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

MICROTUBULES
945
(A)
NH 2
COOH
α-helical region of monomer
(B)
NH 2
NH 2
coiled-coil dimer
48 nm
COOH
COOH
0.1 µm
(C)
NH 2
NH 2
COOH
COOH
COOH
COOH
NH 2
NH 2
staggered tetramer of two coiled-coil dimers
lateral association of 8 tetramers
(D)
(E)
addition of 8 tetramers to growing filament
Figure 16–67 A model of intermediate filament construction. The monomer shown in (A) pairs with another monomer to
form a dimer (B), in which the conserved central rod domains are aligned in parallel and wound together into a coiled-coil.
(C) Two dimers then line up side by side to form an antiparallel tetramer of four polypeptide chains. Dimers and tetramers are
the soluble subunits of intermediate filaments. (D) Within each tetramer, the two dimers are offset with respect to one another,
thereby allowing it to associate with another tetramer. (E) In the final 10-nm ropelike filament, tetramers are packed together in a
helical array, which has 16 dimers (32 coiled-coils) in cross section. Half of these dimers are pointing in each direction. An electron
micrograph of intermediate filaments is shown on the upper left (Movie 16.12). (Electron micrograph courtesy of Roy Quinlan.)
Intermediate Filament Structure Depends on the Lateral Bundling
and Twisting of Coiled-Coils
Although their amino- and carboxy-terminal domains differ, all intermediate filament
family members are elongated proteins with a conserved central α-helical
domain containing 40 or so heptad repeat motifs that form an extended coiledcoil
structure with another monomer (see Figure MBoC6 3–9). m16.19/16.69 A pair of parallel dimers
then associates in an antiparallel fashion to form a staggered tetramer (Figure
16–67). Unlike actin or tubulin subunits, intermediate filament subunits do not
contain a binding site for a nucleotide. Furthermore, since the tetrameric subunit
is made up of two dimers pointing in opposite directions, its two ends are the
same. The assembled intermediate filament therefore lacks the overall structural
polarity that is critical for actin filaments and microtubules. The tetramers pack
together laterally to form the filament, which includes eight parallel protofilaments
made up of tetramers. Each individual intermediate filament therefore has
a cross section of 32 individual α-helical coils. This large number of polypeptides
all lined up together, with the strong lateral hydrophobic interactions typical of
coiled-coil proteins, gives intermediate filaments a ropelike character. They can
be easily bent, with a persistence length of less than one micrometer (compared