Figure 1.

Sequence, location, and roles of switch-2. A) Switch-2 amino acid sequence and conserved communication pathways in different NTPases. Residues and pathways conserved in myosins, kinesins, and G proteins are colored blue. Class-specific residue mutated in the current study is highlighted in red. B) Location of conserved loops of the ATPase active site within the motor domain of myosin-5a (1W7J). C, D) Magnified views of the active site in strongly (1W7J, C) and weakly (1W7I, D) nucleotide-bound states. E) Kinetic scheme of the actomyosin ATPase cycle. Nucleotide-free (rigor) actomyosin (AM) binds ATP in two steps (K₁′ and K₂′). Myosin head (M) then dissociates (K₈) from actin (A) in the main flux pathway indicated by shading and bold characters. After ATP hydrolysis (K₃), M · ADP · P_i rebinds to actin (K₉), and products are released (P_i in K₄′ and ADP in K₅′). Strong and weak actin-binding states are shaded in green and cyan, respectively. All rate and equilibrium constants are defined in the rightward and downward directions.