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A gap junction may also be called a nexus or macula communicans. When found in nerves they are also referred to as an electrical synapse. While an ephapse has some similarities to a gap junction, by modern definition these two entities are different. Gap junctions are a specialized intercellular connection between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells. One gap junction channel is composed of two connexons (or hemichannels), which connect across the intercellular space.〔〔 Gap junctions are analogous to the plasmodesmata that join plant cells. Gap junctions are expressed in virtually all tissues of the body, with the exception of adult fully developed skeletal muscle and mobile cell types such as sperm or erythrocytes. Gap Junctions, however, are not found in simpler organisms such as sponges and slime molds. ==Structure== In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Invertebrate gap junctions comprise proteins from the innexin family. Innexins have no significant sequence homology with connexins. Though differing in sequence to connexins innexins are similar enough to connexins for people to consider innexins form gap junctions in vivo in the same way connexins do. The recently characterized pannexin family, which was originally thought to form intercellular channels (based on similar amino acid sequence similarity to innexins), in fact functions as single-membrane channels that communicate with the extracellular environment, and have been shown to pass calcium and ATP. At gap junctions, the intercellular space is between 2 and 4 nm〔 and unit connexons in the membrane of each cell are lined up with one another. Gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. In turn, hemichannels of uniform connexin composition are called homomeric, while those with differing connexins are heteromeric. Channel composition is thought to influence the function of gap junction channels. Before innexins and pannexins were well characterized, the genes coding for connexin gap junction channels were classified in one of three groups, based on gene mapping and sequence similarity: A, B and C (for example, , ). However, connexin genes do not code directly for the expression of gap junction channels; genes can produce only the proteins that make up gap junction channels. An alternative naming system based on this protein's molecular weight is also popular (for example: connexin43=GJA1, connexin30.3=GJB4). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gap junction」の詳細全文を読む スポンサード リンク
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