Familial Hypertrophic Cardiomyopathy-linked Alterations in Ca2+ Binding of Human Cardiac Myosin Regulatory Light Chain Affect Cardiac Muscle Contraction*

The ventricular isoform of human cardiac regulatory light chain (HCRLC) has been shown to be one of the sarcomeric proteins associated with familial hypertrophic cardiomyopathy (FHC), an autosomal dominant disease characterized by left ventricular and/or septal hypertrophy, myofibrillar disarray, and sudden cardiac death. Our recent studies have demonstrated that the properties of isolated HCRLC could be significantly altered by the FHC mutations and that their detrimental effects depend upon the specific position of the missense mutation. This report reveals that the Ca2+ sensitivity of myofibrillar ATPase activity and steady-state force development are also likely to change with the location of the specific FHC HCRLC mutation. The largest effect was seen for the two FHC mutations, N47K and R58Q, located directly in or near the single Ca2+-Mg2+ binding site of HCRLC, which demonstrated no Ca2+ binding compared with wild-type and other FHC mutants (A13T, F18L, E22K, P95A). These two mutants when reconstituted in porcine cardiac muscle preparations increased Ca2+ sensitivity of myofibrillar ATPase activity and force development. These results suggest the importance of the intact Ca2+ binding site of HCRLC in the regulation of cardiac muscle contraction and imply its possible role in the regulatory light chain-linked pathogenesis of FHC.

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