Usher syndrome (USH) is a human hereditary disorder characterized by profound congenital
deafness,
retinitis pigmentosa, and vestibular dysfunction.
Myosin VIIa has been identified as the responsible gene for USH type 1B, and a number of missense mutations have been identified in the affected families. However, the molecular basis of the dysfunction of USH gene,
myosin VIIa, in the affected families is unknown to date. Here we clarified the effects of USH1B mutations on human
myosin VIIa motor function for the first time. The missense mutations of USH1B significantly inhibited the actin activation of
ATPase activity of
myosin VIIa. G25R, R212C, A397D, and E450Q mutations abolished the
actin-activated ATPase activity completely. P503L mutation increased the basal
ATPase activity for 2-3-fold but reduced the
actin-activated ATPase activity to 50% of the wild type. While all of the mutations examined, except for R302H, reduced the affinity for actin and the
ATP hydrolysis cycling rate, they did not largely decrease the rate of
ADP release from
actomyosin, suggesting that the mutations reduce the duty ratio of
myosin VIIa. Taken together, the results suggest that the mutations responsible for USH1B cause the complete loss of the
actin-activated ATPase activity or the reduction of duty ratio of
myosin VIIa.