Receptor for Dickkopf proteins. Cooperates with DKK1/2 proteins to inhibit Wnt/beta-catenin signaling by promoting the endocytosis of Wnt receptors LRP5 and LRP6. In the absence of DKK1, potentiates Wnt-beta-catenin signaling by maintaining LRP5 or LRP6 at the cell membrane (PubMed:17978005, PubMed:18314504). Interacts with lrp6.