Ndc80 is not an autonomous end-binding complex. (A) Coomassie-stained SDS-PAGE showing the reconstituted S. cerevisiae Ndc80 complex. The Nuf2 subunit is fused to EGFP. (B) The Ndc80 complex was tested for its ability to cosediment with taxol-stabilized microtubules in the presence of increasing amounts of salt. The binding affinity curves plot averaged data from three independent experiments. Error bars represent standard error. (C) Kymographs demonstrating 50 nM Bim1-EGFP tip tracking during microtubule (MT) assembly (top) and faint association of 300 nM Ndc80–EGFP with the microtubule lattice (bottom). CH, calponin homology. Bars, 2 µm.