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.
Connexons made of the same type of connexins are considered homomeric, while connexons made of differing types of connexins are heteromeric.
Structure
Assembly
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 cell's 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.
Degradation
Connexon structure is degraded by its removal from the plasma membrane. Connexons will be internalized by the cell itself as a double membrane channel structure (due to the docking of hemi-channels). Research shows connexons are particularly permeable to soluble second messengers, amino acids, nucleotides, ions and glucose. Cardiovascular disease and possibly type I and II diabetes, are each associated with a major protein connexin that makes up the gap junction.
In cardiovascular disease, Cx43 (connexin 43), a subunit of a connexon, is a general protein of the gap junction stimulating cardio myocyte muscle cells of intercalated discs facilitating synchronized beating of the heart. In the occurrence of cardiovascular disease the Cx43 subunit begins to show signs of oxidative stress, the ability of the heart to counteract the buildup of harmful toxins due to age or diet leading to reduced vascular functions. Overall, these changes in Cx43 expression and oxidant stress can lead to abnormalities in the coordinated beating of the heart, predisposing it to cardiac arrhythmias.