Analysis of mechanical properties of mutant myosin linked to nonsyndromic hereditary deafness, DFNA17

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A replacement of conserved arginine in the SH1 helix with histidine is revealed to be linked to nonsyndromic hereditary deafness, DFNA17. To investigate the involvement of the SH1 helix in mechanical properties of myosin, we have introduced a point mutation into the SH1 helix of Dictyosterium myosin II The mutation resulted in a significant impairment in motile activities, while actin-activated ATPase activity was only slightly affected. Single molecule mechanical measurements using optical trap showed that step size was not shortened, 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 ant mutant myosins also suggested that the mutation affected the elstiity of myosin. A GFP-based fluorescence resonance energy transfer study showed that the activation energy required for conformational transition was significantly reduced by the mutation. The mutation was also shown to reduce thermal stability and induce thermal aggregation of the protein, which might be implicated in the disease process.