Figure 3. Stepping Behavior of Extended Kinesin Mutants(A) Kinesin motors were labeled with a single quantum dot on one head, and head movement was recorded with 70 ms integration time. Stepping traces of WT (blue), 6P (wine), 13P (black), 19P (cyan), 26P (navy), and 14GS (olive), fitted with a step detection program (red lines) (Kerssemakers et al., 2006). Motor velocity was kept in the range of 10–15 nm/s by adding different concentrations of ATP for each construct (1 μM for WT, 15 μM for 6P and 13P, 25 μM for 19P, 40 μM for 26P, and 100 μM for 14GS) (see Experimental Procedures for details). The inset shows the randomness parameter (r) calculated from the dwell time measurements and suggests a coupling defect for extended kinesins (mean ± SD).(B) A histogram of analyzed step sizes along the microtubule axis shows that the WT head moves with a consistent 16 nm step, whereas the head of the neck mutants takes highly variable steps that show peaks at multiples of ∼8 nm. The number of analyzed steps (Nstep) and probability for each construct to take a backward step (pBW) are indicated in each panel.(C) Available binding sites (black circles) for the rear head in the microtubule plus end direction.(D), (E), and (F) Traces showing motor head displacement parallel (black trace) and perpendicular (blue trace) to the long microtubule axis for WT, 14GS, and 26P, respectively. Vertical dotted lines indicate diagonal steps. The inserts show histograms of sideways step sizes, separated into left and right directions. The overall probability of taking a sideways step (psw) is shown on each panel.