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

1048 Chapter 19: Cell Junctions and the Extracellular Matrix
tight
junction
plasma
membranes
of adjacent cells
intercellular
space
LUMEN
OF GUT
apical surface
glucose
glucose
Na + -driven
glucose transporter
LOW
HIGH
glucose
concentration
Figure 19–18 The role of tight junctions
in transcellular transport. For clarity, only
the tight junctions are shown. Transport
proteins are confined to different regions of
the plasma membrane in epithelial cells of
the small intestine. This segregation permits
a vectorial transfer of nutrients across the
epithelium from the gut lumen to the blood.
In the example shown, glucose is actively
transported into the cell by Na + -driven
glucose transporters at its apical surface,
and it leaves the cell through passive
glucose transporters in its basolateral
membrane. Tight junctions are thought
to confine the transport proteins to their
appropriate membrane domains by
acting as diffusion barriers, or “fences,”
within the lipid bilayer of the plasma
membrane; these junctions also block
the backflow of glucose from the basal
side of the epithelium into the gut lumen
(see Movie 11.2).
passive glucose
transporter
basolateral
surface
cell
1
cell
2
cell
3
basal lamina
EXTRACELLULAR
FLUID/CONNECTIVE
TISSUE
glucose
LOW
BLOOD
apical end of each cell in the epithelial sheet (Figure 19–20A and B). In conventional
electron micrographs, the outer leaflets of the two interacting plasma membranes
are tightly apposed where sealing strands are present (Figure 19–20C).
Each sealing strand is composed of a long row of transmembrane homophilic
adhesion proteins embedded in each of the two interacting plasma membranes.
The extracellular domains of these proteins adhere directly to one another to
MBoC6 m19.23/19.18
occlude the intercellular space (Figure 19–21).
The main transmembrane proteins forming these strands are the claudins,
which are essential for tight-junction formation and function. Mice that lack the
claudin-1 gene, for example, fail to make tight junctions between the cells in the
epidermal layer of the skin; as a result, the baby mice lose water rapidly by evaporation
through the skin and die within a day after birth. Conversely, if nonepithelial
cells such as fibroblasts are artificially caused to express claudin genes, they
cell
1
(A)
LUMEN
cell
2
cell
3
tracer
molecule
tight
junction
(B)
tight
junction
0.5 µm 0.5 µm
Figure 19–19 The role of tight junctions
in allowing epithelia to serve as barriers
to solute diffusion. (A) The drawing shows
how a small extracellular tracer molecule
added on one side of an epithelium is
prevented from crossing the epithelium by
the tight junctions that seal adjacent cells
together. Adherens junctions and other
cell junctions are not shown for clarity.
(B) Electron micrographs of cells in an
epithelium in which a small, extracellular,
electron-dense tracer molecule has been
added to either the apical side (on the left)
or the basolateral side (on the right). The
tight junction blocks passage of the tracer
in both directions. (B, courtesy of Daniel
Friend.)