Fig. 8. A model for the arrangement of myofilin and flightin in the IFM thick filament. (A) Cross-sectional top view of half a thick filament showing the myosin sub-filaments (adapted from Beinbrech et al., 1988). In the full thick filament there would be 12 sub-filaments, here we show only numbers 1-3 and 10-12. Myofilin molecules are shown in grey on the inside of the thick filament. Flightin molecules are open symbols on the outside of the filament. (B) A side view of the thick filament lattice, again based on Beinbrech's model. Each sub-filament has myosin molecules with alternating `outer' heads (black ellipses pointing to the left) and `inner' heads pointing to the right. The myofilin molecules only bind to the rod region of the `inner' myosin molecules, with the result that they are arranged at an interval of 29 nm, as shown; this is twice the 14.5 nm spacing of the myosin heads in the filament. Myofilin is placed at an arbitrary position on the myosin rod. Flightin molecules are bound to `outer' myosin molecules about two-thirds of the way along the rod. In order to simplify the representation of the model, only one head of myosin dimers is shown, and the sub-filaments are drawn parallel to the axis of the thick filament, although evidence from X-ray diffraction suggests that they are inclined at a shallow angle to the filament (Wray, 1979b).