29.12: Gap Junctions
The cytoplasm of adjacent animal cells can exchange small molecules, ions, and secondary messengers via the communication channels which form the gap junctions. These junctions comprise a few hundred to thousands of molecular channels, each made of two halves, called the connexon hemichannel. A connexon is a hexamer of six transmembrane connexin proteins, which assemble radially, thus forming a pore or channel in the center. One connexon hemichannel docks with a corresponding connexon on the adjacent cell, completing the channel.
Gap Junction Dynamics
Connexins are translated by the ribosome on the rough endoplasmic reticulum and are cotranslationally inserted into the ER membrane. The monomers are then transported to the Golgi network, where they oligomerize. The hexameric connexons are transported from the trans-Golgi network to the plasma membrane via secretory vesicles. The connexons can freely move along the plasma membrane until they encounter other connexons, forming clusters. Thus a growing cluster has new connexons continually added to its periphery, forming a gap junction plaque. At the center of this plaque, old connexons are endocytosed and marked for degradation. Gap junctions are, therefore, dynamic assemblies with a constant turnover of connexons at the membrane.
Invertebrate Gap Junctions
Gap junctions in invertebrates have a similar structure to those in vertebrates; however, they comprise different proteins called innexins (invertebrate connexins). Though innexins have a similar transmembrane structure, they don't share sequence homology with connexins. They also differ from their vertebrate counterparts in requiring eight units to form an innexon hemichannel instead of six.
Counterparts in Plants
Unlike in animal tissues, where the membranes of adjacent cells are in direct contact, the cell walls in plant tissues prevent direct contact of adjacent plasma membranes. Intercellular communication and exchange of molecules thus occur via specialized junctions called the plasmodesmata. The membrane of one cell is continuous with that of the adjacent cell at these junctions, forming a tubular channel of about 20 to 40nm in diameter. Thus, the cytoplasm of adjacent cells is contiguous, allowing the regulated exchange of various small molecules.