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 (PubMed:16604073). Interacts with BMP6 (PubMed:19252486). Interacts with BMPR1B (By similarity). Interacts with TMPRSS6 (By similarity). Also released in the extracellular space. Muscle cell lineage. Expressed at low levels in somite derived structures. In the developing eye, marked the ocular musculature. Expressed in all differentiating muscles of the limb and the body wall, but not in migrating muscle precursor cells. Not detected in the nervous system, either at 9.5 dpc or at any stage later during development. 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.