Essential Cell Biology 5th edition

700 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
Figure 20–15 An integrin protein switches
to an active conformation when it binds
to molecules on either side of the plasma
membrane. An integrin protein consists
of two different subunits, α (green) and β
(blue), both of which can switch between
a folded, inactive form and an extended,
active form. The switch to the activated
state can be triggered by binding to an
extracellular matrix molecule (such as
fibronectin) or to intracellular adaptor
proteins that then link the integrin to the
cytoskeleton (see Figure 20–14). In both
cases, the conformational change alters
the integrin so that its opposite end
rapidly forms a counterbalancing
attachment to the appropriate structure.
In this way, the integrin establishes a
reversible mechanical linkage across the
plasma membrane. (Based on T. Xiao et al.,
Nature 432:59–67, 2004.)
plasma
membrane
inactive
integrin
CYTOSOL
α subunit
β subunit
BINDING TO
EXTRACELLULAR
MATRIX
BINDING TO
CYTOSKELETON
strong binding to extracellular matrix
(e.g., to collagen via fibronectin)
active
integrin
Gels of Polysaccharides and Proteins Fill Spaces and
Resist Compression
While collagen provides tensile strength to resist stretching, a completely
different group of macromolecules in the extracellular matrix of animal
tissues provides the complementary ECB5 function, e20.16/20.16 resisting compression.
These are the glycosaminoglycans (GAGs), negatively charged polysaccharide
chains made of repeating disaccharide units (Figure 20–16).
Chains of GAGs are usually covalently linked to a core protein to form
proteoglycans, which are extremely diverse in size, shape, and chemistry.
Typically, many GAG chains are attached to a single core protein
that may, in turn, be linked to another GAG, creating a macromolecule
that resembles a bottlebrush. Aggrecan in cartilage, for example, is one
of the most abundant proteoglycans; it has more than 100 GAG chains on
a single core protein, and it interacts extracellularly with another GAG,
hyaluronan (see Figure 20−16), creating an enormous aggregate with a
molecular weight in the millions (Figure 20–17).
5 nm
strong binding to cytoskeleton
(e.g., to actin via adaptor proteins)
In dense, compact connective tissues such as tendon and bone, the
proportion of GAGs is small, and the matrix consists almost entirely of
collagen (or, in the case of bone, of collagen plus calcium phosphate crystals).
At the other extreme, the jellylike substance in the interior of the eye
consists almost entirely of one particular type of GAG, plus water, with
only a small amount of collagen. In general, GAGs are strongly hydrophilic
and tend to adopt highly extended conformations, which occupy a
huge volume relative to their mass (see Figure 20–17). Thus GAGs act as
effective “space fillers” in the extracellular matrix of connective tissues.
CH 2 OH
O
Figure 20–16 Glycosaminoglycans (GAGs)
are built from repeating disaccharide
units. Hyaluronan, a relatively simple GAG,
is shown here. It consists of a single long
chain of up to 25,000 repeated disaccharide
units, each carrying a negative charge
(red). As in other GAGs, one of the sugar
monomers (green) in each disaccharide
unit is an amino sugar. Many GAGs have
additional negative charges, often from
sulfate groups (not shown).
COO
OH
O O N-acetylglucosamine
CH 2 OH
COO HO
O
O O
NHCOCH 3
OH
O OH
HO
glucuronic acid
repeating disaccharide
OH
NHCOCH 3