Catenin

Catenins are a family of proteins found in complexes with cadherin cell adhesion molecules of animal cells. The first two catenins that were identified became known as α-catenin and β-catenin. A-catenin can bind to β-catenin and can also bind actin. B-catenin binds the cytoplasmic domain of some cadherins. Additional catenins such as γ-catenin and δ-catenin have been identified. The name "catenin" was originally selected ('catena' means 'chain' in Latin) because it was suspected that catenins might link cadherins to the cytoskeleton.

All but α-catenin contain armadillo repeats.

Several types of catenins work with N-cadherins to play an important role in learning and memory (For full article, see Cadherin-catenin complex in learning and memory).

Cell-cell adhesion complexes are required for simple epithelia in higher organisms to maintain structure, function and polarity. These complexes, which help regulate cell growth in addition to creating and maintaining epithelial layers, are known as adherens junctions and they typically include at least cadherin, β-catenin, and α-catenin. Catenins play roles in cellular organization and polarity long before the development and incorporation of Wnt signaling pathways and cadherins.

The primary mechanical role of catenins is connecting cadherins to actin filaments, specifically in these adhesion junctions of epithelial cells. Most studies investigating catenin actions focus on α-catenin and β-catenin. β-catenin is particularly interesting as it plays a dual role in the cell. First of all, by binding to cadherin receptor intracellular cytoplasmic tail domains, it can act as an integral component of a protein complex in adherens junctions that helps cells maintain epithelial layers. β-catenin acts by anchoring the actin cytoskeleton to the junctions, and may possibly aid in contact inhibition signaling within the cell. For instance, when an epithelial layer is complete and the adherens junctions indicate that the cell is surrounded, β-catenin may play a role in telling the cell to stop proliferating, as there is no room for more cells in the area. Secondly, β-catenin participates in the Wnt signaling pathway as a downstream target. While the pathway is very detailed and not completely understood, in general, when Wnt is not present, GSK-3B (a member of the pathway) is able to phosphorylate β-catenin as a result of a complex formation that includes β-catenin, AXIN1, AXIN2, APC (a tumor suppressor gene product), CSNK1A1, and GSK3B. Following phosphorylation of the N-terminal Ser and Thr residues of β-catenin, BTRC promotes its ubiquitination, which causes it to be degraded by the TrCP/SKP complex. On the other hand, when Wnt is present, GSK-3B is displaced from the previously mentioned complex, causing β-catenin to not be phosphorylated, and thus not ubiquitinated. As a result, its levels in the cell are stabilized as it builds up in the cytoplasm. Eventually, some of this accumulated β-catenin will move into the nucleus with the help of Rac1. At this point, β-catenin becomes a coactivator for TCF and LEF to activate Wnt genes by displacing Groucho and HDAC transcription repressors. These gene products are important in determining cell fates during normal development and in maintaining homeostasis, or they can lead to de-regulated growth in disorders like cancer by responding to mutations in β-catenin, APC or Axin, each of which can lead to this de-regulated β-catenin level stabilization in cells.

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