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Gap junctions use protein complexes known as connexons to connect one cell to another. The proteins are called connexins. Gap junction proteins include the more than 26 types of connexin, and at least 12 non-connexin components that make up the gap junction complex or nexus. [4]
Gap junctions are the main site of cell-cell signaling or communication that allow small molecules to diffuse between adjacent cells. 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.
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.
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.
A gap junction modulator is a compound or agent that either facilitates or inhibits the transfer of small molecules between biological cells by regulating gap junctions. [1] Various physiological processes including cardiac , neural or auditory , depend on gap junctions to perform crucial regulatory roles, and the modulators themselves are the ...
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 ...
[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.
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]