In contrast to tight and adherens junctions, gap junctions do not seal membranes together, nor do they restrict the passage of material between membranes. Rather, gap junctions are composed of arrays of small channels that permit small molecules to shuttle from one cell to another and thus directly link the interior of adjacent cells. Importantly, gap junctions allow electrical and metabolic coupling among cells because signals inititated in one cell can readily propagate to neighboring cells. In general, the upper limit for passage through gap junctions is roughly 1000 daltons (Da). Aside from ions, important examples of molecules that readily pass include cyclic AMP (329 Da), glucose-6-phosphate (259 Da) and nucleotides (250-300 Da).
Gap junctions are seen with an electron microscope as patches of varying size where the plasma membranes of neighboring cells are separated by a beautifully uniform gap, roughly 2-3 nm in width. The gap reflects areas of the two membranes that are connected by hexagonal tubes called connexons which form aqueous pores roughly 2 nm in diameter between the two cells. The major protein in purified preparations of gap junctions is connexin, which, when expressed in cells that normally do not have gap junctions, allows them to form. Different species of connexin are seen in different organisms and among different tissues within an organism. However, all connexins share a common structure of having four membrane-spanning domains. A connexon is formed from six connexin molecules which extend a uniform distance outside the cells. Alignment of connexons from each cell across the gap results in the formation of the pores which functionally define the gap junction.
Gap junctions are dynamic structures because connexons are able to open and close. Elevated intracellular calcium and low intracellular pH are established stimuli for rapid closing of connexons. This may of importance when one cell within a group becomes damaged - the idea is that closing the gap junctions in the damaged cell would effectively isolate that cell and prevent spreading of the injury.
Gap junctions are seen in virtually all cells that contact other cells in tissues, which of course includes pretty much all cells in the body. Some representative examples of their importance in physiology include:
- Electrical coupling: Gap junctions are abundant in cardiac and smooth muscle. Depolarization of one group of muscle cells rapidly spreads to adjacent cells, leading to well-coordinated contractions of those muscles.
- Metabolic coupling: Many hormones act by elevating intracellular concentrations of cyclic AMP, which initiates a signalling pathway inside the cell. Cyclic AMP readily passes through gap junctions and thus, hormonal stimulation of one cell can lead to signal propagation to a cluster of cells.
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Last updated on January 15, 1997