Mammalian protein found in humans
Catenin beta-1, also known as β-catenin ("beta"-catenin), is a protein that in humans is encoded by the "CTNNB1" gene.
β-Catenin is a dual function protein, involved in regulation and coordination of cell–cell adhesion and gene transcription. In humans, the CTNNB1 protein is encoded by the "CTNNB1" gene. In "Drosophila", the homologous protein is called "armadillo". β-catenin is a subunit of the cadherin protein complex and acts as an intracellular signal transducer in the Wnt signaling pathway. It is a member of the catenin protein family and homologous to γ-catenin, also known as plakoglobin. β-Catenin is widely expressed in many tissues. In cardiac muscle, β-catenin localizes to adherens junctions in intercalated disc structures, which are critical for electrical and mechanical coupling between adjacent cardiomyocytes.
Mutations and overexpression of β-catenin are associated with many cancers, including hepatocellular carcinoma, colorectal carcinoma, lung cancer, malignant breast tumors, ovarian and endometrial cancer. Alterations in the localization and expression levels of β-catenin have been associated with various forms of heart disease, including dilated cardiomyopathy. β-Catenin is regulated and destroyed by the "beta-catenin destruction complex", and in particular by the adenomatous polyposis coli (APC) protein, encoded by the tumour-suppressing "APC" gene. Therefore, genetic mutation of the "APC" gene is also strongly linked to cancers, and in particular colorectal cancer resulting from familial adenomatous polyposis (FAP).
Discovery.
β-Catenin was initially discovered in the early 1990s as a component of a mammalian cell adhesion complex: a protein responsible for cytoplasmatic anchoring of cadherins. But very soon, it was realized that the Drosophila protein "armadillo" – implicated in mediating the morphogenic effects of "Wingless/Wnt" – is homologous to the mammalian β-catenin, not just in structure but also in function. Thus, β-catenin became one of the first examples of moonlighting: a protein performing more than one radically different cellular function.
Structure.
Protein structure.
The core of β-catenin consists of several very characteristic repeats, each approximately 40 amino acids long. Termed armadillo repeats, all these elements fold together into a single, rigid protein domain with an elongated shape – called armadillo (ARM) domain. An average armadillo repeat is composed of three alpha helices. The first repeat of β-catenin (near the N-terminus) is slightly different from the others – as it has an elongated helix with a kink, formed by the fusion of helices 1 and 2. Due to the complex shape of individual repeats, the whole ARM domain is not a straight rod: it possesses a slight curvature, so that an outer (convex) and an inner (concave) surface is formed. This inner surface serves as a ligand-binding site for the various interaction partners of the ARM domains.
The segments N-terminal and far C-terminal to the ARM domain do not adopt any structure in solution by themselves. Yet these intrinsically disordered regions play a crucial role in β-catenin function. The N-terminal disordered region contains a conserved short linear motif responsible for binding of TrCP1 (also known as β-TrCP) E3 ubiquitin ligase – but only when it is phosphorylated. Degradation of β-catenin is thus mediated by this N-terminal segment. The C-terminal region, on the other hand, is a strong transactivator when recruited onto DNA. This segment is not fully disordered: part of the C-terminal extension forms a stable helix that packs against the ARM domain, but may also engage separate binding partners. This small structural element (HelixC) caps the C-terminal end of the ARM domain, shielding its hydrophobic residues. HelixC is not necessary for β-catenin to function in cell–cell adhesion. On the other hand, it is required for Wnt signaling: possibly to recruit various coactivators, such as 14-3-3zeta. Yet its exact partners among the general transcription complexes are still incompletely understood, and they likely involve tissue-specific players. Notably, the C-terminal segment of β-catenin can mimic the effects of the entire Wnt pathway if artificially fused to the DNA binding domain of LEF1 transcription factor.
Plakoglobin (also called γ-catenin) has a strikingly similar architecture to that of β-catenin. Not only their ARM domains resemble each other in both architecture and ligand binding capacity, but the N-terminal β-TrCP-binding motif is also conserved in plakoglobin, implying common ancestry and shared regulation with β-catenin. However, plakoglobin is a very weak transactivator when bound to DNA – this is probably caused by the divergence of their C-terminal sequences (plakoglobin appears to lack the transactivator motifs, and thus inhibits the Wnt pathway target genes instead of activating them).
Partners binding to the armadillo domain.
As sketched above, the ARM domain of β-catenin acts as a platform to which specific linear motifs may bind. Located in structurally diverse partners, the β-catenin binding motifs are typically disordered on their own, and typically adopt a rigid structure upon ARM domain engagement – as seen for short linear motifs. However, β-catenin interacting motifs also have a number of peculiar characteristics. First, they might reach or even surpass the length of 30 amino acids in length, and contact the ARM domain on an excessively large surface area. Another unusual feature of these motifs is their frequently high degree of phosphorylation. Such Ser/Thr phosphorylation events greatly enhance the binding of many β-catenin associating motifs to the ARM domain.
The structure of β-catenin in complex with the catenin binding domain of the transcriptional transactivation partner TCF provided the initial structural roadmap of how many binding partners of β-catenin may form interactions. This structure demonstrated how the otherwise disordered N-terminus of TCF adapted what appeared to be a rigid conformation, with the binding motif spanning many beta-catenin repeats. Relatively strong charged interaction "hot spots" were defined (predicted, and later verified, to be conserved for the β-catenin/E-cadherin interaction), as well as hydrophobic regions deemed important in the overall mode of binding and as potential therapeutic small molecule inhibitor targets against certain cancer forms. Furthermore, following studies demonstrated another peculiar characteristic, plasticity in the binding of the TCF N-terminus to beta-catenin.
Similarly, we find the familiar E-cadherin, whose cytoplasmatic tail contacts the ARM domain in the same canonical fashion. The scaffold protein axin (two closely related paralogs, axin 1 and axin 2) contains a similar interaction motif on its long, disordered middle segment. Although one molecule of axin only contains a single β-catenin recruitment motif, its partner the adenomatous polyposis coli (APC) protein contains 11 such motifs in tandem arrangement per protomer, thus capable to interact with several β-catenin molecules at once. Since the surface of the ARM domain can typically accommodate only one peptide motif at any given time, all these proteins compete for the same cellular pool of β-catenin molecules. This competition is the key to understand how the Wnt signaling pathway works.
However, this "main" binding site on the ARM domain β-catenin is by no means the only one. The first helices of the ARM domain form an additional, special protein-protein interaction pocket: This can accommodate a helix-forming linear motif found in the coactivator BCL9 (or the closely related BCL9L) – an important protein involved in Wnt signaling. Although the precise details are much less clear, it appears that the same site is used by alpha-catenin when β-catenin is localized to the adherens junctions. Because this pocket is distinct from the ARM domain's "main" binding site, there is no competition between alpha-catenin and E-cadherin or between TCF1 and BCL9, respectively. On the other hand, BCL9 and BCL9L must compete with α-catenin to access β-catenin molecules.
Function.
Regulation of degradation through phosphorylation.
The cellular level of β-catenin is mostly controlled by its ubiquitination and proteosomal degradation. The E3 ubiquitin ligase TrCP1 (also known as β-TrCP) can recognize β-catenin as its substrate through a short linear motif on the disordered N-terminus. However, this motif (Asp-Ser-Gly-Ile-His-Ser) of β-catenin needs to be phosphorylated on the two serines in order to be capable to bind β-TrCP. Phosphorylation of the motif is performed by Glycogen Synthase Kinase 3 alpha and beta (GSK3α and GSK3β). GSK3s are constitutively active enzymes implicated in several important regulatory processes. There is one requirement, though: substrates of GSK3 need to be pre-phosphorylated four amino acids downstream (C-terminally) of the actual target site. Thus it also requires a "priming kinase" for its activities. In the case of β-catenin, the most important priming kinase is Casein Kinase I (CKI). Once a serine-threonine rich substrate has been "primed", GSK3 can "walk" across it from C-terminal to N-terminal direction, phosphorylating every 4th serine or threonine residues in a row. This process will result in dual phosphorylation of the aforementioned β-TrCP recognition motif as well.
The beta-catenin destruction complex.
For GSK3 to be a highly effective kinase on a substrate, pre-phosphorylation is not enough. There is one additional requirement: Similar to the mitogen-activated protein kinases (MAPKs), substrates need to associate with this enzyme through high-affinity "docking motifs". β-Catenin contains no such motifs, but a special protein does: axin. What is more, its GSK3 docking motif is directly adjacent to a β-catenin binding motif. This way, "axin" acts as a true scaffold protein, bringing an enzyme (GSK3) together with its substrate (β-catenin) into close physical proximity.
But even "axin" does not act alone. Through its N-terminal regulator of G-protein signaling (RGS) domain, it recruits the adenomatous polyposis coli (APC) protein. "APC" is like a huge "Christmas tree": with a multitude of β-catenin binding motifs (one "APC" molecule alone possesses 11 such motifs ), it may collect as many β-catenin molecules as possible. "APC" can interact with multiple "axin" molecules at the same time as it has three "SAMP motifs" (Ser-Ala-Met-Pro) to bind the RGS domains found in "axin". In addition, axin also has the potential to oligomerize through its C-terminal DIX domain. The result is a huge, multimeric protein assembly dedicated to β-catenin phosphorylation. This complex is usually called the "beta-catenin destruction complex", although it is distinct from the proteosome machinery actually responsible for β-catenin degradation. It only marks β-catenin molecules for subsequent destruction.
Wnt signaling and the regulation of destruction.
In resting cells, axin molecules oligomerize with each other through their C-terminal DIX domains, which have two binding interfaces. Thus they can build linear oligomers or even polymers inside the cytoplasm of cells. DIX domains are unique: the only other proteins known to have a DIX domain are Dishevelled and DIXDC1. (The single "Dsh" protein of "Drosophila" corresponds to three paralogous genes, "Dvl1", "Dvl2" and "Dvl3" in mammals.) Dsh associates with the cytoplasmic regions of Frizzled receptors with its PDZ and DEP domains. When a Wnt molecule binds to "Frizzled", it induces a poorly known cascade of events, that result in the exposure of dishevelled's DIX domain and the creation of a perfect binding site for "axin". Axin is then titrated away from its oligomeric assemblies – the β-catenin destruction complex – by "Dsh". Once bound to the receptor complex, "axin" will be rendered incompetent for β-catenin binding and GSK3 activity. Importantly, the cytoplasmic segments of the Frizzled-associated LRP5 and LRP6 proteins contain GSK3 pseudo-substrate sequences (Pro-Pro-Pro-Ser-Pro-x-Ser), appropriately "primed" (pre-phosphorylated) by CKI, as if it were a true substrate of GSK3. These false target sites greatly inhibit GSK3 activity in a competitive manner. This way receptor-bound "axin" will abolish mediating the phosphorylation of β-catenin. Since β-catenin is no longer marked for destruction, but continues to be produced, its concentration will increase. Once β-catenin levels rise high enough to saturate all binding sites in the cytoplasm, it will also translocate into the nucleus. Upon engaging the transcription factors LEF1, TCF1, TCF2 or TCF3, β-catenin forces them to disengage their previous partners: Groucho proteins. Unlike "Groucho", that recruit transcriptional repressors (e.g. histone-lysine methyltransferases), β-catenin will bind transcriptional activators, switching on target genes.
Role in cell–cell adhesion.
Cell–cell adhesion complexes are essential for the formation of complex animal tissues. β-catenin is part of a protein complex that form adherens junctions. These cell–cell adhesion complexes are necessary for the creation and maintenance of epithelial cell layers and barriers. As a component of the complex, β-catenin can regulate cell growth and adhesion between cells. It may also be responsible for transmitting the contact inhibition signal that causes cells to stop dividing once the epithelial sheet is complete. The E-cadherin – β-catenin – α-catenin complex is weakly associated to actin filaments. Adherens junctions require significant protein dynamics in order to link to the actin cytoskeleton,
thereby enabling mechanotransduction.
An important component of the adherens junctions are the cadherin proteins. Cadherins form the cell–cell junctional structures known as adherens junctions as well as the desmosomes. Cadherins are capable of homophilic interactions through their extracellular cadherin repeat domains, in a Ca2+-dependent