Beta-catenin (CTNNB1)| Gene Protein structure| Functions and Mutation.

Beta-catenin (CTNNB1) location and structure

Beta-catenin (CTNNB1) also known as catenin beta-1, is located on the short arm of chromosome 3 at position 3p22.1. The size of the complete gene was determined to be 23.2 kb. CTNNB1 has 16 exons according sequence analysis, the exon size ranged from 61 to 790 bp, half of the introns were smaller than 550 bp, with the smallest being 84 bp and the longest being 6700 bp.  Alternative splicing within exon 16 produced a splice variant that is 159-bp shorter in the 3-prime untranslated region. The promoter region was shown to be GC-rich and to contain a TATA box.

Protein structure

The primary structure of β-catenin consists of three domains: central domain known as armadillo repeat domain, an N-terminal domain, and a C-terminal.

• Armadillo repeat domain constitute of 550-amino-acid repeats occupy between N- terminus and C-terminus. The central armadillo domain composed 12 repeats of three helices, which is known as armadillo repeats, each of which contains approximately 42 amino acid residues. This structure forms a super helix for the binding of many factors, including the Tcf transcription factor, the cell adhesion protein cadherin, APC, Axin and other.
• N-terminal domain is composed of 150-amino-acid, the N-terminal domain is the phosphorylation site for GSK-3β and casein kinase-1. The N-terminus of b-catenin is phosphorylated when b-catenin is bound to the destruction complex.
• C-terminal domain is composed of approximately 100-amino-acid residues. The C-terminal segment of β-catenin can mimic the effects of the entire Wnt pathway.

       The N- and C-terminal regions are much smaller, and form flexible regions that interact with transcriptional activating factors. Both of terminal domain combine with the armadillo repeat domain and may regulate the partner-binding properties of the armadillo repeats.

Figure 1: represet the gene protein structure of catenin beta_1 along with its protein interacting domain.

The basic biological function of β-catenin

β-catenin is crucial for two important developmental processes:

  1: Establishment and maintenance of cell-type-specific through cell-to-cell adhesion and

  2: Regulation of target gene expression via the Wnt signaling pathway.

1: Role in cell-to-cell adhesion   

In most cases, β-catenin perform its functions in combination with several other proteins to. β-catenin is normally present in the cell membrane and can play a role in cell to cell adhesion by forming a complex, β-catenin associates with the cytoplasmic domain of E-cadherin (also known as uvomorulin that expressed by epithelial cells) and also with α-catenin, which in turn binds to F-actin protein. β-Catenin, therefore, provides the physical linkage between transmembrane adhesion proteins and the cytoskeleton proteins in the cells. This linkage to the cytoskeleton play an important role to the cell adhesion function. In summary, β-catenin provides obvious connections among extracellular signals, cell-cycle management, and gene transcription.

2: Role in Wnt signaling pathway

β-catenin-dependent Wnt signaling path way is also called the ‘canonical’ Wnt signaling pathway. The canonical Wnt/β-catenin pathway is more complex because high number of ligands and receptors involved in signaling that involve in variety of intracellular responses. Activity of β-catenin is mediated by the destruction complex, consisting of APC, AXIN-1, AXIN-2, casein kinase-1α (CK-1), protein phosphatase 2A (PP2A), and glycogen synthase kinase (GSK)-3β in the cytoplasm.

This pathway has two states dependent upon the presence or absence of Wnt ligands

1: In the absence of Wnt ligands

In normal inactivated cells GSK3-β kinases bind to and phosphorylates β-catenin in the APC/Axin destruction complex, leading to subsequent degradation of β-catenin in the proteasome.

 2: In the presence of Wnt ligands

However, upon Wnt stimulation, the GSK3-β kinase activity is inhibited, so the stable and nonphosphorylated β-catenin accumulates in the cytoplasm and then translocates into the nucleus. Nuclear localized β-catenin binds to the T Cell-Factor/Lymphoid-Enhancer Factor (TCF/LEF) DNA-binding proteins and regulates the transcription of many target genes, depending on its developmental stages and context.

      

Figure 2: represents β-catenin Wnt signaling pathway, in A the absence of wnt ligant and B presence of wnt ligant.

Mutation in β-catenin

Exon 3 of CTNNB1 is a key exon encoding serine-threonine phosphorylation sites for GSK-3β (Glycogen synthase kinase 3 beta) that activates degradation of β-catenin. The CTNNB1 mutations are frequently missense mutations, mostly localized in the hot-spot exon 3, and most of them have affect the phosphorylation sites for GSK-3β; S45 is the phosphorylation site for casein kinase-1; and D32 and G34 are essential for the interaction of β-catenin with Fbw1.

Gene mutations lead to activate the Wnt/β-catenin signaling pathway, which play a role in the development of some cancer cases. Moreover, a high level of β-catenin activity is required for cancer initiation. Initial characterization of mutations of CTNNB1 and deregulation of the canonical Wnt pathway were in colorectal cancer cases.  Similarly, another mutation of β-catenin (S37F) activates Wnt signaling in several melanoma cell lines. Such mutations have been shown to result in the accumulation of nuclear β-catenin and stabilization of the protein and tumorigenesis. These mutations stabilize β-catenin, as they cease the phosphorylation-dependent interaction of β-catenin with Fbw 1 (is a type of F-box protein), involve in degradation of β-catenin in association with casein kinase-1α and GSK-3β. In addition, other relatively benign tumors, such as desmoid-type fibromatosis, also have CTNNB1 mutations and abnormal nuclear β-catenin expression.