Functional role of loop 2 in myosin V

Area of Science:

• Molecular biology
• Biochemistry
• Cellular dynamics

Background:

• Myosin V is a processive molecular motor that moves along actin filaments.
• Its kinetics differ from nonprocessive myosin II, with higher actin affinity and rapid phosphate release.
• ADP release is the rate-limiting step in myosin V's ATPase cycle.

Purpose of the Study:

• To investigate the role of loop 2 in the actin-binding region of myosin V.
• To determine how loop 2 mutations affect the kinetic tuning of myosin V's motor function.
• To elucidate the contribution of loop 2 to actin affinity and phosphate release.

Main Methods:

• Generation of two myosin V loop 2 mutants.
• Analysis of ATPase kinetics, including apparent actin affinity (K(ATPase)) and maximum ATPase rate (V(MAX)).
• Transient kinetic analysis to determine the rates of individual steps in the ATPase cycle.

Main Results:

• Loop 2 mutations altered apparent actin affinity (K(ATPase)) but not V(MAX).
• Actin binding rate and affinity were dependent on the net positive charge of loop 2.
• Mutations significantly altered actin affinity in the M.ADP.Pi-state but not phosphate release rate.

Conclusions:

• Loop 2 is crucial for myosin V's high affinity binding to actin in weak binding states.
• Loop 2 does not directly influence product release steps (phosphate or ADP).
• Maintaining high weak-binding affinity via loop 2 may enhance processive motion by preventing diffusion from actin.