Essential Cell Biology 5th edition

576 CHAPTER 17 Cytoskeleton
(A)
NH 2
COOH
α-helical region of monomer
(B)
NH 2
NH 2
coiled-coil dimer
COOH
COOH
48 nm
0.1 µm
(C)
NH 2
COOH
NH 2
NH 2
COOH
NH 2
staggered antiparallel tetramer of two coiled-coil dimers
lateral association of 8 tetramers
(D)
(E)
addition of 8 tetramers to growing filament
Figure 17–4 Intermediate filaments are
like ropes made of long, twisted strands
of protein. (A) The intermediate filament
monomer consists of an α-helical central
rod domain shown with unstructured
terminal domains at either end (not shown).
The C-terminal end of the monomer is
marked in dark blue to make its position
within the assembled filament. (B) Pairs
of monomers associate to form a dimer,
and (C) two dimers then line up to form a
staggered, antiparallel tetramer.
(D) Tetramers can pack together into a
helical array containing eight tetramer
strands; (E) these in turn assemble into
the final ropelike intermediate filament.
These filaments can elongate by the
addition of tetramer arrays to either end.
An electron micrograph of intermediate
filaments is shown on the upper left.
(U. Aebi et al. Protoplasma 145:73–81, 1988.
With permission from Springer Science and
Business Media.)
end (Figure 17–4A). The rod domain consists of an extended α-helical
region that enables pairs of intermediate filament proteins to form stable
dimers by wrapping around each other in a coiled-coil configuration
(Figure 17–4B), as described in Chapter 4. Two of these coiled-coil dimers,
running in opposite directions, associate to form a staggered tetramer
(Figure 17–4C). These dimers and tetramers are the soluble subunits of
intermediate filaments. The tetramers associate with each other sideby-side
(Figure 17–4D) and then assemble to generate the final ropelike
intermediate ECB5 e17.04-17.04 filament (Figure 17–4E).
Because the two dimers point in opposite directions, both ends of the
tetramer are the same, as are the two ends of assembled intermediate
filaments; as we will see, this distinguishes these filaments from microtubules
and actin filaments, whose structural polarity is crucial for their
function. Almost all of the interactions between the intermediate filament
proteins depend on noncovalent bonding; it is the combined strength of
the overlapping lateral interactions along the length of the proteins that
gives intermediate filaments their great tensile strength.
The central rod domains of different intermediate filament proteins are all
similar in size and amino acid sequence, so that when they pack together
they always form filaments of similar diameter and internal structure. By
contrast, the terminal head and tail domains vary greatly in both size and
amino acid sequence from one type of intermediate filament protein to
another. These unstructured domains are exposed on the surface of the
filament, where they allow it to interact with specific components in the
cytoplasm.