Fluorescence-Based Assays for Microtubule Architecture - Methods in Cell Biology - page 335

A B

β

13-pf MT

}

3-Start

β

α

11-pf 13-pf 16-pf

α

“Seam”

C D

θ

13

1

FIGURE 21.1
Microtubule architecture. (A) Schematic of a 13-pf microtubule showing the ab-tubulin
dimer, the 3-start helix, and the “seam.” (B) Cross-sections of electron density maps for three
different pf numbers (11-pf, EMD-5191; 13-pf, EMD-5193; and 16-pf, EMD-5196). (C)
Schematic representation of the 13-pf lattice, viewed from the outside of the microtubule,
indicating the left-handed helix. Dotted rows are used to indicate the 3-start helix. Note the
horizontal line shows that one dotted row ends where another begins.(D) Schematic
representation of a 14-pf lattice. The lattice was rotated by a small angle (

y ) such that the

dotted rows retained their alignment. This angle is the supertwist.
Panels C and D adapted from Chretien and Wade (1991)

bonds may account for the unique mechanical properties of microtubules ( Sui &
Downing, 2010 ).
A consequence of changing the pf number is a change in the pitch and supertwist
of the microtubule lattice. Tubulin dimers associate laterally in a “B-lattice” config-
uration, in which b -tubulins bind laterally to b - and a -tubulins bind to a -tubulins
( Fig. 21.1 A). The B-lattice bond has a small longitudinal offset, however, which
gives the microtubule lattice a helical pitch. The helix is described by its monomer
repeat; a 13-pf microtubule, for example, has a “3-start” helix based on a 3-monomer
repeat ( Fig. 21.1 A, labeled). The 3-start helix creates a discontinuity in the B-lattice,
in which one b -tubulin binds laterally to an a -tubulin, a feature known as the “seam”