The myosin filament. XII. Effect of MgATP on assembly

Summary

The effect of MgATP on myosin filament assembly has been studied. Filaments were assembled by a standard dilution procedure involving two steps, dilution from 0.6 to 0.3m KCl and from 0.3 to 0.15m KC1 with a different rate of dilution in each step. This standard dilution procedure gives filaments which are structurally similar to native filaments in that they have a sharp length distribution around 1.5 μm, a diameter of 16 nm and they vary in length with KCl concentration in a similar manner to native filaments. The addition of 1mm MgATP leads to a sharpening of the length distribution around 1.5 μm without change in the 16 nm diameter. Filaments assembled by dialysis or by rapid dilution are not similarly affected by the presence of MgATP indicating that the standard dilution procedure produces filaments which are more closely similar to native filaments than those produced by these other methods. MgAMPPNP and magnesium pyrophosphate have the same effect as MgATP thus eliminating the possibility that phosphorylation of the myosin is involved in the effect. The effect of MgATP is not directly related to its binding to the active site of the myosin molecule since a 500∶1 mole ratio of MgATP to myosin is required for the effect. It is therefore likely that the effect of MgATP is related to other binding sites on the myosin molecule.

The presence of MgATP leads to molecular rearrangements which finely tune the molecular organization of the filaments formed by the standard dilution procedurein vitro. It is likely that the MgATP present in living cells may similarly be responsible for the fine tuning of the molecular assembly of the myosin filaments to produce uniform lengthsin vivo.