Adherens junction

Overview
Adherens junctions (or zonula adherens) are protein complexes that occur at cell-cell junctions in epithelial tissues, usually more basal than tight junctions.

They can appear as bands encircling the cell (zonula adherens) or as spots of attachment to the extracellular matrix (adhesion plaques).

Proteins
They are composed of three major proteins:
 * cadherins. The cadherins are a family of transmembrane proteins that form homodimers in a calcium-dependent manner with other cadherin molecules on adjacent cells.
 * β-catenin and α-catenin. The catenins both form a complex with the intracellular portion of the cadherin molecule.

Models
Adherens junctions were, for many years, thought to share the characteristic of anchoring 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) from 2 labs at Stanford University published in the journal Cell demonstrates that membrane associated actin is several fold less stable compared to components of the adherens junctional complex.

Additionally, the authors 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.