Kinetic mechanism of human myosin IIIA

Myosin IIIA is specifically expressed in photoreceptors and cochlea and is important for the phototransduction and hearing processes. In addition, myosin IIIA contains a unique N-terminal kinase domain and C-terminal tail actin-binding motif. We examined the kinetic properties of baculovirus expressed human myosin IIIA containing the kinase, motor, and two IQ domains. The maximum actin-activated ATPase rate is relatively slow (k(cat) = 0.77 +/- 0.08 s(-1)), and high actin concentrations are required to fully activate the ATPase rate (K-ATPase = 34 +/- 11 mu m). However, actin co-sedimentation assays suggest that myosin III has a relatively high steady-state affinity for actin in the presence of ATP (K-actin similar to 7 mu m). The rate of ATP binding to the motor domain is quite slow both in the presence and absence of actin (K(1)k(+2) = 0.020 and 0.001 mu m(-1.)s(-1), respectively). The rate of actin-activated phosphate release is more than 100-fold faster (85 s(-1)) than the k(cat), whereas ADP release in the presence of actin follows a two-step mechanism (7.0 and 0.6 s(-1)). Thus, our data suggest a transition between two actomyosin-ADP states is the rate-limiting step in the actomyosin III ATPase cycle. Our data also suggest the myosin III motor spends a large fraction of its cycle in an actomyosin ADP state that has an intermediate affinity for actin (K-d similar to 5 mu m). The long lived actomyosin-ADP state may be important for the ability of myosin III to function as a cellular transporter and actin crosslinker in the actin bundles of sensory cells.