Cryo-EM structure of a GMPCPP microtubule. (A) A low-pass–filtered GMPCPP microtubule seen from the outside with a 10-Å cutoff for the fair comparison with the GDP-taxol microtubule (Fig. S1). The plus end is up. α and β show the positions of α- and β-tubulins, respectively. The dashed orange circles and squares show the holes in the microtubule wall. The purple and cyan planes show sectioning planes for views in B and Fig. 3, respectively. Arrowheads indicate OCs. (B) Contour plot of a cross-section at 10-Å resolution observed from the plus end. Most of the α helices are well resolved as the local maxima of the densities. The arrowheads show OCs, and the dashed circle shows the low-density region at the middle of the tubulin monomers. Also see Figs. S1 and S2 and Video 1 for the comparison between GMPCPP and GDP-taxol microtubules. Bars, 2 nm.