Connexin, C-terminal (Connexin_C)
Type: Homologous_superfamily

This superfamily represents the C-terminal domain of Connexins .

The connexins are a family of integral membrane proteins that oligomerise to form intercellular channels that are clustered at gap junctions. These channels are specialised sites of cell-cell contact that allow the passage of ions, intracellular metabolites and messenger molecules (with molecular weight less than 1-2kDa) from the cytoplasm of one cell to its opposing neighbours. They are found in almost all vertebrate cell types, and somewhat similar proteins have been cloned from plant species. Invertebrates utilise a different family of molecules, innexins, that share a similar predicted secondary structure to the vertebrate connexins, but have no sequence identity to them .

Vertebrate gap junction channels are thought to participate in diverse biological functions. For instance, in the heart they permit the rapid cell-cell transfer of action potentials, ensuring coordinated contraction of the cardiomyocytes. They are also responsible for neurotransmission at specialised 'electrical' synapses. In non-excitable tissues, such as the liver, they may allow metabolic cooperation between cells. In the brain, glial cells are extensively-coupled by gap junctions; this allows waves of intracellular Ca
2+
to propagate through nervous tissue, and may contribute to their ability to spatially-buffer local changes in extracellular K
+
concentration .

The connexin protein family is encoded by at least 13 genes in rodents, with many homologues cloned from other species. They show overlapping tissue expression patterns, most tissues expressing more than one connexin type. Their conductances, permeability to different molecules, phosphorylation and voltage-dependence of their gating, have been found to vary. Possible communication diversity is increased further by the fact that gap junctions may be formed by the association of different connexin isoforms from apposing cells. However, in vitro studies have shown that not all possible combinations of connexins produce active channels .

Hydropathy analysis predicts that all cloned connexins share a common transmembrane (TM) topology. Each connexin is thought to contain 4 TM domains, with two extracellular and three cytoplasmic regions. This model has been validated for several of the family members by
in vitro
biochemical analysis. Both N-and C-termini are thought to face the cytoplasm, and the third TM domain has an amphipathic character, suggesting that it contributes to the lining of the formed-channel. Amino acid sequence identity between the isoforms is ~50-80%, with the TM domains being well conserved. Both extracellular loops contain characteristically conserved cysteine residues, which likely form intramolecular disulphide bonds. By contrast, the single putative intracellular loop (between TM domains 2 and 3) and the cytoplasmic C terminus are highly variable among the family members. Six connexins are thought to associate to form a hemi-channel, or connexon. Two connexins then interact (likely via the extracellular loops of their connexins) to form the complete gap junction channel.

NH2-*** *** *************-COOH
** ** ** **
** ** ** ** Cytoplasmic
---**----**-----**----**----------------
** ** ** ** Membrane
** ** ** **
---**----**-----**----**----------------
** ** ** ** Extracellular
** ** ** **
** **