Fig. 5.

A novel mechanism of regulation. Schematic diagrams of the thin filament assembly for simplicity shown as a linear array (i.e. not helical). Protein components colour coded: (yellow), tropomyosin (blue) and troponin (TnC, speckled blue; TnI, red; TnT green). When the thin filament changes from the Ca²⁺ free state (a) to the Ca²⁺ bound state (b) the tropomyosin strands on actins g, a and b move further than those on actins c-f. Actins g, a and b may be exposed enough for some myosin heads to bind (dashed outline). Bound heads going over to strong states may then activate the whole filament (c). The body of troponin may move out of the way (dashed arrows) to permit binding of further heads. (a and b) also illustrate a potential mechanism whereby the distal arm of TnT1 acting on the tropomyosin overlap region may swing to expose the binding sites on actins g, a and b when TnC binds Ca ²⁺.