Essential Cell Biology 5th edition

The Shape and Structure of Proteins
135
Figure 4−27 A single type of protein subunit can pack together
to form a filament, a hollow tube, or a spherical shell. Actin
subunits, for example, form actin filaments (see Figure 4–26), whereas
tubulin subunits form hollow microtubules, and some virus proteins
form a spherical shell (capsid) that encloses the viral genome
(see Figure 4−28).
spherical
shell
One large class of intracellular fibrous proteins resembles α-keratin,
which we met earlier when we introduced the α helix. Keratin filaments
are extremely stable: long-lived structures such as hair, horns, and nails
are composed mainly of this protein. An α-keratin molecule is a dimer
of two identical subunits, with the long α helices of each subunit forming
a coiled-coil (see Figure 4−16). These coiled-coil regions are capped
at either end by globular domains containing binding sites that allow
them to assemble into ropelike intermediate filaments—a component
of the cytoskeleton that gives cells mechanical strength (discussed in
Chapter 17).
filament
subunit
hollow
tube
Fibrous proteins are especially abundant outside the cell, where they form
the gel-like extracellular matrix that helps bind cells together to form tissues.
These proteins are secreted by the cells into their surroundings,
where they often assemble into sheets or long fibrils. Collagen is the most
abundant of these fibrous extracellular proteins in animal tissues. A collagen
molecule consists of three long polypeptide chains, each containing
the nonpolar amino acid glycine at every third position. This regular structure
allows the chains to wind around one another to generate a long,
regular, triple helix with glycine at its core (Figure 4−29A). Many such
collagen molecules bind to one another, side-by-side and end-to-end, to
create long, overlapping arrays called collagen fibrils, which are extremely
strong and help hold tissues together, as described in Chapter 20.
ECB5 e4.27/4.26
In complete contrast to collagen is another fibrous protein in the extracellular
matrix, elastin. Elastin molecules are formed from relatively loose
and unstructured polypeptide chains that are covalently cross-linked into
a rubberlike elastic meshwork. The resulting elastic fibers enable skin and
other tissues, such as arteries and lungs, to stretch and recoil without
tearing. As illustrated in Figure 4−29B, the elasticity is due to the ability
of the individual protein molecules to uncoil reversibly whenever they
are stretched.
Extracellular Proteins Are Often Stabilized by Covalent
Cross-Linkages
Many protein molecules are attached to the surface of a cell’s plasma
membrane or secreted as part of the extracellular matrix, which exposes
them to the potentially harsh conditions outside the cell. To help maintain
their structures, the polypeptide chains in such proteins are often stabilized
by covalent cross-linkages. These linkages can either tie together
two amino acids in the same polypeptide chain or join together many
polypeptide chains in a large protein complex—as for the collagen fibrils
and elastic fibers just described. A variety of different types of cross-links
exist.
Figure 4−28 Many viral capsids are essentially spherical protein
assemblies. They are formed from many copies of a small set of
protein subunits. The nucleic acid of the virus (DNA or RNA) is
packaged inside. The structure of the simian virus SV40, shown here,
was determined by x-ray crystallography and is known in atomic detail.
(Courtesy of Robert Grant, Stephan Crainic, and James M. Hogle.)
20 nm