The canonical Wnt-β-catenin signaling pathway is initiated by induction of phosphorylation of one of the Wnt receptors, low density lipoprotein receptor-related protein (LRP) 5/6, at Thr 1479 and Ser 1490 . We identified, by screening a human kinase siRNA library, phosphatidylinositol 4-kinase type II (PI4KII) α and phosphatidylinositol-4-phosphate 5-kinase type I (PIP5KI) as required for Wnt3a-induced LRP6 phosphorylation at Ser 1490 in mammalian cells and confirmed that these kinases are important for Wnt signaling in Xenopus embryos. Wnt3a stimulates the formation of phosphatidylinositol 4,5-bisphosphates [PtdIns (4,5)P 2 ] through frizzled (Fz) and dishevelled (Dvl), the latter of which directly interacted with and activated PIP5KI. PtdIns (4,5)P 2 in turn regulated phosphorylation of LRP6 at Thr 1479 and Ser 1490 . Therefore, our study reveals a new signaling mechanism for Wnt to regulate LRP6 phosphorylation.