162
chapter 10
Heteropolysaccharides I: Glycoproteins and Glycolipids
Fibrin Collagen
Cells Heparin Fibrin
F I G U R E 1 0 -9
Schematic representation of a fibronectin molecule. It is a dimer of similar
subunits (M.W. of each ~ 250,000) joined by a pair of disulfide linkages.
The various functional domains by which fibronectin can interact with
other protein and membrane components are indicated.
been altered. Reduced adhesion of cancer cells to a matrix
has been related to reduced synthesis of fibronectin and
collagen.
Fibronectin,
a glycoprotein abundant on the cell sur-
face of normal cells, promotes that attachment and sub-
sequent spreading of many cell types. Known also as
a cell surface protein, fibronectin is a large, external,
transformation-sensitive protein that binds to a num-
ber of substances (e.g., collagen and fibrin). The name
derives from Latin
fibra
(“fiber”) and Latin
nectere
(“to tie”). Two types of fibronectin are recognized: cell
surface fibronectin, which occurs as dimers and mul-
timers, and plasma fibronectin, which occurs primar-
ily as dimers. Fibronectin is a multifunctional molecule
(Figure 10-9) containing regions that recognize glyco-
conjugates (e.g., glycolipids) of the cell surface, pro-
teoglycans, and collagen fibers. A schematic model of
an adhesion site is shown in Figure 10-10. Fibronectin
contains about
5%
carbohydrate by weight. Plasma fi-
bronectin plays several roles in wound repair: in the
formation of a fibrin clot as cross-linked fibronectin,
in some reactions of platelets, in the enhancement of
the opsonic activity of macrophages (important for re-
moval of foreign material and necrotic tissue), and in
attracting fibroblasts (which participate in the produc-
tion of repair components such as collagen and pro-
teoglycans in the extracellular matrix). A unique form
of fibronectin, known as
fetal fibronectin,
is found in
the extracellular matrix surrounding the extravillous tro-
phoblast at the uteroplacental junction. The presence
of fetal fibronectin in cervicovaginal secretions may be
used as a marker in assessing the risk for preterm deliv-
ery. It is measured by enzyme immunoassay procedures
(Chapter
8
).
In some cell types, fibronectin is not involved in
cell adhesion. For example, the extracellular matrix ad-
jacent to epithelial cells and chondrocytes does not
contain fibronectin but rather two other glycoproteins,
laminin
and
chondronectin.
Laminin mediates in adhe-
sion of epithelial cells, whereas chondronectin mediates
F I G U R E 1 0 -1 0
Fibronectin’s role in adhesion of a cell with its extracellular collagenous matrix. Fibronectin, through one of its sites (see
Figure 10-9), binds to specific regions of collagen fibers. At another site, fibronectin binds to a cell surface receptor
protein complex (140K complex) that interacts with the actin filaments inside the cell. [Reproduced with permission
from R. O. Hynes: Fibronectins.
Sci. A m .
2 5 4 ,42 June 1986. ©1986 by Scientific American, Inc., All rights reserved.]
