Acts as a bone morphogenetic protein (BMP) coreceptor. Through enhancement of BMP signaling regulates hepcidin (HAMP) expression and regulates iron homeostasis. Interacts with BMP2 and BMP4 (By similarity). Interacts with BMP6 (By similarity). Interacts with BMPR1B. Interacts with TMPRSS6 (By similarity). Also released in the extracellular space. Autocatalytically cleaved at low pH; the two chains remain linked via two disulfide bonds. Also proteolytically processed by TMPRSS6, several fragments being released in the extracellular space; regulates HJV activity in BMP signaling and thefore iron homeostasis. Belongs to the repulsive guidance molecule (RGM) family. Was originally (Ref.1) thought to originate from human.