Microtubule structure at improved resolution

Microtubule architecture can vary with eukaryotic species, with different c ell types, and with the presence of stabilizing agents. For in vitro assemb led microtubules, the average number of protofilaments is reduced by the pr esence of sarcodictyin A, epothilone B, and eleutherobin (similarly to taxo l) but increased by taxotere, Assembly with a slowly hydrolyzable GTP analo gue GMPCPP is known to give 96% 14 protofilament microtubules. We have used electron cryomicroscopy and helical reconstruction techniques to obtain th ree-dimensional maps of taxotere and GMPCPP microtubules incorporating data to 14 Angstrom resolution. The dimer packing within the microtubule wall i s examined by docking the tubulin crystal structure into these improved mic rotubule maps. The docked tubulin and simulated images calculated from "ato mic resolution" microtubule models show tubulin heterodimers are aligned he ad to tail along the protofilaments with the beta subunit capping the micro tubule plus end. The relative positions of tubulin dimers in neighboring pr otofilaments are the same for both types of microtubule, confirming that co nserved lateral interactions between tubulin subunits are responsible for t he surface lattice accommodation observed for different microtubule archite ctures. Microtubules with unconventional protofilament numbers that exist i n vivo are likely to have the same surface lattice organizations found in v itro. A curved "GDP" tubulin conformation induced by stathmin-like proteins appears to weaken lateral contacts between tubulin subunits and could bloc k microtubule assembly or favor disassembly. We conclude that lateral conta cts between tubulin subunits in neighboring protofilaments have a decisive role for microtubule stability, rigidity, and architecture.