High concentration of the cosolvent poly(ethylene glycol) (PEG) induces reversible aggregation of skeletal myosin subfragment 1 (S1) and inhibition of its Mg-ATPase activity [Highsmith et al. (1998) Biophys. J. 74, 1465-1472]. In the present work the effect of aggregation on the various steps of the ATPase cycle was studied. The isomerization and hydrolysis steps of the cycle were not affected by S1 aggregation since the formation of the "trapped" S1·MgADP·phosphate analogue complexes, which mimic the prehydrolysis M*ATP and posthydrolysis M**ADP·P i transition states, proceeded without any hindrance. Similar conclusions could be reached from the chemical modification of Lys-83 and Cys-707 in the presence of MgATP and MgATPγS, which indicated that the most populated intermediate of the cycle in solubilized and aggregated S1 is M**ADP·Pi. The dissociation of the trapped S1·MgADP· phosphate analogue complexes resembling the M**ADP·Pi state was strongly inhibited by PEG-6000, showing that the transition from this intermediate is prevented by the aggregation. This step is presumably inhibited because the coupled swinging of the lever arm from the closed to the open position is constrained by the close packing of aggregated S1.
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