Endosomal ferrireductase required for efficient transferrin-dependent iron uptake in erythroid cells. Participates in erythroid iron homeostasis by reducing Fe(3+) to Fe(2+). Also mediates reduction of Cu(2+) to Cu(1+), suggesting that it participates in copper homeostasis. Uses NADP(+) as acceptor. May play a role downstream of p53/TP53 to interface apoptosis and cell cycle progression. Indirectly involved in exosome secretion by facilitating the secretion of proteins such as TCTP. Homodimer. Interacts with BNIP3L, MYT1, RHBDL4/RHBDD1 and TCTP (By similarity). Highly expressed in fetal liver (the site of midgestational hematopoiesis). Proteolytically cleaved by RHBDL4/RHBDD1. RHBDL4/RHBDD1-induced cleavage occurs at multiple sites in a glycosylation-independent manner (By similarity). Glycosylated. Mice display iron deficiency anemia, due to a defect in iron release through the transferrin cycle. Belongs to the STEAP family.