Fig. 6.

The three Classes of head coupling described by Squire et al. (2005). Invertebrate myosin filaments were originally thought to be Class III, with heads interacting between adjacent crowns along the long-pitched helices shown in the helical nets in Fig. 4. Now, they have been shown to be Class I structures (e.g. Woodhead et al. 2005; heads within the same myosin molecule interacting). The Class II structure with heads from two different myosin molecules interacting around a single crown has been proposed for insect flight muscle myosin filaments (Lethocerus; AL-Khayat et al. 2003). An unlikely fourth possibility is that the heads do not interact with each other at all (Class IV)