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Downloaded from on August 1, 2011 - Published by Cold Spring Harbor Laboratory Press

Gap Junctions
Daniel A. Goodenough and David L. Paul Cold Spring Harb Perspect Biol 2009;1:a002576


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Downloaded from on August 1, 2011 - Published by Cold Spring Harbor Laboratory Press

Gap Junctions
Daniel A. Goodenough1 and David L. Paul2
1 2

Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115


Gap junctions are aggregates of intercellular channels that permit direct cell– cell transfer of ions and small molecules. Initially described as low-resistance ion pathways joining excitable cells (nerve and muscle), gap junctions are found joining virtuallyall cells insolid tissues. Their long evolutionary history has permitted adaptation of gap-junctional intercellular communication to a variety of functions, with multiple regulatory mechanisms. Gap-junctional channels are composed of hexamers of medium-sized families of integral proteins: connexins in chordates and innexins in precordates. The functions of gap junctions have been explored by studying mutationsin flies, worms, and humans, and targeted gene disruption in mice. These studies have revealed a wide diversity of function in tissue and organ biology.

ap junctions are clusters of intercellular channels that allow direct diffusion of ions and small molecules between adjacent cells. The intercellular channels are formed by head-to-head docking of hexameric assemblies (connexons) of tetraspanintegral membrane proteins, the connexins (Cx) (Goodenough et al. 1996). These channels cluster into polymorphic maculae or plaques containing a few to thousands of units (Fig. 1). The close membrane apposition required to allow the docking between connexons sterically excludes most other membrane proteins, leaving a narrow 2 nm extracellular “gap” for which the junction is named (Fig. 2). Gapjunctions in prechordates are composed of innexins (Phelan et al. 1998; Phelan 2005). In chordates, connexins arose by convergent evolution (Alexopoulos et al. 2004), to expand by gene duplication (Cruciani and Mikalsen 2007)


into a 21-member gene family. Three innexinrelated proteins, called pannexins, have persisted in vertebrates, although it is not clear if they form intercellular channels(Panchin et al. ˚ 2000; Bruzzone et al. 2003). 7A-resolution electron crystallographic structures of intercellular channels composed of either a carboxyterminal truncation of Cx43 (Unger et al. 1999; Yeager and Harris 2007) or an M34A mutant of Cx26 (Oshima et al. 2007) are available. The overall pore morphologies are similar with the exception of a “plug” in the Cx26 channel pore. The density ofthis plug is substantively decreased by deletion of amino acids 2– 7, suggesting that the amino-terminus contributes ˚ to this structure (Oshima et al. 2008). A 3.5-A X-ray crystallographic structure has visualized the amino-terminus of Cx26 folded into the mouth of the channel without forming a plug, thought to be an image of the open channel

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