The in vitro motility activity of β-cardiac myosin depends on the nature of the β-myosin heavy chain gene mutation in hypertrophic cardiomyopathy

Several mutations in the β-myosin heavy chain gene cause hypertrophic cardiomyopathy. This study investigates (1) the in vitro velocities of translocation of fluorescently-labelled actin by β-myosin purified from soleus muscle of 30 hypertrophic cardiomyopathy patients with seven distinct β-myosin heavy chain gene mutations: Thr124Ile, Tyr162Cys, Gly256Glu, Arg403Gln, Val606Met, Arg870His, and Leu908Val mutations; and (2) motility activity of β-myosin purified from cardiac and soleus muscle biopsies in the same patients. The velocity of translocation of actin by β-myosin purified from soleus or cardiac muscle of 22 normal controls was 0.48 ± 0.09 μm s−1. By comparison, the motility activity was reduced in all 30 patients with β-myosin heavy chain gene mutations (range, 0.112 ± 0.041 to 0.292 ± 0.066 μm s−1). Notably, the Tyr162Cys and Arg403Gln mutations demonstrated significantly lower actin sliding velocities: 0.123 ± 0.044, and 0.112 ± 0.041 μm s−1, respectively. β-myosin purified from soleus muscle from four patients with the Arg403Gln mutation had a similar actomyosin motility activity compared to β-myosin purified from their cardiac biopsies (0.127 ± 0.045 μm s−1 versus 0.119 ± 0.068 μm s−1, respectively). Since these seven mutations lie in several distinct functional domains, it is likely that the mechanisms of their inhibitions of motility are different