Transmembrane protein which can modulate activity of various signaling pathways, probably via binding to integrin ITGAV:ITGB3. Mediates heterophilic cell-cell interactions in vitro. Inhibits osteoclastogenesis downstream of TNFSF11/RANKL and CSF1, where it may function by attenuating signaling via integrin ITGB3 and MAP kinase p38. Plays a role in cartilage formation where it promotes proliferation and maturation of growth plate chondrocytes. Stimulates formation of primary cilia in chondrocytes. Enhances expression of genes involved in the hedgehog signaling pathway in chondrocytes, including the hedgehog signaling molecule IHH; may also promote signaling via the PTHLH/PTHrP pathway. Plays a role in angiogenesis where it suppresses migration of endothelial cells and also promotes their apoptosis. Inhibits VEGF-induced activation of AKT and p38 MAP kinase in endothelial cells. Also inhibits VTN (vitronectin)-mediated integrin ITGAV:ITGB3 signaling and activation of PTK2/FAK. May play a role in the maturation and maintenance of the blood-brain barrier.; (Microbial infection) Contributes to arthritogenic alphavirus pathogenesis and acts as a receptor for these viruses.