The Eukaryotic Linear Motif resource for
Functional Sites in Proteins
Functional site class:
The GSK-3beta docking motif in Axin
Functional site description:
The scaffold protein Axin acts as a critical component of the β-catenin destruction complex which negatively regulates the Wnt/β-catenin signaling. Axin simultaneously binds two components of the Wnt pathway: β-catenin and its negative regulator, GSK-3β. Binding of Axin to GSK-3β is via a docking motif. Axin bound GSK-3β phosphorylates β-catenin and controls its turnover. Upon Wnt stimulation, Axin bound GSK-3β is translocated to the plasma membrane (MacDonald,2009). This inhibits the phosphorylation and subsequent degradation of β-catenin. Accumulation of β-catenin then leads to its nuclear entry to modulate transcription. Wnt signaling plays an important role in metazoan development. Studies have shown that the misregulation of this pathway causes serious birth defects such as reduced embryonic axis formation in xenopus ( Hedgepeth,1999) defects in brain formation, bone formation, kidney and lung development in mice and various cancers in human (McCubrey,2014).
ELM Description:
The GSK-3β docking motif in Axin binds as a single amphipathic 3α-helix into the hydrophobic channel formed by GSK-3β. The Axin binding site on GSK-3β is shared by other substrates such as FRAT and GSKIP. But due to the surface flexibility, the motif and binding modes are distinct. The Axin docking is an evolutionary conserved mechanism and the highly conserved motif in vertebrates can be represented as VEP^P.FA..LI^PRL.^P[VIL]. The conserved Phenylalanine is acommodated very well in hydrophobic cavity on GSK-3β and seems very important for the docking interaction (1O9U). Only Alanine is permitted immediately after Phenylalanine as helix takes the turn and long side chains can not be accomodated at this position. Further next two Leucines from the motif also hang in the deep hydrophobic pocket formed by GSK-3β. These residues seem to provide additional specificity to the motif and hence conserved.
Pattern: V[ED]P[^P][RK]FA[^P]ELI[^P]RLE[^P][VIL]
Pattern Probability: 7.238e-16
Present in taxon: Vertebrata
Interaction Domain:
Pkinase (PF00069) Protein kinase domain (Stochiometry: 1 : 1)
o See 6 Instances for DOC_GSK3_Axin_1
o Abstract
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.
o 16 selected references:

o 14 GO-Terms:

o 6 Instances for DOC_GSK3_Axin_1
(click table headers for sorting; Notes column: =Number of Switches, =Number of Interactions)
Acc., Gene-, NameStartEndSubsequenceLogic#Ev.OrganismNotes
P57094 axin1
386 402 HVEPEKFAAELISRLEGVLR TP 1 Danio rerio (Zebrafish)
O70239 Axin1
383 399 RVEPQKFAEELIHRLEAVQR TP 1 Rattus norvegicus (Norway rat)
O42400 AXIN1
383 399 HVEPEKFAAELINRLEEVQK TP 1 Gallus gallus (Chicken)
O35625 Axin1
383 399 RVEPQKFAEELIHRLEAVQR TP 1 Mus musculus (House mouse)
Q9YGY0 axin1
383 399 HVDPEKFAAELISRLEGVLR TP 3 Xenopus laevis (African clawed frog)
O15169 AXIN1
383 399 RVEPQKFAEELIHRLEAVQR TP 4 Homo sapiens (Human)
Please cite: ELM 2016-data update and new functionality of the eukaryotic linear motif resource. (PMID:26615199)

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