Structure
In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Invertebrate gap junctions comprise proteins from the hypothetical innexin family. However, the recently characterized pannexin family, which was originally thought to form intercellular channels (based on similar amino acid sequence similarity to innexins), in fact functions as single-membrane channels that communicate with the extracellular environment, and have been shown to pass calcium and ATP.
At gap junctions, the intercellular space is 4 nm and unit connexons in the membrane of each cell are lined up with one another.
Gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. In turn, hemichannels of uniform connexin composition are called homomeric, while those with differing connexins are heteromeric. Channel composition is thought to influence the function of gap junction channels.
Generally, the genes coding for gap junction channels are classified in one of three groups, based on sequence similarity: A, B and C (for example, GJA1, GJC1). However, genes do not code directly for the expression of gap junction channels; genes can only produce the proteins which make up gap junction channels (connexins). An alternative naming system based on this protein's molecular weight is also popular (for example: connexin43, connexin30.3).
Read more about this topic: Gap Junction
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