The Eukaryotic Linear Motif resource for
Functional Sites in Proteins
Accession:
Functional site class:
GSK-3 binding motif in LRP6 and LRP5
Functional site description:
Inhibition of the GSK-3 kinase through pseudo-substrate binding to phosphorylated motifs on the transmembrane receptor LRP6 is a crucial step in the canonical Wnt signaling pathway. Inhibited GSK-3 fails to phosphorylate β-catenin, disabling its degradation via the β-TrCP E3 ligase and the proteosomal machinery. This finally leads to accumulation of β-catenin in the cytoplasm, which subsequently translocates to the nucleus where it regulates transcription factors (TCF) and consequently expression of genes controlling proliferation and cell cycle progression. In the presence of Wnt ligands the pathway is activated and a complex of the membrane receptors Frizzled and LDL receptor related protein 6 (LRP6) is formed. This triggers the phosphorylation of the LRP6 intracellular domain at five conserved PPPSP motifs. It is also known that phosphorylation of these motifs occurs sequentially by GSK-3 and CK1.
ELM Description:
Upon activation of the Wnt pathway in the presence of Wnt ligands, a complex of the membrane receptors Frizzled and LDL receptor related proteins 6 (LRP6, also LRP5) is formed. This triggers the phosphorylation of the LRP6 intracellular domain at five strongly conserved PPPSP motif repeats. In the phosphorylated state, these motifs are able to bind to GSK3, thereby inhibiting GSK3 phosphorylation activity. A second Thr/Ser is almost always present at +2 after the PPPSP motif. Therefore the motif is often called PPPSPxS in the literature. The phosphorylation of both S/T residues is considered a requirement for biological activity, however the first phosphosite is sufficient for direct GSK3 inhibition by LRP6. This phosphate makes strong charged contacts to two arginines and a lysine. Using peptides derived from the LRP6 PPPSP motifs, it was shown that peptide binding to GSK3 results in drastic conformational changes of the highly conserved C-loop, resulting in its movement towards the C-terminal lobe of GSK3 while fastening the peptides in a clamp-like structure (4NM5, 4NM7, Stamos,2014). As well as the phosphate contacts, the peptide path is anchored by some backbone H-bonding, placing the third Pro of the motif deepest into the groove, making tight hydrophobic contacts. The second PPPSP motif has a Thr in the third position while the fourth PPPSP has a Cys in the first position. The regular expression for the motif in ELM captures these variants.
Pattern: (([CP]PP)|(PP[TP]))[ST]P[^P][TS]{0,1}
Pattern Probability: 0.0000373
Present in taxon: Metazoa
Interaction Domain:
Pkinase (PF00069) Protein kinase domain (Stochiometry: 1 : 1)
o See 8 Instances for LIG_GSK3_LRP6_1
o Abstract
The canonical Wnt signalling pathway plays a key role during embryonic development and is also crucial for stem cell renewal and tissue homeostasis in adults. Misregulations in the Wnt pathway are known to be involved in multiple diseases including cancers. (Dees,2014) In the absence of secreted Wnt ligands, β-catenin is downregulated by the APC/Axin1 destruction complex. The destruction complex is comprised of glycogen synthase kinase 3β (GSK-3β), casein kinase 1 (CK1), the Adenomatous Polyposis Coli (APC) tumour suppressor protein and the scaffolding protein Axin (Dees,2014).

The phosphorylation of β-catenin takes place sequentially by CK1 at Ser45, which is followed by GSK-3 phosphorylation at Thr41, Ser37 and Ser33 (Wu,2009). Among these residues, Ser33 and Ser37 constitute the key recognition sites of the degron (Stamos,2013). In short, the phosphorylation of β-catenin on Ser33 and Ser37 generates a DSGxxS phosphodegron, which enables it to be targeted by the SCF β-TrCP E3 ubiquitin ligase complex. Once β-catenin is ubiquitinylated, it is degraded by the proteosomal machinery (Wu,2009).

Upon activation of the Wnt pathway in the presence of Wnt ligands, a complex of the membrane receptors Frizzled and LDL receptor related protein 6 (LRP6) is formed. This triggers the phosphorylation of the LRP6 intracellular domain at five conserved PPPSP motifs. Phosphorylation of these motifs occurs sequentially by GSK-3 and CK1 (Zeng,2005). The phosphorylation of both S/T residue of the PPPSP motif is a requirement for biological activity, however the first site is sufficient for GSK-3 inhibition by LRP6 (Stamos,2014). One model suggests that the phosphorylation of LRP6 at these motifs enables Axin to bind in a pseudo-substrate manner, thereby recruiting its associated proteins (including GSK-3) to LRP6 leading to the indirect inhibition of β-catenin phosphorylation. (Wu,2009) More recent studies suggest that phosphorylated LRP6 inhibits GSK-3 directly by recruiting it as a pseudo-substrate (Stamos,2014).

The inhibition of β-catenin phosphorylation terminates its degradation and leads to the accumulation of unphosphorylated β-catenin in the cytoplasm. Subsequently it translocates to the nucleus where it regulates transcription factors (TCF) and expression of genes controlling proliferation and cell cycle progression (Wolf,2008).
o 7 selected references:

o 13 GO-Terms:

o 8 Instances for LIG_GSK3_LRP6_1
(click table headers for sorting; Notes column: =Number of Switches, =Number of Interactions)
Acc., Gene-, NameStartEndSubsequenceLogic#Ev.OrganismNotes
A0A0B8RSN6 LRP6
A0A0B8RSN6_PIG
1492 1497 YFPAILNPPPSPATERSHYT TP 1 Sus scrofa domesticus (Domestic pig)
1 
O88572 Lrp6
LRP6_MOUSE
1527 1532 YSYRHFAPPTTPCSTDVCDS TP 1 Mus musculus (House mouse)
1 
F4X5W2 G5I_13753
F4X5W2_ACREC
1512 1517 YPQETLNPPPSPATIVSSTR TP 1 Acromyrmex echinatior (Panamanian leafcutter ant)
1 
O75581 LRP6
LRP6_HUMAN
1604 1609 ERSYSHHLYPPPPSPCTDSS TP 4 Homo sapiens (Human)
1 
O75581 LRP6
LRP6_HUMAN
1587 1592 AEENYESCPPSPYTERSYSH TP 3 Homo sapiens (Human)
1 
O75581 LRP6
LRP6_HUMAN
1569 1574 YDSEPVPPPPTPRSQYLSAE TP 4 Homo sapiens (Human)
1 
O75581 LRP6
LRP6_HUMAN
1527 1532 YSYRHFAPPTTPCSTDVCDS TP 3 Homo sapiens (Human)
1 
O75581 LRP6
LRP6_HUMAN
1487 1492 YFPAILNPPPSPATERSHYT TP 3 Homo sapiens (Human)
1 
Please cite: ELM 2016-data update and new functionality of the eukaryotic linear motif resource. (PMID:26615199)

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