FIGURE 18-3 Structure of tubulin dimers and their organization into microtubules. (a) Ribbon diagram of the tubulin dimer. The GTP bound to the α-tubulin monomer is nonexchangeable, whereas the GDP bound to the β-tubulin monomer is exchangeable with free GTP. (b) The organization of tubulin subunits in a microtubule. The dimers are aligned end to end into protofilaments, which pack side by side to form the wall of the microtubule. The protofilaments are slightly staggered so that α-tubulin in one protofilament is in contact with α-tubulin in the neighboring protofilaments, except at the seam, where an α-subunit contacts a β-subunit. The microtubule displays a structural polarity in that subunits are added preferentially at the end where β-tubulin monomers are exposed. This end of the microtubule is known as the (+) end.

[Part (a) data from E. Nogales et al., 1998, Nature 391:199, PDB 1D 1tub.]