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

1054 Chapter 19: Cell Junctions and the Extracellular Matrix
cytoplasm
smooth
endoplasmic
reticulum
desmotubule
plasma
membrane
endoplasmic
reticulum
cytosol
cell walls
of adjacent
plant cells
desmotubule
cell
wall
plasmodesmata
(A)
plasma membrane lining
plasmodesma, connecting
two adjacent cells
(B)
100 nm
Figure 19–27 Plasmodesmata. (A) The cytoplasmic channels of
plasmodesmata pierce the plant cell wall and connect cells in a plant
together. (B) Each plasmodesma is lined with plasma membrane that is
common to the two connected cells. It usually also contains a fine tubular
structure, the desmotubule, derived from smooth endoplasmic reticulum.
(C) Electron micrograph of a longitudinal section of a plasmodesma from
a water fern. The plasma membrane lines the pore and is continuous from
one cell to the next. Endoplasmic reticulum and its association with the
central desmotubule can also be seen. (D) A similar plasmodesma seen in
cross section. (C and D, from R. Overall, J. Wolfe and B.E.S. Gunning, in
Protoplasma 111, pp. 134–150. Heidelberg: Springer-Verlag, 1982.)
(C)
0.1 µm
is similar to the molecular-weight cutoff for gap junctions. As with gap junctions,
transport through plasmodesmata is regulated. Dye-injection experiments, for
example, show that there can be barriers to the movement of even low-molecularweight
molecules between certain cells, or groups of cells, that are connected by
apparently normal plasmodesmata; the mechanisms that restrict communication
in these cases are not understood.
(D)
cell
wall
desmotubule
plasma membrane
25 nm
Selectins Mediate Transient Cell–Cell Adhesions in the
Bloodstream
We now complete our overview of cell–cell junctions and adhesion by briefly
describing some of the more specialized adhesion MBoC6 m19.38/19.28 mechanisms used in some tissues.
In addition to those we have already discussed, at least three other superfamilies
of cell–cell adhesion proteins are important: the integrins, the selectins,
and the adhesive immunoglobulin (Ig) superfamily members. We shall discuss
integrins in more detail later: their main function is in cell–matrix adhesion, but a
few of them mediate cell–cell adhesion in specialized circumstances. Ca 2+ dependence
provides one simple way to distinguish among these classes of adhesion
proteins experimentally. Selectins, like cadherins and integrins, require Ca 2+ for
their adhesive function; Ig superfamily members do not.
Selectins are cell-surface carbohydrate-binding proteins (lectins) that mediate
a variety of transient cell–cell adhesion interactions in the bloodstream. Their
main role, in vertebrates at least, is in governing the traffic of white blood cells
into normal lymphoid organs and any inflamed tissues. White blood cells lead a
nomadic life, roving between the bloodstream and the tissues, and this necessitates
special adhesive behavior. The selectins control the binding of white blood
cells to the endothelial cells lining blood vessels, thereby enabling the blood cells
to migrate out of the bloodstream into a tissue.
Each selectin is a transmembrane protein with a conserved lectin domain that
binds to a specific oligosaccharide on another cell (Figure 19–28A). There are at
least three types: L-selectin on white blood cells, P-selectin on blood platelets and
on endothelial cells that have been locally activated by an inflammatory response,
and E-selectin on activated endothelial cells. In a lymphoid organ, such as a lymph