Cell Junctions
Most cadherins function as transmembrane adhesion proteins that indirectly link the cytoskeleton of the cells they join. This occurs in Cell Junctions like the adherens junctions where the cytoplasmic tails of cadherins interacts indirectly with actin filaments by means of a group of intracellular anchor proteins called "catenins" and with desmosomes where the cytoskeleton is intermediate filaments.
-
Occluding junctions: include the tight junctions whose role is particularly prominent in the small intestine where they prevent transport proteins at the apical surface of the epithelia cell (surface facing the lumen) from mixing with those in the basolateral surfaces of the cell. In addition tight junctions insure that spaces between epithelia cells are sealed so that transported molecules cannot diffuse back into the gut. Major transmembrane proteins in a tight junction include the claudins and occludins.
-
Anchoring junctions: connect the cytoskeleton of a cell to the cytoskeleton of other cells using the transmembrane protein cadherin or to the extracellular matrix (basal lamina) through the use of the TM protein integrin. In both cases, there is an intracellular coupling to either actin or intermediate filaments depending on the type of intracellular anchor proteins involved. The various anchoring junctions are contained in the chart below:
Cell-Cell |
TM protein |
Extracellular Ligand |
Intracellular Ligand |
Intracellular anchor proteins |
| Adherens junction | cadherin | cadherin in other cell | actin filaments | α actinin, catenins, more |
| Desmosome | cadherin | desmogleins & desmocolins | intermediate filaments | desmoplakins |
| Cell-Matrix | ||||
| Focal adhesion | integrins | extracellular matrix proteins | actin filaments | α actinin, more |
| Hemidesosome | integrins | extracellular matrix proteins | intermediate filaments |
- Communicating junctions: include the gap junctions which form channels through transmembrane proteins called connexins. 6 connexins are required to form a channel (called a "connexon") and when the connexons in the PM of 2 cells in contact are aligned, they form a continuous aqueous channel that connects the two cell interiors.
