Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

926 Chapter 16: The Cytoskeleton
Microtubules Are Hollow Tubes Made of Protofilaments
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
β-tubulin
GTP
β
α
tubulin heterodimer
(= microtubule subunit)
protofilament
lumen
(E)
10 nm
plus
end
50 nm
minus
end
(A)
α-tubulin
(B)
(C)
microtubule
(D)
50 nm
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 α- and 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; MBoC6 E, courtesy m16.11/16.42 of Richard Linck.)