Connexon

Connexon
Connexon and connexin structure
Details
Latin	connexona
TH	H1.00.01.1.02025
Anatomical terminology []

In biology, a connexon, also known as a connexin hemichannel or a pannexin channel, 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. However, 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.

Connexons made of the same type of connexins are considered homomeric, while connexons made of differing types of connexins are heteromeric.

The assembly of connexons 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.

Connexons contribute to the formation of gap junctions, and are an essential component of the electric synapses in neural pathways. In a single gap junction, connexons will assemble around an aqueous porous membrane, forming a hemi-channel that is composed of connexins. Connexins are the smaller protein molecules that make up connexons and play a crucial part to the formation of gap junctions. Structurally, connexins are made up of 4 alpha helical transmembrane domains connected by 2 extracellular loops and 1 cytoplasmic loop, while both N and C terminals reside intracellularly. Connexin types can be further differentiated by using their predicted molecular weight (ex: Connexin 43 is Cx 43 due to its molecular weight of 43 kDa). Connexons will form the gap junction by docking a hemi-channel to another hemi-channel in an adjacent cell membrane. During this phase, the formation of an intercellular channels spanning both of the plasma membranes, occurs. Subsequently, this process leads to a better understanding of how electric synapses are facilitated between neurons.