What is the structure and function of a gap junction, desmosome, and a gap junction?

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 Ca2+ 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 junctions 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 junctions 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 junctions; genes can only produce the proteins which make up gap junctions (connexins). An alternative naming system based on this protein's molecular weight is also popular (for example: connexin43, connexin30.3).

It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells.
gap junctionOne gap junction is composed of two connexons (or hemichannels) which connect across the intercellular space. Gap junctions are analogous to the plasmodesmata that join plant cells.
A notable use of gap junctions is in the electrical synapse found in some neurons.


Desmosomes are molecular complexes of cell adhesion proteins and linking proteins that attach the cell surface adhesion proteins to intracellular keratin cytoskeletal filaments.
The cell adhesion proteins of the desmosome, desmoglein and desmocollin, are members of the cadherin family of cell adhesion molecules. They are transmembrane proteins that bridge the space between adjacent epithelial cells by way of homophilic binding of their extracellular domains to other desmosomal cadherins on the adjacent cell. Both have five extracellular domains, and have calcium-binding motifs.
The extracellular domain of the desmosome is called the Extracellular Core Domain (ECD) or the Desmoglea, and is bisected by an electron-dense midline where the desmoglein and desmocollin proteins bind to each other. These proteins can bind in a W, S, or ? manner.
On the cytoplasmic side of the plasma membrane, there are two dense structures called the Outer Dense Plaque (ODP) and the Inner Dense Plaque (IDP). These are spanned by the Desmoplakin protein. The Outer Dense Plaque is where the cytoplasmic domains of the cadherins attach to desmoplakin via plakoglobin and plakophillin. The Inner Dense Plaque is where desmoplakin attaches to the intermediate filaments of the cell.

A desmosome, also known as macula adherens or macula adherentes, is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes.
Desmosomes help to resist shearing forces and are found in simple and stratified squamous epithelium. The intercellular space is very wide (about 30 nm). Desmosomes are also found in muscle tissue where they bind muscles cells to one another.