Je. Molloy et al., SINGLE-MOLECULE MECHANICS OF HEAVY-MEROMYOSIN AND S1 INTERACTING WITHRABBIT OR DROSOPHILA ACTINS USING OPTICAL TWEEZERS, Biophysical journal, 68(4), 1995, pp. 298-305
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).