On the mechanism of actin monomer-polymer subunit exchange at steady state.

The rate of exchange of G-actin with subunits of F-actin and the rate of hydrolysis of ATP in solutions of F-actin at steady state have been measured simultaneously. Subunit exchange kinetics were analyzed by both a treadmill model and an exchange-diffusion model. The best fit to a treadmill model of the data obtained in 0.5 mM MgCl2 and 0.2 mM ATP at 30 degrees C gave a treadmill efficiency (net monomers incorporated per ATP hydrolyzed) of 0.26, in good agreement with the previously reported s-value of 0.25 (Wegner, A. (1976) J. Mol. Biol. 108, 139-150) for similar ionic conditions. However, in this and other conditions with excess free divalent cations (Ca2+ or Mg2+), the observed exchange kinetics were in better agreement with an exchange-diffusion model than with a treadmilling model over the entire time course of the experiment. In the absence of excess divalent cations (50 mM KCl), exchange was too slow to be analyzed adequately by either model. Using the measured filament length distribution and the observed fit of the exchange-diffusion model to the data in 0.5 mM MgCl2, an on-rate constant of 2.8 x 10(6) M-1 S-1 and an off-rate constant of 5.8 s-1 were calculated. These values, while in good agreement with previously measured pre-steady state polymerization rate constants under different ionic conditions (Pollard, T. D., and Mooseker, M. S. (1981) J. Cell Biol. 88, 654-659), are about 30-fold higher than the rate constants predicted from the rate of ATP hydrolysis at steady state. To rationalize these discrepancies, a model is proposed in which a segment of F-actin subunits at one or both ends of the filament contains bound ATP at steady state.