Fig. 2
Mechanical output of the myosin-II nanomachine in 2 mM ATP. a Schematic representation of three snapshots during the phases of the interaction between the actin filament and the motors numbered as in b. Components of the system coloured as in Fig. 1a. b Recording of the relative sliding (red trace) and force (blue trace) during the interaction. Phase 1, following the formation of the first bonds between the actin filament and myosin motors, the force rises in position feedback to the maximum isometric value (F0 ~ 17 pN). Phases 2–6, shortening response to a staircase of stepwise reductions in force (4 pN) separated by 1 s imposed in force feedback. c Another experiment in which in phase 1 in position feedback isometric force rises to F0 (~21 pN) with some force fluctuations and in frames 2–7 the staircase in force feedback is made by 5-pN steps separated by 1 s (3–5) and 0.5 s (6, 7). In phase 8, after a total active sliding of 2.7 µm, the control is switched back to position feedback, and the force redevelops up to the original F0. Phases 9–11, force response to a series of rapid lengthenings and shortenings of ~500 nm superimposed on F0. In both b, c, black lines are superimposed on the length trace to better appreciate the slope. d Frequency distribution of F0. Data are plotted in classes of 2 pN and fitted with a Gaussian (continuous line): centre = 15.5 ± 0.4 pN, σ = 4.7 ± 0.4 pN (mean ± SEM, n = 46). e Force–velocity (F–V) relation. The black filled symbol on the ordinate is the Vf value from IVMA (see related Supplementary Fig. 4). The dashed line is Hill’s hyperbolic fit to the data. f Power–force (P–F) relation from the fit in e
