Figure 19–25 Gap junctions. (a) a drawing of the interacting plasma membranes of two adjacent cells connected by gap junctions. Each lipid bilayer is shown as a pair of red sheets. protein assemblies called connexons (green), each of which is formed by six connexin subunits, penetrate the apposed lipid bilayers. two connexons join across the intercellular gap to form a continuous aqueous channel connecting the two cells. (B) the organization of connexins into connexons, and connexons into intercellular channels. the connexons can be homomeric or heteromeric, and the intercellular channels can be homotypic or heterotypic. (C) the high-resolution structure of a homomeric gap-junction channel, determined by x-ray crystallography of human connexin 26. In this view, we are looking down on the pore, formed from six connexin subunits. the structure illustrates the general features of the channel and suggests a pore size of about 1.4 nm, as predicted from studies  of gap-junction permeability with molecules of various sizes (see Figure 19–24).  (pDB code: 2ZW3.)
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 4or 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.) 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.