Is a positive regulator of the BMP signaling pathway, required for proper chondrogenesis, osteogenesis and skeletal development. It acts as coreceptor for BMP ligands, particularly BMP2 and BMP4, facilitating their interactions with BMP type I receptors (PubMed:33308444). It is required for ligand-induced recruitment of BMP receptors to lipid rafts (By similarity). Binds to TGFBR2 and activates TGFB signaling. In lung cancer cells, could serve as an endogenous autocrine and paracrine ligand of TGFBR2, which could regulate TGFBR2 signaling and hence modulate epithelial-mesenchymal transition and cancer progression. Forms homooligomers (PubMed:33308444). Forms heterooligomers with SCUBE1 and SCUBE2 (PubMed:33308444). Interacts with TGFBR2 through the CUB domain; this interaction does not affect TGFB1-binding to TGFBR2. Interacts with BMP2, BMP4 and BMP7; the interaction is mediated by the CUB domain (PubMed:33308444). Interacts with BMPR1A, BMPR1B and BMPR2; the interaction with BMPR1A and BMPR1B is BMP2- and BMP4-dependent (PubMed:33308444). Highly expressed in osteoblasts. In normal lung, mainly expressed in bronchial epithelial cells. Tends to be up-regulated in lung cancer cells. N-glycosylated. Proteolytic cleavage produces a CUB-containing C-terminal fragment that retains the ability to bind to TGFBR2. This reaction is catalyzed in vitro by MMP2 and, to a lesser extent, by MMP9. The disease is caused by variants affecting the gene represented in this entry. Truncated N-terminus.