Conformational change in actin filament induced by the interaction with heavy meromyosin: effects of pH, tropomyosin and deoxy-ATP.

Conformational changes in pure and tropomyosin-containing F-actin during interaction with heavy meromyosin in the absence and presence of deoxy-ATP, were studied by measurements of the changes in fluorescence intensity of e-ADP² incorporated into the F-actin instead of ADP. The actin filaments were found to be stabilized by tropomyosin and were more stable at pH 7 than at pH 8. The rigor binding of HMM to F-actin caused an increase in the fluorescence intensity. The increase with F-actin containing TM was higher than that with pure F-actin at each HMM concentration. A linear relation between the fluoresence change and moles of HMM per actin was found regardless of the presence of TM, with a maximum value of 0.5 moles of HMM per actin. In the presence of deoxy-ATP, (which is a substrate for acto-HMM but cannot bind to actin) no changes in fluorescence intensity of e-ADP bound to pure F-actin were observed. In the case of F-actin containing TM, the fluorescence intensity increased with increasing HMM concentration, although the light scattering intensity of the acto-HMM solutions indicated that almost all the HMM was dissociated from the F-actin. This suggests that the conformational change in F-actin-TM induced by the interaction with HMM in the presence of deoxy-ATP has a long lifetime which continues for some time even after the detachment of the HMM.