The effect of polyethylene glycol on the mechanics and ATPase activity of active muscle fibers

We have used polyethylene glycol (PEG) to perturb the actomyosin interactio n in active skinned muscle fibers. PEG is known to potentiate protein-prote in interactions, including the binding of myosin to actin. The addition of 5% w/v PEG (MW 300 or 4000) to active fibers increased fiber tension and de creased shortening velocity and ATPase activity, all by 25-40%. Variation i n [ADP] or [ATP] showed that the addition of PEG had little effect on the d issociation of the cross-bridge at the end of the power stroke. Myosin comp lexed with ADP and the phosphate analog V-i or AIF(4) binds weakly to actin and is an analog of a pre-power-stroke state. PEG substantially enhances b inding of these states both in active fibers and in solution. Titration of force with increasing [P-i] showed that PEG increased the free energy avail able to drive the power stroke by about the same amount as it increased the free energy available from the formation of the actomyosin bond. Thus PEG potentiates the binding of myosin to actin in active fibers, and it provide s a method for enhancing populations of some states for structural or mecha nical studies, particularly those of the normally weakly bound transient st ates that precede the power stroke.