A microtubule is a hollow cylindrical structure built from 13 parallel protofilaments, each composed of αβ-tubulin heterodimers stacked head to tail and then folded into a tube (Figure 16–42B–D). Microtubule assembly generates two new types of protein–protein contacts. Along the longitudinal axis of the microtubule, the “top” of one β-tubulin molecule forms an interface with the “bottom” of the α-tubulin molecule in the adjacent heterodimer. This interface is very similar to the interface holding the α and β monomers together in the dimer subunit, and the binding energy is high. Perpendicular to these interactions, neighboring protofilaments form lateral contacts. In this dimension, the main lateral contacts are between monomers of the same type (α–α and β–β). As longitudinal and lateral contacts are repeated during assembly, a slight stagger in lateral contacts gives rise to the helical microtubule lattice. Because multiple contacts within the lattice hold most of the subunits in a microtubule in place, the addition and loss of subunits occurs almost exclusively at the microtubule ends (see Figure 16–5). These multiple contacts among subunits make microtubules stiff and difficult to bend. The persistence length of a microtubule is several millimeters, making microtubules the stiffest and straightest structural elements found in most animal cells.
The subunits in each protofilament in a microtubule all point in the same direction, and the protofilaments themselves are aligned in parallel (see Figure
Figure 16–42 The structure of a microtubule and its subunit. (A) The subunit of each protofilament is a tubulin heterodimer, formed from a tightly linked pair of αand β-tubulin monomers. The GTP molecule in the α-tubulin monomer is so tightly bound that it can be considered an integral part of the protein. The GTP molecule in the β-tubulin monomer, however, is less tightly bound and has an important role in filament dynamics. Both nucleotides are shown in red. (B) One tubulin subunit (αβ-heterodimer) and one protofilament are shown schematically. Each protofilament consists of many adjacent subunits with the same orientation. (C) The microtubule is a stiff hollow tube formed from 13 protofilaments aligned in parallel.
(D) A short segment of a microtubule viewed in an electron microscope. (E) Electron micrograph of a cross section of a microtubule showing a ring  of 13 distinct protofilaments. (D, courtesy of Richard Wade; E, courtesy of Richard Linck.)
Figure 16–43 The preferential growth of microtubules at the plus end.
Microtubules grow faster at one end than at the other. A stable bundle of microtubules obtained from the core of a cilium (called an axoneme) was incubated for a short time with tubulin subunits under polymerizing conditions. Microtubules grew fastest from the plus end of the microtubule bundle, the end at the top in this micrograph. (Courtesy of Gary Borisy.) 16–42). Therefore, the microtubule lattice itself has a distinct structural polarity, with α-tubulins exposed at the minus end and β-tubulins exposed at the plus end. As for actin filaments, the regular, parallel orientation of their subunits gives microtubules structural and dynamic polarity (Figure 16–43), with plus ends growing and shrinking more rapidly.