Propagation of Acto-S-1 ATPase Reaction-Coupled Conformational Change in Actin along the Filament1

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Abstract
F-Actin was partially cross-linked to myosin subfragment-1 (S-1) at various molar ratios (r=S-1/actin) with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The cross-linked acto-S-1 ATPase showed so called “super-activation, ” Vx. S-1 was added further to the cross-linked acto-S-1 and the ATPase activity, Vy, was measured. Since the added S-1 can interact only with the bare actin protomers within the cross-linked actin filament, the difference, ΔV = Vy − Vx − Vs (where VS is the ATPase activity of the additional S-1 alone), can indicate the state of the bare actin protomers while the cross-linked acto-S-1 is hydrolyzing ATP. With increasing r, ΔV decreased much more rapidly than ΔVo(1−r) (where ΔVo is ΔV at r=0) and reached a minimum around r=0.15. As r increased further, ΔV approached the level of ΔVo(1−r). When SH1/SH2-blocked S-1 was cross-linked to F-actin, ΔV decreased according to ΔVo(1−r). Therefore, the large reduction of ΔV, observed when intact S-1 was cross-linked, was coupled to the high ATPase activity of the cross-linked acto-S-1. Combining these data with other kinetic data, we could deduce that structural distortion in a cross-linked actin induced by the ATPase reaction of the S-1 partner propagated over several bare actin protomers along the filament and reduced their affinity for the S-1-ADP-P1, complex. A model is presented to interpret these data, hypothesizing an asymmetry in the propagation of actin conformational change.