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Hetero-hexamers at gap junction plaques, help form a uniform intercellular space of 2-4 nm. [21] In this way hemichannels in the membrane of each cell are aligned with one another forming an intercellular communication path. [22] Invertebrate gap junctions comprise proteins from the innexin family.
Connexins are commonly named according to their molecular weights, e.g. Cx26 is the connexin protein of 26 kDa. A competing nomenclature is the gap junction protein system, where connexins are sorted by their α (GJA) and β (GJB) forms, with additional connexins grouped into the C, D and E groupings, followed by an identifying number, e.g. GJA1 corresponds to Cx43.
In vertebrates, gap junctions are composed of transmembrane proteins called connexins. They form hexagonal pores or channels through which ions, sugars, and other small molecules can pass. Each pore is made of 12 connexin molecules; 6 form a hemichannel on one cell membrane and interact with a hemichannel on an adjacent cell membrane.
Cell junctions are also especially important in enabling communication between neighboring cells via specialized protein complexes called communicating (gap) junctions. Cell junctions are also important in reducing stress placed upon cells. In plants, similar communication channels are known as plasmodesmata, and in fungi they are called septal ...
Gap junctions are often present at nerve endings such as in cardiac muscle and are important in maintaining homeostasis in the liver and proper function of the kidneys. The gap junction itself is a structure that is a specialized transmembrane protein formed by a connexon hemichannel. [8]
Innexins are transmembrane proteins that form gap junctions in invertebrates. Gap junctions are composed of membrane proteins that form a channel permeable to ions and small molecules connecting the cytoplasm of adjacent cells. Although gap junctions provide similar functions in all multicellular organisms, it was not known what proteins ...
[19] [20] [21] In heart muscle gap junctions function to coordinate the beating of the heart. Adding even further to their versatility gap junctions can also function to form a direct connection to the exterior of a cell paralleling the functioning of the protein cousin the pannexins which are explained elsewhere.
Phosphorylation of gap junctions and their subunits is typically achieved through protein kinases, enzymes that add phosphates to the amino acids of proteins. [ 11 ] [ 12 ] [ 13 ] Serine/threonine kinases , which phosphorylate the hydroxyl group of serine or threonine residues, form the bulk of the Connexin phosphorylation kinases.