Figure 1.

(a) The interacting head motif (IHM) of the two myosin heads in a myosin molecule (pdb accession code 3DTP) shown as a stereo pair (wall-eyed stereo). One head in each pair is in shades of blue and the other in yellow. The motor domains are towards the top, and the lever arms with a long central α–helix (each with two light chains, yellow and red on the right head) are below the motor domains. The coiled-coil of part of the subfragment 2 (S2) of the myosin rod, where the two heads join together, is shown at the lower left. The actin binding site on the right head abuts the left head. The right hand head is called the blocked head and the other the free head. (b) Stereo image of the reconstruction from electron microscopy and single particle analysis of the myosin filaments in human heart muscle, with heads fitted to the density. The total length of filament shown here is around 43 nm. There are three crowns of heads with six heads in each crown. The head pair configuration is similar to that in (a), but the whole structure, for example of the heads nearest to the viewer, has been rotated 180° top to bottom roughly around a horizontal rotation axis in the plane of the page. Other head pairs are then rotated around the filament axis by varying amounts.