E22K mutation of RLC that causes familial hypertrophic cardiomyopathy in heterozygous mouse myocardium: Effect on cross-bridge kinetics

Familial hypertrophic cardiomyopathy is a disease characterized by left ventricular and/or septal hypertrophy and myofibrillar disarray. It is caused by mutations in sarcomeric proteins, including the ventricular isoform of myosin regulatory light chain (RLC). The E22K mutation is located in the RLC Ca ²⁺-binding site. We have studied transgenic (Tg) mouse cardiac myofibrils during single-turnover contraction to examine the influence of E22K mutation on 1) dissociation time (τ₁) of myosin heads from thin filaments, 2) rebinding time (τ₂) of the cross bridges to actin, and 3) dissociation time (τ₃) of ADP from the active site of myosin. τ₁ was determined from the increase in the rate of rotation of actin monomer to which a cross bridge was bound. τ₂ was determined from the rate of anisotropy change of the recombinant essential light chain of myosin labeled with rhodamine exchanged for native light chain (LC1) in the cardiac myofibrils. τ₃ was determined from anisotropy of muscle preloaded with a stoichiometric amount of fluorescent ADP. Cross bridges were induced to undergo a single detachment-attachment cycle by a precise delivery of stoichiometric ATP from a caged precursor. The times were measured in Tg-mutated (Tg-m) heart myofibrils overexpressing the E22K mutation of human cardiac RLC. Tg wild-type (Tg-wt) and non-Tg muscles acted as controls. τ₁ was statistically greater in Tg-m than in controls. τ₂ was shorter in Tg-m than in non-Tg, but the same as in Tg-wt. τ₃ was the same in Tg-m and controls. To determine whether the difference in τ₁ was due to intrinsic difference in myosin, we estimated binding of Tg-m and Tg-wt myosin to fluorescently labeled actin by measuring fluorescent lifetime and time-resolved anisotropy. No difference in binding was observed. These results suggest that the E22K mutation has no effect on mechanical properties of cross bridges. The slight increase in τ₁ was probably caused by myofibrillar disarray. The decrease in τ₂ of Tg hearts was probably caused by replacement of the mouse RLC for the human isoform in the Tg mice.