Communication plays an important role in the rapidConnexin Structure progress of modern society. We live in an age when Gap junctions exhibit a hierarchy of assembly. The prininformation is transmitted through many different pathcipal structural component, the membrane protein conways and in many different forms to influence our daily nexin, is organized into the basic unit of structure, the decisions. However, although we live in communities, connexon, which is a hexameric structure with a toroid we still treasure our individuality. By analogy, the surappearance in negative-stained preparations. The famvival of multicellular organisms depends on each cell ily of connexin proteins includes at least 12 members in type retaining its individuality, even though all cellular rodents (Table 1; Kumar and Gilula, 1992). An individual activities must be coordinated with other cells. Organconnexon from one cell docks or associates with a corisms have evolved multiple strategies to achieve this responding connexon on a neighboring cell to form a goal, which include long-range interactions mediated gap junction channel, and multiple channels, in turn, by neural or endocrine mechanisms or short-range intercluster or aggregate in the plane of the membrane to actions that include direct physical or cell-cell contact. form gap junction plaques. The properties of the gap While the first strategy involves interactions at a disjunction channels are defined by the connexins. Structance, one mode of direct communication involves the tural and biophysical studies are being used to define cell-to-cell transmission of molecules through channels the mechanism by which connexins function. in a specialized cell surface membrane structure, the Connexins are the principal protein component of gap gap junction. junctions. There is much evidence to support the fact Gap junctions contain channels that connect neighthat the connexins alone (assembled in a lipid bilayer) boring cells. They differ from other membrane channels are responsible for the generation of gap junctional since they exist between two cells, they are relatively channels. This evidence includes the following: connonspecific, and the molecular movement through the nexin sequences are consistent with an integral memchannels occurs by passive diffusion. The gap junctional brane protein that has a transmembrane domain conchannels have an apparent selectivity based principally taining polar amino acids that would contribute to the on molecular size, allowing the movement of molecules formation of a channel lining; reconstitution of purified smaller than 1000 Da, such as cAMP, but preventing connexins into artificial membranes yields functional the movement of proteins or nucleic acids. Small inforchannels (reviewed by Buehler et al., 1995); expression mational molecules, such as certain morphogens, could of connexin cDNAs in heterologous systems (including be directly transmitted between cells via gap junctions. yeast) yields not only functional gap junction channels, Consequently,...

Gap junction membrane channels mediate electrical and metabolic coupling between adjacent cells. The structure of a recombinant cardiac gap junction channel was determined by electron crystallography at resolutions of 7.5 angstroms in the membrane plane and 21 angstroms in the vertical direction. The dodecameric channel was formed by the end-to-end docking of two hexamers, each of which displayed 24 rods of density in the membrane interior, which is consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. The transmembrane alpha-helical rods contrasted with the double-layered appearance of the extracellular domains. Although not indicative for a particular type of secondary structure, the protein density that formed the extracellular vestibule provided a tight seal to exclude the exchange of substances with the extracellular milieu.

Gap junction channels formed by alpha3 (Cx46) and alpha8 (Cx50) connexin provide pathways for communication between the fiber cells in the normal transparent lens. To determine the specific role of alpha3 connexin in vivo, the alpha3 connexin gene was disrupted in mice. Although the absence of alpha3 connexin had no obvious influence on the early stages of lens formation and the differentiation of lens fibers, mice homozygous for the disrupted alpha3 gene developed nuclear cataracts that were associated with the proteolysis of crystallins. This study establishes the importance of gap junctions in maintaining normal lens transparency by providing a cell-cell signaling pathway or structural component for the proper organization of lens membrane and cytoplasmic proteins.

Abstract. An extended synthetic oligonucleotide (58-mer) has been used to identify and characterize a human liver gap junction eDNA. The eDNA is 1,574 bases long and contains the entire coding region for a gap junction protein. In vitro translation of the RNA products of this eDNA is consistent with it coding for a 32,022-D protein. Southern blot analysis indicates that the gap junction gene is present as a single copy, and that it can be detected in a variety of organisms using the human liver eDNA as a probe. The human cDNA has been used to screen a rat liver eDNA library, and a rat liver junction eDNA clone has been isolated. The rat liver clone is 1,127 bases in length, and it has strong sequence homology to the human cDNA in the protein-coding region, but less extensive homology in the 3'-untranslated region.

Synthetic peptides corresponding to sequences in the human liver gap junction protein were chemically synthesized and used for generation of peptide antisera to defined sequences in the protein. The antibodies were affinity purified and characterized by demonstrating that they specifically recognized both their corresponding synthetic peptide (as indicated by dot blot analysis) and the native 32‐kd gap junction protein (by immunoblotting). The specificity of a subset of the different site‐specific antibodies was subsequently confirmed by demonstration of their binding to specific gap junction fragments produced by treatment with a lysine‐specific endoproteinase. Immunoelectron microscopy was used to localize the specific peptide antibody epitopes to either the cytoplasmic or extracellular surfaces of the gap junction. Results indicate a transmembrane orientation for the protein with the amino and carboxyl termini located on the cytoplasmic side of the membrane. Based on these data, a model is proposed for the transmembrane folding of the gap junction protein.