Phosphorylation of XIAP at threonine 180 controls its activity in Wnt signaling

X-linked inhibitor of apoptosis (XIAP) plays an important role in preventing apoptotic cell death. XIAP has been shown to participate in signaling pathways, including Wnt signaling. XIAP regulates Wnt signaling by promoting the monoubiquitylation of the co-repressor Groucho/TLE family proteins, decreasing its affinity for the TCF/Lef family of transcription factors and allowing assembly of transcriptionally active beta-catenin-TCF/Lef complexes. We now demonstrate that XIAP is phosphorylated by GSK3 at threonine 180, and that an alanine mutant (XIAPT(180A)) exhibits decreased Wnt activity compared to wild-type XIAP in cultured human cells and in Xenopus embryos. Although XIAPT(180A) ubiquitylates TLE3 at wildtype levels in vitro, it exhibits a reduced capacity to ubiquitylate and bind TLE3 in human cells. XIAPT(180A) binds Smac (also known as DIABLO) and inhibits Fas-induced apoptosis to a similar degree to wild-type XIAP. Our studies uncover a new mechanism by which XIAP is specifically directed towards a Wnt signaling function versus its anti-apoptotic function. These findings have implications for development of anti-XIAP therapeutics for human cancers.