(A) Kymograph showing acceleration of microtubule growth with increasing length at 40 nM Kip2. (B) Porcine microtubule growth rate as a function of length without Kip2 (black) and binned for 1–2 nM Kip2 (purple), 5–10 nM Kip2 (blue) and 20–40 nM Kip2 (green). Lengths are binned for 0–2 μm, 2–3 μm, 3–4 μm, 4–6 μm, 6–8 μm, 8–12 μm, 12–16 μm and 16–24 μm. The data were fit with the equation, where v0 = 0.294 ± 0.009 μm/min is the initial growth rate; vmax = 1.03 ± 0.03 μm/min is the maximum growth rate; L is microtubule length and A = 39.8 ± 5.4 μm·nM. (C) Mean catastrophe length at various Kip2 concentrations for short (light gray) and long (dark gray) microtubules. In the short microtubule bins, we summed the total distance that microtubules grew while shorter than 4 μm and divided by the number of catastrophes that occurred at lengths < 4 μm. In the long microtubule bin, we summed the total distance that microtubules grew while longer than 4 μm and divided by the number of catastrophes that occurred at lengths > 4 μm (Figure 4C, inset). The number of catastrophes was 120 (0 nM Kip2), 102 (1 nM Kip2), 111 (2 nM Kip2) and 23 (5 nM Kip2). The number of catastrophes at higher Kip2 concentrations was too small to make statistically significant comparisons. Error bars are standard errors of the mean.