Essential Cell Biology 5th edition

Epithelial Sheets and Cell Junctions
707
hemidesmosome
plaque of
linker proteins
keratin filaments
integrin proteins
CYTOSOL
basal plasma
membrane of
epithelial cell
basal
lamina
Figure 20–27 Hemidesmosomes anchor
the keratin filaments in an epithelial
cell to the basal lamina. The linkage is
mediated by a transmembrane attachment
complex containing integrins, rather than
cadherins.
domains of these integrins ECB5 bind e20.28/20.28
to laminin in the basal lamina; inside
the cell, the integrin tails are bound via linker proteins to keratin filaments,
creating a structure that looks superficially like half a desmosome.
These attachments of epithelial cells to the basal lamina beneath them
are therefore called hemidesmosomes (Figure 20–27).
Gap Junctions Allow Cytosolic Inorganic Ions and Small
Molecules to Pass from Cell to Cell
The final type of epithelial cell junction, found in virtually all epithelia
and in many other types of animal tissues, serves a totally different purpose
from the junctions discussed so far. In the electron microscope, gap
junctions appear as regions where the plasma membranes of two cells
lie close together and exactly parallel, with a very narrow gap of 2–4 nm
between them. The gap, however, is not entirely empty; it is spanned
by the protruding ends of many identical, transmembrane protein complexes
that reside in the plasma membranes of the two apposed cells.
These complexes, called connexons, are aligned end-to-end to form narrow,
water-filled channels across the interacting membranes (Figure
20–28). The channels allow inorganic ions and small, water-soluble molecules
(up to a molecular mass of about 1000 daltons) to move directly
from the cytosol of one cell to the cytosol of the other. This flow creates
an electrical and a metabolic coupling between the cells. Gap junctions
between cardiac muscle cells, for example, provide the electrical coupling
that allows electrical waves of excitation to spread synchronously
through the heart, triggering the coordinated contraction of the cells that
produces each heart beat.
Gap junctions in many tissues can be opened or closed in response to
extracellular or intracellular signals. The neurotransmitter dopamine, for
(A)
CELL 1
CELL 2
gap junction
100 nm
gap of
2–4 nm
two connexons in
register forming a
cytosolic channel
between adjacent cells
(B)
interacting plasma membranes
of cells 1 and 2
channel
1.5 nm in
diameter
connexon
composed of
six protein
subunits
QUESTION 20–4
Analogs of hemidesmosomes are
the focal contacts described in
Chapter 17, which are also sites
where the cell attaches to the
extracellular matrix. These junctions
are prevalent in fibroblasts but
largely absent in epithelial cells. On
the other hand, hemidesmosomes
are prevalent in epithelial cells but
absent in fibroblasts. In focal contact
sites, intracellular connections are
made to actin filaments, whereas,
in hemidesmosomes, connections
are made to intermediate filaments.
Why do you suppose these two
different cell types attach differently
to the extracellular matrix?
Figure 20–28 Gap junctions provide
neighboring cells with a direct channel of
intercytosolic communication. (A) Electron
micrograph of a gap junction between
two cells in culture. (B) A model of a gap
junction. The drawing shows the interacting
plasma membranes of two adjacent cells.
The apposed membranes are penetrated
by protein assemblies called connexons
(green), each of which is formed from six
identical protein subunits. Two connexons
join across the intercellular gap to form an
aqueous channel connecting the cytosols
of the two cells. (A, from N.B. Gilula, in Cell
Communication [R.P. Cox, ed.], pp. 1–29.
New York: Wiley, 1974. With permission
from John Wiley & Sons, Inc.)