Gap Junction

Gap junctions are collections of intercellular channels that allow ions and tiny molecules to move directly between cells. Gap junctions are found connecting nearly all cells in solid tissues and were first identified as low-resistance ion channels interconnecting excitable cells (nerve and muscle). Gap-junctional intercellular communication has been adapted to a range of roles with diverse regulation mechanisms according to their long evolutionary history. Hexamers of the medium-sized families of integral proteins connexins in chordates and innexins in precordates make up the gap-junctional channels. By examining mutations in flies, worms, and humans as well as targeted gene disruption in mice, researchers have investigated the roles of gap junctions.

The direct transport of ions and small molecules between adjacent cells is made possible by gap junctions, which are collections of intercellular channels. Hexameric clusters (connexons) of tetraspan integral membrane proteins, the connexins, bind head-to-head to form the intercellular channels (Cx). These channels group together to form polymorphic plaques or maculae that range in size from a few to thousands of units. The tight membrane apposition necessary for connexon docking sterically excludes most other membrane proteins, leaving the junction’s distinctive tiny extracellular “gap” of 2 nm. Prechordates have innexin-based gap junctions. Connexins were created by convergent evolution in chordates and then expanded into a 21-member gene family by gene duplication.

It is unclear if the three pannexins, which are linked to innexins, form intercellular channels, yet they have persisted in vertebrates. Intercellular channels made of either a carboxy-terminal truncation of Cx43 or an M34A mutant of Cx26 have 7Å-resolution electron crystallographic structures available. The Cx26 channel pore has a “plug,” but otherwise the pore morphologies are comparable. Deletion of amino acids 2–7 significantly reduces the density of this plug, indicating that the amino terminus contributes to its structure.

The amino-terminus of Cx26 was seen folded into the channel mouth without producing a plug in a 3.5Å- X-ray crystallographic structure, which is assumed to be a representation of the open channel conformation. A role for the amino-terminus as a gating structure has been supported by the physiological evidence linking it to voltage-gating of the Cx26 and Cx32 channels. However, Cx43 also exhibits voltage-gating, and the mystery behind its lack of a plug-like structure continues to this day. A quarter-worth century of X-ray advancement can be summed up by comparing an intercellular channel structure from 1985 with the 3.5Å structure from 2009.

The fundamental structural and operational unit of all living things is the cell. Each cell has a cytoplasm that is surrounded by a membrane and is home to a variety of biomolecules, including proteins and nucleic acids.

Cells can develop specialised functions and perform a variety of tasks within the cell, including protein synthesis, DNA repair, replication, and motility. Within the cell, cells can specialise and move around. Due to their small size, the majority of cells are measured in micrometres.