Figure 1. Structure of microtubules. (a) Atomic structure of the -tubulin heterodimer shown as a ribbon diagram; helices in blue, sheet in green. Taxol and GDP (bound to -tubulin) and GTP (bound to -tubulin in a position equivalent to that of GDP in -tubulin) are shown as space-filling atoms. Each subunit has two globular domains separated by the core helix. (b) Assembly of heterodimers into longitudinal protofilaments, which in turn associate into sheets and microtubules. Side-to-side association of the protofilaments can produce an A-lattice (symmetrical for the 13-protofilament microtubule shown) or a B-lattice with an A-lattice-like seam, or a more mixed lattice. (c) Stabilization of the polymer by microtubule-associated proteins (MAPs). A widespread family, including tau, contains sequence repeats (coloured purple in the model shown here) that bind to the inside surface of a microtubule in a similar site to taxol (which is present in the solved atomic structure). Flanking regions (blue and grey) also help tau bind to tubulin. Tubulin's negatively-charged C-termini (small red projections) interact with positive charges on tau and other MAPs. The large red projections represent the negatively-charged projection domain of tau.

Figure 2. Vertebrate mitotic spindle in metaphase (chromosomes paired at the centre) and anaphase (chromosomes moving towards the poles). Each pole acts as the focus for a microtubule aster and contains a pair of centrioles surrounded by a cloud of protein. Microtubule minus ends are closer to the poles.

Figure 3. (a) Cross-section through a typical axoneme, viewed from the tip, i.e. from the plus ends of the microtubules. (b) Enlarged cross-section of one of the doublet microtubules. Nine doublet microtubules and a central core structure are linked together by a complex variety of accessory molecules.