DOC_GSK3_Axin_1

The Wnt signaling pathway plays an essential role in the determination of cell fate, proliferation, polarity, and cell death during embryonic development, as well as for tissue homeostasis in adults. Abnormalities in the Wnt signalling pathway are associated with a large variety of human diseases, ranging from birth defects to cancers (Saito-Diaz,2013). The canonical Wnt signaling pathway affects/modulates the expression of genes via transcription factor β-catenin. In the absence of Wnt signaling, β-catenin is present in the cytoplasm as a part of the destruction complex which includes GSK-3β, Axin and adenomatous polyposis coli tumor suppressor protein (APC). Axin acts as a scaffold protein which brings GSK-3β in close proximity to β-catenin for the latter’s efficient phosphorylation and its subsequent ubiquitin dependent degradation via β-TRCP (MacDonald,2009). Upon Wnt stimulation, binding of Wnt to its transmembrane receptors (LRP5/6 and Frizzled) leads to the disruption of the destruction complex mainly by the recruitment of Axin to the plasma membrane (Mao,2001;Stamos,2014) which in turn leads to inhibition of β-catenin phosphorylation and degradation. Subsequently, β-catenin accumulates and enters in the nucleus, where it controls the expression of genes important for development (Saito-Diaz,2013;MacDonald,2009). So, Axin acts as a docking station mediating negative regulation of β-catenin by GSK-3β. Importantly, it is worth to mention that GSK-3β is inhibited by the phosphorylated P-P-P-S/T-P-X-S/T motifs present in cytoplasmic tail of LRP5/6 receptor. The proposed inhibition model suggests that these motifs act as a psudosubstrate and engage GSK-3β(Stamos,2014).

Additionally, it is also known that there are other GSK-3β binding proteins such as FRAT and GSKIP which compete with Axin for GSK-3β binding and move GSK-3β away from the scaffold complex (Howng,2010). The coordinated regulation of GSK-3β also occurs in part through a competition between its activating partner (Axin) and its inhibiting partner (FRAT) (Bax,2001). The docking surface on GSK-3β is rather flexible and consists of mainly an α-helix and an extended loop. Different proteins can use overlapping sites for binding with high specificity. The different binding affinities of docking motifs are mainly due to the type of interaction they are involved in. FRAT demonstrated a strong electrostatic interaction resulting in a higher binding affinity. Axin mainly uses hydrophobic interaction while GSKIP showed a combination of both types and its binding mode is more similar to that of Axin (Tang,2011). The GSK-3β binding motif present in Axin is different from other binding partners. The motif composes of an amphipathic helix that packs into a hydrophobic groove formed between a helix and an extended loop of GSK-3β (Dajani,2003). It is highly conserved in vertebrates.