Connexins, also known as gap junction proteins, are four-pass transmembrane proteins with cytoplasmic C and N termini. Six connexins combine together to form a hemichannel called a connexon.

The connexin gene family is diverse, with 20 identified members in the sequenced human genome. Different connexin gene products combine to form gap junctions with different properties, including pore conductance, size selectivity, charge selectivity, voltage gating properties, and chemical gating properties. In recent literature, connexins are most commonly named according to their molecular weights, e.g. Cx26 is the connexin protein of 26 KAMU. This leads to confusion when connexin genes from different species are compared, e.g. human Cx36 is homologous to zebrafish Cx35. There was also an earlier classification of connexins according to functional properties, e.g. Cx-α1, etc.

As they are being translated by ribosomes, connexins are inserted into the membrane of the endoplasmic reticulum (Bennet and Zukin, 2004). There they gather to form hemichannels (connexons), which are carried to the cell membrane in vesicles and diffuse through the membrane until they meet a hemichannel from the other cell, with which they can dock to form a channel (Bennet and Zukin, 2004). Molecules on a connexin allow it to “recognize” the other connexins in their hemichannel and those of the other cell’s hemichannel, and cause correct alignment and formation of the channel (Kandel et al., 2000, p. 178-179).

Connexin gap junctions are found only in vertebrates. A functionally analogous but genetically unrelated group of proteins, the pannexins are expressed in both vertebrate and invertebrate species. The innexin proteins, invertebrate gap junction proteins, are probably pannexins. They have a similar structure, but don't share any sequence homology.

References

• Bennett MV, Zukin RS. Electrical coupling and neuronal synchronization in the Mammalian brain. Neuron. 2004 Feb 19;41(4):495-511. PMID 14980200
• Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed., pp.178-180. McGraw-Hill, New York (2000). ISBN 0838577016

Proteins | ion channels | Connexin