Phase separation of Axin organizes the β-catenin destruction complex

Abstract

In Wnt/β-catenin signaling, the β-catenin protein level is deliberately controlled by the assembly of the multiprotein β-catenin destruction complex composed of Axin, adenomatous polyposis coli (APC), glycogen synthase kinase 3β (GSK3β), casein kinase 1α (CK1α), and others. Here we provide compelling evidence that formation of the destruction complex is driven by protein liquid–liquid phase separation (LLPS) of Axin. An intrinsically disordered region in Axin plays an important role in driving its LLPS. Phase-separated Axin provides a scaffold for recruiting GSK3β, CK1α, and β-catenin. APC also undergoes LLPS in vitro and enhances the size and dynamics of Axin phase droplets. The LLPS-driven assembly of the destruction complex facilitates β-catenin phosphorylation by GSK3β and is critical for the regulation of β-catenin protein stability and thus Wnt/β-catenin signaling.

Reconstitution of the destruction complex defines roles of AXIN polymers and APC in β-catenin capture, phosphorylation, and ubiquitylation

“…The dominance of events below 0.2 MDa indicated DC dissociation upon dilution for mass photometry (<100 nM) ( Sonn-Segev et al., 2020 ), even within the short time frame (≈5 min). This generated multiple lower molecular weight events from the parental complex, in line with the dynamic, concentration-dependent assembly of the DC ( Barua and Hlavacek, 2013 ; Nong et al., 2021 ; Pronobis et al., 2017 ; Schaefer and Peifer, 2019 ). Nonetheless, we still detected high-molecular weight species ( Figure 1 F).…”

Liquid-liquid phase separation: Orchestrating cell signaling through time and space

“…In the absence of Wnt ligands, b-catenin is basally degraded by the degradosome complex (Gammons and Bienz, 2018). The degradosome is assembled through LLPS of Axin (Nong et al, 2021), which serves as a scaffold to recruit downstream components such as adenomatous polyposis coli (APC) and glycogen synthase kinase 3b (GSK3b) (Figure 4). Cytosolic b-catenin functions as a degradosome client protein, partitioning into this phase-separated compartment through interactions with APC, which enhances phosphorylation by GSK3b (Nong et al, 2021).…”

“…The degradosome is assembled through LLPS of Axin (Nong et al, 2021), which serves as a scaffold to recruit downstream components such as adenomatous polyposis coli (APC) and glycogen synthase kinase 3b (GSK3b) (Figure 4). Cytosolic b-catenin functions as a degradosome client protein, partitioning into this phase-separated compartment through interactions with APC, which enhances phosphorylation by GSK3b (Nong et al, 2021). LLPS thus promotes efficient phosphorylation and degradation of b-catenin to suppress Wnt signaling at the basal state (Nong et al, 2021).…”

“…Cytosolic b-catenin functions as a degradosome client protein, partitioning into this phase-separated compartment through interactions with APC, which enhances phosphorylation by GSK3b (Nong et al, 2021). LLPS thus promotes efficient phosphorylation and degradation of b-catenin to suppress Wnt signaling at the basal state (Nong et al, 2021). APC also enhances LLPS of the degradosome (Nong et al, 2021), potentially through self-oligomerization and multivalent interactions with Axin (Kunttas-Tatli et al, 2014;Schaefer et al, 2018).…”

“…LLPS thus promotes efficient phosphorylation and degradation of b-catenin to suppress Wnt signaling at the basal state (Nong et al, 2021). APC also enhances LLPS of the degradosome (Nong et al, 2021), potentially through self-oligomerization and multivalent interactions with Axin (Kunttas-Tatli et al, 2014;Schaefer et al, 2018). Disrupting Axin-mediated LLPS impedes normal cell development (Nong et al, 2021).…”