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Gap junctions are specialized membrane channels between neighboring animal cells. They support intercellular signaling by allowing the exchange of ions, second messengers, sugars, and other small molecules between cells.

These channels are made of transmembrane proteins called connexins, or CX proteins.

Six connexin molecules assemble into a hemichannel, also called a connexon. A connexon makes up one half of a complete gap junction channel.

Connexons can be classified into two types: homomeric and heteromeric. When all six connexins are the same type, the structure is called a homomeric connexon.

For instance, in the heart, CX40 can form homomeric connexons. However, CX40 can also combine with another heart connexin, CX43, to form a heteromeric connexon. This heteromeric connexon may have different functions, such as selectivity for specific molecules.

Connexins are first made in the rough endoplasmic reticulum. Many connexins then move to the Golgi apparatus, where they assemble into connexons. The cell then transports the connexons to the plasma membrane. There, they pair with connexons from neighboring cells.

Together, the two hemichannels form a complete gap junction channel t