FIGURE 5.21 • Molecular structure of the macula adherens (desmosome). a. Electron micrograph of a macula adherens, showing the intermediate filaments (arrows) attaching into a dense, intracellular attachment plaque located on the cytoplasmic side of the plasma membrane. The intercellular space is also occupied by electron-dense material (arrowheads) containing desmocollins and desmogleins. The intercellular space above and below the macula adherens is not well defined because of extraction of the plasma membrane to show components of this structure. 40,000. (Courtesy of Dr. Ernst Kallenbach.) b. Schematic diagram showing the structure of a macula adherens. Note the intracellular attachment plaque with anchored intermediate filaments. The extracellular portions of desmocollins and desmogleins from opposing cells interact with each other in the localized area of the desmosome, forming the cadherin “zipper.”
A variety of methods are used to study structure and function of gap junctions.
Various procedures have been used to study gap junctions, including the injection of dyes and fluorescent or radiolabeled compounds and measuring the flow of an electric current between cells. In dye studies, a fluorescent dye is injected with a micropipette into one cell. After a short period, the dye can be readily visualized in immediately adjacent cells. Electrical conductance studies show that neighboring cells joined by gap junctions exhibit a low electrical resistance between them and current flow is high; therefore, gap junctions are also called low-resistance junctions.
Current molecular biology techniques allow for isolation of cDNA clones encoding a family of gap junction proteins (connexins) and expressing them in tissue culture cells. Connexins expressed in transfected cells produce gap junctions, which can be isolated and studied by molecular and biochemical methods as well as by the improved imaging techniques of electron crystallography and atomic force microscopy.
Gap junctions are formed by 12 subunits of the connexin protein family.
When viewed with the TEM, the gap junction appears as an area of contact between the plasma membranes of adjacent cells (Fig. 5.22a). High-resolution imaging techniques such as cryo-electron microscopy have been used to examine the structure of gap junctions. These studies reveal groups of tightly packed channels, each formed by two half-channels called connexons embedded in the facing membranes. These channels are represented by pairs of connexons that bridge the extracellular space between adjacent cells. The connexon in one cell membrane is precisely aligned to dock with a corresponding connexon on the membrane of an adjacent cell, thus, as the name implies, allowing communication between the cells.
Each connexon contains six symmetrical subunits of an integral membrane protein called connexin (Cx) that is paired with a similar structure from the adjacent membrane. Therefore, the entire channel consists of 12 subunits. The subunits are configured in a circular arrangement to surround a 10-nm-long cylindrical transmembrane channel with a diameter of 2.8 nm (Fig. 5.22b).
About 21 members of the connexin family of proteins have been identified. All traverse the lipid bilayer four times (i.e., they have four transmembrane domains). Most connexons pair with identical connexons (homotypic interaction) on the adjacent plasma membrane. These channels allow molecules to pass evenly in both directions; however, heterotypic channels can be asymmetrical in function, passing certain molecules faster in one direction than in another.