Introduction to Adherens Junctions
Adherens junctions (or zonula adherens, intermediate junction, or "belt desmosome") are protein complexes that occur at cell–cell junctions in epithelial tissues. They are usually more basal than tight junctions. An adherens junction is defined as a cell junction whose cytoplasmic face is linked to the actin cytoskeleton. They can appear as bands encircling the cell (zonula adherens) or as spots of attachment to the extracellular matrix (adhesion plaques). A similar cell junction in non-epithelial cells is the fascia adherens. It is structurally the same, but appears in ribbon-like patterns that do not completely encircle the cells. One example is in cardiomyocytes.
Adherens junctions are composed of the following proteins (Figure 1):
- cadherins. The cadherins are a family of transmembrane proteins that form homodimers in a calcium-dependent manner with other cadherin molecules on adjacent cells.
- p120 (sometimes called delta catenin) binds the juxtamembrane region of the cadherin.
- β-catenin or gamma-catenin (plakoglobin) binds the catenin-binding region of the cadherin.
- α-catenin binds the cadherin indirectly via β-catenin or plakoglobin and links the actin cytoskeleton with cadherin.
Adherens junctions were, for many years, thought to share the characteristic of anchor cells through their cytoplasmic actin filaments. The accepted model has been that adherens junctions serve as a bridge connecting the actin cytoskeleton of neighboring cells through direct interaction. However, scientists have not been able to isolate the quaternary complex of cadherin-βcatenin-αcatenin-actin in vitro. Recent data (2005) demonstrate that membrane associated actin is several fold less stable compared to components of the adherens junctional complex. Additionally, it was found that monomeric α-catenin preferentially binds to the cadherin junction complex through β-catenin. Dimeric α-catenin preferentially binds to actin and suppresses Arp2/3 complex-mediated actin branching, thus acting as a molecular switch to regulate actin polymerization. Adherens junctions may serve as a regulatory module to maintain the actin contractile ring with which it is associated in microscopic studies.