Figure 2.

The thin filaments slide by using the mechanics of a drill-borer.

(a) Surface view of a reconstruction of a thin filament of frog skeletal muscle in EGTA, and (b) of Limulus muscle in Ca²⁺. The bright structures wound about the right-handed helical actin filaments are the two tropomyosin coiled-coils, that in (b) are shifted a little to the left. Arrow A₀ indicates the position without Ca²⁺, arrow A_i the position with Ca²⁺ (from Vibert et al. [35], with permission). (c) Screw of a right-handed drill-borer, shown in (d) and (e). The inclination angle of the thin filament helix and drill-borer screw is about 70°. (d) The drill-borer works by moving up and down (arrows) the handle with a nut-coil. This causes clockwise and counter-clockwise rotation (arrows) of the screw. (e) When the handle with the nut-coil is fixed, pulling up the screw (arrow up) causes counter-clockwise drilling rotation (arrow cc), as seen from above. (f) High EM magnification of two thin filaments on both sides of a thick filament (“myac” view, Z-band is topwards) of an insect flight muscle in rigor (quick-freeze and deep-etched preparation of John Heuser [36], with permission), When ATP is added to an intact rigor muscle, a stretch can pull out the thin filaments off the cross-bridges (arrows up). That means a counter-clockwise drilling rotation of the thin filaments (arrows cc) as demonstrated by the drill-borer model in (e).