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

1038 Chapter 19: Cell Junctions and the Extracellular Matrix
There are some exceptions to these rules. Some integrins, for example, mediate
cell–cell rather than cell–matrix attachment. Moreover, there are other types
of cell adhesion molecules that can provide transient cell–cell attachments more
flimsy than anchoring junctions, but sufficient to stick cells together in special
circumstances.
We begin the chapter with a discussion of the major forms of cell–cell junctions.
We then consider in turn the extracellular matrix of animals, the structure
and function of integrin-mediated cell–matrix junctions, and, finally, the plant
cell wall, a special form of extracellular matrix.
CELL–CELL JUNCTIONS
Cell–cell junctions come in many forms and can be regulated by a variety of
mechanisms. The best understood and most common are the two types of cell–
cell anchoring junctions, which employ cadherins to link the cytoskeleton of one
cell with that of its neighbor. Their primary function is to resist the external forces
that pull cells apart. The epithelial cells of your skin, for example, must remain
tightly linked when they are stretched, pinched, or poked. Cell–cell anchoring
junctions must also be dynamic and adaptable, so that they can be altered or rearranged
when tissues are remodeled or repaired, or when there are changes in the
forces acting on them.
In this section, we focus primarily on the cadherin-based anchoring junctions.
We then briefly describe tight junctions and gap junctions. Finally, we consider
the more transient cell–cell adhesion mechanisms employed by some cells in the
bloodstream.
Cadherins Form a Diverse Family of Adhesion Molecules
Cadherins are present in all multicellular animals whose genomes have been
analyzed. They are also present in the choanoflagellates, which can exist either as
free-living unicellular organisms or as multicellular colonies and are thought to
be representatives of the group of protists from which all animals evolved. Other
eukaryotes, including fungi and plants, lack cadherins, and they are also absent
from bacteria and archaea. Cadherins therefore seem to be part of the essence of
what it is to be an animal.
The cadherins take their name from their dependence on Ca 2+ ions: removing
Ca 2+ from the extracellular medium causes adhesions mediated by cadherins to
come apart. The first three cadherins to be discovered were named according to
the main tissues in which they were found: E-cadherin is present on many types
of epithelial cells; N-cadherin on nerve, muscle, and lens cells; and P-cadherin on
cells in the placenta and epidermis. All are also found in other tissues. These and
other classical cadherins are closely related in sequence throughout their extracellular
and intracellular domains.
There are also a large number of nonclassical cadherins that are more distantly
related in sequence, with more than 50 expressed in the brain alone. The
nonclassical cadherins include proteins with known adhesive function, such as
the diverse protocadherins found in the brain, and the desmocollins and desmogleins
that form desmosomes (see Table 19–1). Other family members are involved
primarily in signaling. Together, the classical and nonclassical cadherin proteins
constitute the cadherin superfamily (Figure 19–4), with more than 180 members
in humans.
Cadherins Mediate Homophilic Adhesion
Anchoring junctions between cells are usually symmetrical: if the linkage is to
actin in the cell on one side of the junction, it will be to actin in the cell on the
other side. In fact, the binding between cadherins is generally homophilic (liketo-like,
Figure 19–5): cadherin molecules of a specific subtype on one cell bind to
cadherin molecules of the same or closely related subtype on adjacent cells.