XMAP215 does not decrease the GTP-cap size. (A) Mean EB1 comet length as a function of microtubule growth rate over a range of XMAP215 concentrations (0–50 nM) in the presence of 20 µM tubulin and 200 nM EB1-GFP. Dim points represent individual 30-s growth segments. Error bars, 95% CI. Bold points are weighted means; error bars, weighted error. For −XMAP215 condition (0 nM XMAP215), all individual segments from a single experiment were averaged. For +XMAP215 condition (3.13–50 nM XMAP215), individual segments were binned using growth rate into 25-nm/s bins and averaged. 20 microtubule kymographs were analyzed from each condition. A total of six experimental conditions were performed over 2 d. (B) Microtubules were polymerized with either 60 µM tubulin and 200 nM EB1-GFP (condition I) or 20 µM tubulin, 200 nM EB1-GFP, and 25 nM XMAP215 (condition II) to obtain growth rate–matched conditions. Left: 69 growth segments for condition I and 53 growth segments for condition II, with no significant difference in growth rate (P = 0.28, unpaired t test), were analyzed. Means and SD are shown. Right: Representative kymographs of EB1 localization. (C) The super-average EB1 comet profiles were fitted to an exponential decay (dark lines) to determine the average comet lengths (see Materials and methods). Error, 95% CI of the fit.