Effect of myosin on actin-bound nucleotide exchange in the presence and absence of ATP

Abstract

We have continued our investigation of bound nucleotide exchange in F-actin and actomyosin by measuring the release of actin-bound [ 3H]ADP using a rapid ultrafiltration technique. In agreement with previous work, the rate of actin-bound nucleotide release was found to be greatly increased by myosin in the presence of ATP under conditions of superprecipitation. However, we now find that the same effect of myosin is observed not only in the presence of ATP but also in the presence of ADP where no superprecipitation occurs. This effect of myosin is abolished at high ionic strength, and heavy meromyosin does not show the effect, suggesting that aggregation of the myosin is essential. We conclude that the increased exchangeability of the actin-bound nucleotide in actomyosin is unrelated to the ATPase activity or contraction of the actomyosin, but is caused simply by the binding of F-actin to myosin filaments.