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

CELL–CELL JUNCTIONS
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Figure 19–26 Connexin turnover at a gap junction. Cells were transfected
with a slightly modified connexin gene, coding for a connexin with a short
amino acid tag containing four cysteines in the sequence Cys-Cys-X-X-
Cys-Cys (where X denotes an arbitrary amino acid). This tetracysteine tag
can bind strongly to certain small fluorescent dye molecules, which can be
added to the culture medium and will readily enter cells by diffusing across
the plasma membrane. In the experiment shown, a green dye was added
first to label all the connexin molecules in the cells, and the cells were then
washed and incubated for 4 or 8 hours. At the end of this time, a red dye
was added to the medium and the cells were washed again and fixed.
Connexin molecules already present at the beginning of the experiment are
labeled green (and take up no red dye because their tetracysteine tags are
already saturated with green dye), while connexins synthesized subsequently,
during the 4- or 8-hour incubation, are labeled red. The fluorescence images
show gap junctions between pairs of cells treated in this way. The central
part of the gap-junction plaque is green, indicating that it consists of old
connexin molecules, while the periphery is red, indicating that it consists
of connexins synthesized during the previous 4 or 8 hours. The longer the
time of incubation, the smaller the green central patch of old molecules, and
the larger the peripheral ring of new molecules that have been recruited to
replace the old ones. (From G. Gaietta et al., Science 296:503–507, 2002.
With permission from AAAS.)
CROSS SECTIONS
4 h incubation
8 h incubation
EN-FACE VIEW
1 µm
leakage through them. But there is also evidence that in some circumstances they
can open and serve as channels for the release of small signal molecules.
In Plants, Plasmodesmata Perform Many of the Same Functions
as Gap Junctions
The tissues of a plant are organized on different principles from those of an animal.
This is because plant cells are imprisoned within tough cell walls composed
of an extracellular matrix rich in cellulose and other polysaccharides, as we discuss
later. The cell walls of adjacent cells are firmly cemented to those of their
neighbors, which eliminates the need for anchoring junctions to hold the cells
in place. But a need for direct cell–cell communication remains. Thus, plant cells
have only one class of intercellular junctions, plasmodesmata. Like gap junctions,
they directly connect the cytoplasms of adjacent cells.
In plants, the cell wall between a typical pair of adjacent cells is at least 0.1 μm
thick, and so a structure very different from a gap junction is required to mediate
communication across it. Plasmodesmata solve the problem. With a few specialized
exceptions, every living cell in a higher plant is connected to its living
neighbors by these structures, which form fine cytoplasmic channels through the
intervening cell walls. As shown in Figure 19–27A, the plasma membrane of one
cell is continuous with that of its neighbor at each plasmodesma, which connects
the cytoplasms of the two cells by a roughly cylindrical channel with a diameter
of 20–40 nm.
Running through the center of the channel in most plasmodesmata is a narrower
cylindrical structure, the desmotubule, which is continuous with elements
of the smooth endoplasmic reticulum (ER) in each of the connected cells (Figure
19–27B–D). Between the outside of the desmotubule and the inner face of
the cylindrical channel formed by plasma membrane is an annulus of cytosol
through which small molecules can pass from cell to cell. As each new cell wall
is assembled during the cytokinesis phase of cell division, plasmodesmata are
created within it. They form around elements of smooth ER that become trapped
across the developing cell plate (discussed in Chapter 17). They can also be
inserted de novo through preexisting cell walls, where they are commonly found
in dense clusters called pit fields. When no longer required, plasmodesmata can
be removed.
In spite of the radical difference in structure between plasmodesmata and gap
junctions, they seem to function in remarkably similar ways. Evidence obtained
by injecting tracer molecules of different sizes suggests that plasmodesmata allow
the passage of molecules with a molecular weight of less than about 800, which
8 h incubation
MBoC6 m19.36/19.27
2 µm