Single-headed binding of a spin-labeled-HMM-ADP complex to F-actin. Saturation transfer electron paramagnetic resonance and sedimentation studies.

The interaction of actin and spin-labeled heavy meromyosin (MSL-HMM) was studied in the presence and absence of adenosine diphosphate or 5'-adenyl-yl-imidodiphosphate (AMPPNP) to determine the contributions of single and double-headed binding. The extent of single-headed binding to actin was deduced from a comparison of the fraction of immobilized heads (fi) with the fraction of bound molecules (fs) determined by saturation-transfer EPR (ST-EPR) and sedimentation, respectively. The ST-EPR measurements depend on the reduced motion of the spin label rigidly bound to the HMM heads upon the interaction of the latter with actin. During titration of acto-MSL-HMM with nucleotide, we measured changes in fi and fs brought about by dissociation of MSL-HMM from actin. On titration with ADP, fs changed very little, remaining above 0.8, while fi decreased to approximately 0.5 at 10mM ADP, a result consistent with extensive single-headed binding of MSL-HMM to actin. On titration with AMPPNP, single-headed binding was not detected; viz., fi and fs decreased in parallel. It was not necessary to postulate a nucleotide induced state of the bound heads, differing in motional properties from that of rigor heads, to account for the results.