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 μM). 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 ∼ 7 μM). The rate of ATP binding to the motor domain is quite slow both in the presence and absence of actin (K₁ k₊₂ = 0.020 and 0.001 μM^-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 ∼ 5 μM). The long lived actomyosin-ADP state may be important for the ability of myosin III to function as a cellular transporter and actin cross-linker in the actin bundles of sensory cells.