Structural basis of MAP stabilization of the MT lattice. (A and B) View of the inside surface of the 8.2 Å cryo-EM map (violet surface) of B-lattice contacts between four dimers in DCX–MTs. The paclitaxel binding pocket in β-tubulin is empty (dotted circle; Fig. S3 A), whereas the equivalent area in α-tubulin is occupied by loop S9-S10 (density labeled with an asterisk; Fig. S3 A). The structures of α- and β-tubulin from zinc-induced sheets are fitted in the map (1JFF.pdb, α in blue and β in cyan). However, for α-tubulin, the N and M loops from a tubulin–stathmin complex match the reconstruction better (3HKE.pdb, pink; Dorléans et al., 2009). Arrows in A indicate equivalent H10-H7 contacts at the intradimer interfaces. (C) The reconstruction is well defined around the nucleotide (ball and stick) bound to β-tubulin, particularly loop T7. There is continuous density between α₁-H10 and β₄-H7 at the interdimer interface (also see Video 1), which is consistent with the straight conformation of β₄-tubulin but is incompatible with a curved conformation (Fig. S3 B). (D and E) In the asymmetric reconstruction, these lateral contacts are present at both the B lattice (D) and A lattice (E) contacts, despite the absence of DCX on the outer surface of the seam (arrow).