Connexon

In biology, a connexon, also known as a connexin hemichannel, is an assembly of six proteins called connexins that form the pore for a gap junction between the cytoplasm of two adjacent cells. This channel allows for bidirectional flow of ions and signaling molecules. The connexon is the hemichannel supplied by a cell on one side of the junction; two connexons from opposing cells normally come together to form the complete intercellular gap junction channel. In some cells, the hemichannel itself is active as a conduit between the cytoplasm and the extracellular space, allowing the transference of ions and small molecules lower than 1-2 KDa. Little is known about this function of connexons besides the new evidence suggesting their key role in intracellular signaling. In still other cells connexons have been shown to occur in mitochondrial membranes and appear to play a role in heart ischaemia. Connexons made of the same type of connexins are considered homomeric, while connexons made of differing types of connexins are heteromeric. The assembly of connexins destined for gap junction plaques begins with synthesis of connexins within the cell and ends with the formation of gap junction channel plaques on the cell membrane. The connexin subunit proteins that make up connexons are synthesized on the membranes of the cells endoplasmic reticulum. These subunits are then oligomerized, or combined with other smaller parts, into connexons in the golgi apparatus. The connexons are then delivered to their proper location on the plasma membrane. Connexons then dock with compatible connexons from the neighboring cell to form gap junction channel plaques. A large part of this process is mediated by phosphorylation of different enzymes and proteins, allowing and preventing interaction between certain proteins. The connexons forming channels to the cell exterior or in mitochondria will require a somewhat altered path of assembly. Connexons contribute to the formation of gap junctions, and are an essential component of the electric synapses in neural pathways.