Figure 1. Connection of the myosin head to the filaments and its working stroke.

A, conformation of the myosin head attached to actin during isometric contraction. The myosin head (or subfragment 1, S1) is formed of a catalytic domain (blue) attached to actin (black and grey) according to Holmes et al. (2003) and a light chain domain (red) that has an average orientation of θ = 60 deg relative to the filament axis and is connected to the filament backbone (violet) via the coiled-coil subfragment 2 (S2, yellow). The partner head, assumed not to be attached during isometric contraction, and its S2 component are in light grey. B, the crystallographic model of the working stroke. The catalytic domain (CD, blue) of the myosin head is firmly attached to the actin filament, here represented by only three monomers (black and grey). The working stroke consists of an ∼70 deg rotation of the light chain domain (LCD) with the fulcrum near Cys707 (yellow circle). The orientation of the LCD is represented as the vector joining the fulcrum to the head–rod junction (residue Lys 843, grey circle). Red and green lines represent the pre- and post-working stroke conformations of the LCD, respectively (adapted from Irving et al. 2000).