Figure 2. The myosin motor domain presented in the rigor-like conformation.

(A) The motor subdomains and its functional sites. The nearly rigid motor subdomains are shown in space-filling models (on the left) and cartoons (on the right). The N-terminal (N), the upper 50 kDa (U50), the lower 50 kDa (L50), the converter (C), the first IQ motif (IQ), and the essential light chain (ELC) are colored in orange, blue, red, lime, pale green, and yellow, respectively. In the space-filling representation the location of the myosin functional sites is indicated: the actin-binding site at the interface of the U50 and L50 subdomains; the nucleotide-binding site at the interface of the N and U50 subdomains; and the beginning of the lever arm, whose position is controlled by the rotation of the converter. (B) The subdomain connectors in the myosin motor domain. The various connectors are color-coded as follows: the P-loop, switch I, switch II, the strut, the relay, helix SH1, and loop 76–81 are cyan, magenta, orange, red, yellow, slate, and violet. The P-loop, switch I and switch II contribute to the formation of the active site involved in nucleotide binding and hydrolysis at the interface of the N, U50, and L50 subdomains; the strut joins U50 and L50 in the upper part of the U50/L50 cleft; the relay group connects L50 to C; helix SH1 and loop 76–81 connect the converter to the N-terminal subdomain. In the text, the terms “upward” and “downward” are used to indicate motion in the direction of the top and bottom of the figure.