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
1061
1 µm aggrecan aggregate
core protein
(aggrecan)
link protein
hyaluronan
molecule
keratan
sulfate
chondroitin sulfate
(B)
(A)
fibroblasts, syndecans can be found in cell–matrix adhesions, where they modulate
integrin function by interacting with fibronectin on the cell surface and with
cytoskeletal and signaling proteins inside the cell. As we discuss later, syndecan
and other proteoglycans also interact with soluble peptide growth factors, influencing
their effects on cell growth and proliferation.
Collagens Are the Major Proteins of the Extracellular Matrix
The collagens are a family of fibrous proteins found in all multicellular animals.
MBoC6 m19.60/19.38
They are secreted in large quantities by connective-tissue cells, and in smaller
quantities by many other cell types. As a major component of skin and bone, collagens
are the most abundant proteins in mammals, where they constitute 25% of
the total protein mass.
The primary feature of a typical collagen molecule is its long, stiff, triple-stranded
helical structure, in which three collagen polypeptide chains, called
α chains, are wound around one another in a ropelike superhelix (Figure 19–39).
1 µm
Figure 19–37 An aggrecan aggregate
from fetal bovine cartilage. (A) An
electron micrograph of an aggrecan
aggregate shadowed with platinum. Many
free aggrecan molecules are also visible.
(B) A drawing of the giant aggrecan
aggregate shown in (A). It consists of about
100 aggrecan monomers (each like the
one shown in Figure 19–36) noncovalently
bound through the N-terminal domain of
the core protein to a single hyaluronan
chain. A link protein binds both to the core
protein of the proteoglycan and to the
hyaluronan chain, thereby stabilizing the
aggregate. The link proteins are members
of a family of hyaluronan-binding proteins,
some of which are cell-surface proteins.
The molecular mass of such a complex can
be 10 8 daltons or more, and it occupies a
volume equivalent to that of a bacterium,
which is about 2 × 10 –12 cm 3 . (A, courtesy
of Lawrence Rosenberg.)
collagen
fibril
0.5 µm
Figure 19–38 Proteoglycans in the
extracellular matrix of rat cartilage.
The tissue was rapidly frozen at –196°C,
and fixed and stained while still frozen
(a process called freeze substitution) to
prevent the GAG chains from collapsing. In
this electron micrograph, the proteoglycan
molecules are seen to form a fine
filamentous network in which a single
striated collagen fibril is embedded.
The more darkly stained parts of the
proteoglycan molecules are the core
proteins; the faintly stained threads are
the GAG chains. (Reproduced from
E.B. Hunziker and R.K. Schenk, J. Cell
Biol. 98:277–282, 1984. With permission
from The Rockefeller University Press.)