Fig. 2. Current model of microtubule structure at atomic resolution. The crystal structure of tubulin, represented as a ribbon model, is docked onto a low-resolution 3-D image of tubulin protofilaments. The figure shows a section through a microtubule; on the left is a view from inside the microtubule and on the right from one side of a protofilament. Lateral interactions between protofilaments occur through the M-loops on one side contacting the helix H3 in the adjacent protofilament. At the top of each monomer is a guanine-nucleotide-binding site; GTP (bound to -tubulin) and GDP (bound to ß-tubulin) is shown occupying sites at the interface between tubulin subunits. The nucleotide is contacted by loop T7 of the next subunit in the protofilament. Also shown (as a space-filling molecule) in a pocket on the inside surface of the ß-tubulin subunit is the microtubule-binding drug taxol. At the tubulin C-terminus, the last residues visible by electron microscopy (440 and ß437) are indicated by black circles; the conformations of the last 8-10 residues are unknown. Figure reproduced from Amos, 2000, with permission from Elsevier Science.