Functional Role of the Extended Loop 2 in the Myosin 9b Head for Binding F-actin

The mammalian class IX myosin Myo9b can move considerable distances along actin filaments before it dissociates. This is remarkable, because it is single headed and because the rate-limiting step in its ATPase cycle is ATP hydrolysis. Thus, it spends most of its cycling time in the ATP-bound state that has a weak affinity for F-actin in other myosins. It has been speculated that the very extended loop 2 in the Myo9b head domain comprises an additional actin-binding site that prevents it from dissociation in the weak binding states. Here we show that two regions in the loop 2 determine the F-actin concentrations needed to maximally activate the steady-state ATPase activity. Together these two regions regulate the amount capable of binding F-actin and the affinity of the nucleotide-free state. The isolated loop 2 behaved like an entropic spring and bound stoichiometrically and with high affinity to F-actin. Subfragment 1 from skeletal muscle myosin II bound to F-actin simultaneously with the isolated loop 2 of Myo9b and could not displace it. Furthermore, the present results imply also a regulatory role for the tail region. Taken together, the results demonstrate that the extended loop 2 in Myo9b binds F-actin and influences the binding of the conventional stereo-specific actin-binding site.