Impacts of Usher syndrome type IB mutations on human myosin VIIa motor function

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 113, and a number of missense mutations have been identified in the affected families. However, the molecular basis of the dysfunction of USH gene, myosin Vila, in the affected families is unknown to date. Here we clarified the effects of USH1B mutations on human myosin Vila motor function for the first time. The missense mutations of USH I B significantly inhibited the actin activation of ATPase activity of myosin VI-la. 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 USH I B cause the complete loss of the actin-activated ATPase activity or the reduction of duty ratio of rnyosin VIIa.