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

THE EXTRACELLULAR MATRIX OF ANIMALS
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Figure 19–44 Elastic fibers. These
scanning electron micrographs show
(A) a low-power view of a segment of a
dog’s aorta and (B) a high-power view
of the dense network of longitudinally
oriented elastic fibers in the outer layer
of the same blood vessel. All the other
components have been digested away with
enzymes and formic acid. (From K.S. Haas
et al., Anat. Rec. 230:86–96, 1991. With
permission from Wiley-Liss.)
(A)
1 mm
(B)
100 µm
Elastin Gives Tissues Their Elasticity
Many vertebrate tissues, such as skin, blood vessels, and lungs, need to be both
strong and elastic in order to function. A network of elastic fibers in the extracellular
matrix of these tissues gives them the resilience to recoil after transient
stretch (Figure 19–44). Elastic fibers are at least five times more extensible than a
rubber band of the same cross-sectional area. Long, inelastic collagen fibrils are
interwoven with the elastic fibers
MBoC6
to
m19.70/19.45
limit the extent of stretching and prevent the
tissue from tearing.
The main component of elastic fibers is elastin, a highly hydrophobic protein
(about 750 amino acids long), which, like collagen, is unusually rich in proline and
glycine but, unlike collagen, is not glycosylated. Soluble tropoelastin (the biosynthetic
precursor of elastin) is secreted into the extracellular space and assembled
into elastic fibers close to the plasma membrane, generally in cell-surface infoldings.
After secretion, the tropoelastin molecules become highly cross-linked to
one another, generating an extensive network of elastin fibers and sheets.
The elastin protein is composed largely of two types of short segments that
alternate along the polypeptide chain: hydrophobic segments, which are responsible
for the elastic properties of the molecule; and alanine- and lysine-rich α-helical
segments, which are cross-linked to adjacent molecules by covalent attachment
of lysine residues. Each segment is encoded by a separate exon. There is
still uncertainty concerning the conformation of elastin molecules in elastic fibers
and how the structure of these fibers accounts for their rubberlike properties.
However, it seems that parts of the elastin polypeptide chain, like the polymer
chains in ordinary rubber, adopt a loose “random coil” conformation, and it is the
random coil nature of the component molecules cross-linked into the elastic fiber
network that allows the network to stretch and recoil like a rubber band (Figure
19–45).
Elastin is the dominant extracellular matrix protein in arteries, comprising
50% of the dry weight of the largest artery—the aorta (see Figure 19–44). Mutations
in the elastin gene causing a deficiency of the protein in mice or humans
result in narrowing of the aorta and other arteries and excessive proliferation of
smooth muscle cells in the arterial wall. Apparently, the normal elasticity of an
artery is required to restrain the proliferation of these cells.
Elastic fibers do not consist solely of elastin. The elastin core is covered with a
sheath of microfibrils, each of which has a diameter of about 10 nm. The microfibrils
appear before elastin in developing tissues and seem to provide scaffolding
to guide elastin deposition. Arrays of microfibrils are elastic in their own right,
and in some places they persist in the absence of elastin: they help to hold the
lens in its place in the eye, for example. Microfibrils are composed of a number
of distinct glycoproteins, including the large glycoprotein fibrillin, which binds to