TMEM59 potentiates Wnt signaling by promoting signalosome formation

Wnt/β-catenin signaling controls development and adult tissue homeostasis by regulating cell proliferation and cell fate decisions. Wnt binding to its receptors Frizzled (FZD) and low-density lipoprotein-related 6 (LRP6) at the cell surface initiates a signaling cascade that leads to the transcription of Wnt target genes. Upon Wnt binding, the receptors assemble into large complexes called signalosomes that provide a platform for interactions with downstream effector proteins. The molecular basis of signalosome formation and regulation remains elusive, largely due to the lack of tools to analyze its endogenous components. Here, we use internally tagged Wnt3a proteins to isolate and characterize activated, endogenous Wnt receptor complexes by mass spectrometry-based proteomics. We identify the single-span membrane protein TMEM59 as an interactor of FZD and LRP6 and a positive regulator of Wnt signaling. Mechanistically, TMEM59 promotes the formation of multi-meric Wnt–FZD assemblies via intramembrane interactions. Subsequently, these Wnt–FZD–TMEM59 clusters merge with LRP6 to form mature Wnt signalosomes. We conclude that the assembly of multiprotein Wnt signalosomes proceeds along well-ordered steps that involve regulated intramembrane interactions.
This work is part of the Oncode Institute which is partly financed by the Dutch Cancer Society. This work was supported by European Research Council Starting Grant 242958 (to M.M.M.), European Union Grant FP7 Marie Curie ITN 608180 “WntsApp” (to M.M.M.), and the Netherlands Organization for Scientific Research NWO VICI Grant 91815604 and ECHO Grant 711.013.012 (to M.M.M.). T.Y.L. and A.J.R.H. are supported by large-scale proteomics facility Proteins@Work Project 184.032.201 embedded in the Netherlands Proteomics Centre and supported by the Netherlands Organization for Scientific Research. They were additionally supported through European Union Horizon 2020 Program FET-OPEN Project MSmed, Project 686547. F.X.P. was supported by Grants SAF2014-53320R and SAF2017-88390R from the Spanish Government.