SINGLE-MOLECULE MECHANICS OF HEAVY-MEROMYOSIN AND S1 INTERACTING WITHRABBIT OR DROSOPHILA ACTINS USING OPTICAL TWEEZERS

Single-molecule mechanical interactions between rabbit heavy meromyosi n (HMM) or subfragment 1 (S1) and rabbit actin were measured with an o ptical tweezers piconewton, nanometer transducer. Similar intermittent interactions were observed with HMM and S1. The mean magnitude of the single interaction isotonic displacements was 20 nm for HMM and 15 nm with S1. The mean value of the force of single-molecule interactions was 1.8 pN for HMM and 1.7 pN with S1. The stiffness of myosin S1 was determined by applying a sinusoidal length change to the thin filament and measuring the corresponding force; the mean stiffness was 0.13 pN nm(-1). By moving an actin filament over a long distance past an isol ated S1 head, we found that cross-bridge attachment occurred preferent ially at a periodicity of about 40 nm, similar to that of the actin he lical repeat. Rate constants for the probability of detachment of HMM from actin were determined from histograms of the lifetime of the atta ched state. This gave a value of 8 s(-1) or 0.8 x 10(6) M(-1) S-1 for binding of ATP to the rigor complex. We conclude (1) that our HMM-acti n interactions involve just one head, (2) that compliance of the cross -bridge is not in myosin subfragment 2, although we cannot say to what extent contributions arise from myosin S1 or actin, and (3) that the elemental movement can be caused by a change of shape of the S1 head, but that this would have to be much greater than the movements suggest ed from structural studies of S1 (Rayment et al., 1993).