Movement generated by the
myosin motor is generally thought to be driven by distortion of an elastic
element within the
myosin molecule and subsequent release of the resulting strain. However, the location of this elastic
element in
myosin remains unclear. The
myosin motor domain consists of four major subdomains connected by flexible joints. The SH1 helix is the joint that connects the converter subdomain to the other domains, and is thought to play an important role in arrangements of the converter relative to the motor. To investigate the involvement of the SH1 helix in elastic distortion in
myosin, we have introduced a point mutation into the SH1 helix of Dictyostelium
myosin II (R689H), which in human nonmuscle
myosin IIA causes nonsyndromic hereditary
deafness,
DFNA17. The mutation resulted in a significant impairment in motile activities, whereas
actin-activated ATPase activity was only slightly affected. Single molecule mechanical measurements using optical trap showed that the step size was not shortened by the mutation, suggesting that the slower motility is caused by altered kinetics. The single molecule measurements demonstrated that the mutation significantly reduced cross-bridge stiffness. Motile activities produced by mixtures of wild-type and mutant
myosins also suggested that the mutation affected the elasticity of
myosin. These results suggest that the SH1 helix is involved in modulation of
myosin elasticity, presumably by modulating the converter flexibility. Consistent with this, the mutation was also shown to reduce thermal stability and induce thermal aggregation of the
protein, which might be implicated in the disease process.