Figure S2.

Plus-end preference is not an artifact of differential labeling. (A) Polarity-marked microtubules (magenta), polymerized from bright seeds with dim extensions on both plus and minus ends, were nearly always captured via their plus ends by assembled Ndc80-GFP kinetochores (cyan). To polymerize dim extensions from both ends of bright seeds, NEM-treated tubulin was omitted from the polymerization mix (see Materials and methods). Polymerization at plus ends is faster than at minus ends, so plus ends were distinguishable by their longer lengths relative to minus ends. (B) Percentages of tip-captured, polarity-marked microtubules that were bound by their plus ends. A strong preference for plus ends occurred irrespective of whether the minus ends were more brightly labeled, via polymerization with a small amount of NEM-treated tubulin (with NEM, at left), or whether the minus and plus ends were both dimly labeled, via polymerization without NEM-treated tubulin (without NEM, at right). See Materials and methods for details about how polarity-marked microtubules were generated. Bars represent percentages of polarity-marked microtubules that were bound by their plus ends (mean ± SD, from N = 4 experiments with NEM and N = 4 without NEM, examining a total of 86 and 110 tip-captured microtubules, respectively).