Figure 4.

Slip events. (a and b) Typical slip events and backward steps of a Kip3 motor under a 3 pN hindering force. (a) Time trace of force (raw data (light grey line), boxcar filtered to 400 Hz (dark grey line) and 100 Hz (magenta line)). The start and end of the large slip event are marked with green lines. The dashed green line shows the mean force $F_{\text{mean}}$ during that slip. The threshold force for detecting slips in the raw data is marked with a blue horizontal line. (b) Time trace of microsphere position. We determined the slip time τ_s and distance x_s based on the rise time of the force spike. After a slip, kinesin-8 made regular 8 nm (here, backward) steps. (c) Histogram of slip distance x_s of all observed slip events (Kip3 (magenta bars): 1402 events, Kif18A (black bars): 475 events). The mean of the Gaussian fits was 16.5 ± 0.2, 23.6 ± 0.3, and 31.9 ± 0.2 nm for Kip3 (magenta line), and 16.0 ± 0.3, 24.1 ± 0.2, and 32.4 ± 0.6 nm for Kif18A (grey line), corresponding to multiples of ≈8 nm. Slips of 8 nm cannot be distinguished from 8 nm steps and therefore could not be detected.