Component of a progesterone-binding protein complex. Binds progesterone. Has many reported cellular functions (heme homeostasis, interaction with CYPs). Required for the maintenance of uterine histoarchitecture and normal female reproductive lifespan. Intracellular heme chaperone. Regulates heme synthesis via interactions with FECH and acts as a heme donor for at least some hemoproteins. Homodimer. Forms stable homodimer through hydrophobic heme-heme stacking interactions. Interacts with FECH; the interaction results in decreased FECH activity (By similarity). Interacts with EGFR, CYP1A1 and CYP3A4; the interactions require PGRMC1 homodimerization (By similarity). The cytochrome b5 heme-binding domain lacks the conserved iron-binding His residues at positions 106 and 130. Non-classical progesterone receptors involved in extranuclear signaling are classified in 2 groups: the class II progestin and adipoQ receptor (PAQR) family (also called mPRs) (PAQR5, PAQR6, PAQR7, PAQR8 and PAQR9) and the b5-like heme/steroid-binding protein family (also called MAPRs) (PGRMC1, PGRMC2, NENF and CYB5D2). Belongs to the cytochrome b5 family. MAPR subfamily.