cardiac muscle fibers showed that these mutations had much greater Ca2+ sensitizing effects on force generation than the cTnI mutations in hypertrophic cardiomyopathy (HCM). The mutation K178E in the second actin-tropomyosin (Tm) binding region showed a particularly potent Ca2+-sensitizing effect among the six RCM-causing mutations nuclear magnetic resonance spectroscopy revealed that this Mutation does not extensively affect the Structure of the whole cTnI molecule, but induces all unexpectedly subtle change in the structure of a region around the Mutated residue. The results indicate that the K178E mutation has a localized effect on a structure that is critical to the regulatory function of the second actin-Tin binding region of cTnI. The present study also Suggests that both HCM and RCM involving cTnI Mutations share a common feature of increased Ca2+ sensitivity of cardiac myofilament, but more severe change in Ca2+ sensitivity is associated with the clinical phenotype of RCM. (c) 2005 Elsevier Inc. All rights reserved.