Three-Dimensional Reconstruction of Thin Filaments Containing Mutant Tropomyosin

Interactions of the components of reconstituted thin filaments were investigated using a tropomyosin internal deletion mutant, D234, in which actin-binding pseudo-repeats 2, 3, and 4 are missing. D234 retains regions of tropomyosin that bind troponin and form end-to-end tropomyosin bonds, but has a length to span only four instead of seven actin monomers. It inhibits acto-myosin subfragment 1 ATPase (acto-S-1 ATPase) and filament sliding in vitro in both the presence and absence of Ca 2+ (Landis et al., 1997, J. Biol. Chem. 272:14051–14056) and lowers the affinity of S-1·ADP for actin while increasing its cooperative binding. Electron microscopy and three-dimensional reconstruction of reconstituted thin filaments containing actin, troponin, and wild-type or D234 tropomyosin were carried out to determine if Ca 2+-induced movement of D234 occurred in the filaments. In the presence and absence of Ca 2+, the D234 position was indistinguishable from that of the wild-type tropomyosin, demonstrating that the mutation did not affect normal tropomyosin movement induced by Ca 2+ and troponin. These results suggested that, in the presence of Ca 2+ and troponin, D234 tropomyosin was trapped on filaments in the Ca 2+-induced position and was unable to undergo a transition to a completely activated position. By adding small amounts of rigor-bonded N-ethyl-maleimide-treated S-1 to mutant thin filaments, thus mimicking the myosin-induced “open” state, inhibition could be overcome and full activation restored. This myosin requirement for full activation provides support for the existence of three functionally distinct thin filament states (off, Ca 2+-induced, myosin-induced; cf. Landis et al., 1997; Vibert et al., 1997, J. Mol. Biol. 266:8–14). We propose a further refinement of the three-state model in which the binding of myosin to actin causes allosteric changes in actin that promote the binding of tropomyosin in an otherwise energetically unfavorable “open” state.