Figure 1: Harmonic force spectroscopy with human β-cardiac S1.
(a–c) An actin dumbbell held by two fixed optical traps interacts with a β-cardiac S1 molecule that is surface attached to the top of a platform bead on a sinusoidally translating piezoelectric stage. Upon binding, the motor strokes towards the right (blue arrows). Depending on whether the binding occurs left of the centre (red asterisk in a), at the centre (red asterix on grey dashed line in b) or right of the centre (red asterisk in c), the motor experiences backward (a), near zero (b) or forward (c) mean load F0. (d–f) Three examples of time traces of the displacement x1 of a dumbbell bead in its trap. The other bead has coordinate x2 (not shown). Large sinusoidal oscillations caused by being attached to the stage by a motor are observed in the middle portion of the traces. Small oscillations before and after attachments are caused by drag from buffer moving past the unattached dumbbell. (g–i) Change in phase shift ϕ1(t) relative to the unbound state (black lines, dashed black lines showing its time average) and amplitude Δx1(t) of oscillations (red lines, dashed red lines showing its time average obtained from longer stretches of unbound state times than shown) of bead positions in trap shown in d–f reveal binding and unbinding of motor (Methods). For each binding event, we determine the duration (ts=t2–t1) of the attached state, the mean load (F0) and the amplitude of load oscillations (ΔF) (Methods). (j–l) Data in d–f fitted with two harmonic functions, red for attached state and orange for unattached.
