Progesterone receptor membrane component-1 (PGRMC1) and its binding partner, plasminogen activator inhibitor 1 RNA binding protein (PAIRBP1) are thought to form a complex that functions as membrane receptor for progesterone (P4). The present investigations confirm PGRMC1's role in this membrane receptor complex by demonstrating that depleting PGMRC1 with PGRMC1 siRNA results in a 60% decline in ³H-P4 binding and the loss of P4's anti-apoptotic action. Studies conducted on partially purified GFP-PGRMC1 fusion protein indicate that ³H-P4 specifically binds to PGRMC1 at a single site with an apparent K_d of 35 nM. In addition experiments utilizing various deletion mutations reveal that the entire PGRMC1 molecule is required for maximum ³H-P4 binding and P4 responsiveness. Analysis of the binding data also suggests that the P4 binding site is within a segment of PGRMC1 that is composed of the transmembrane domain and the initial segment of the C-terminus. Interestingly, PAIRBP1 appears to bind to the C-terminus between amino acids 70-130, which is distal to the putative P4 binding site. Taken together, these data provide compelling evidence that PGRMC1 is the P4 binding protein that mediates P4's anti-apoptotic action. Moreover, the deletion mutation studies indicate that each domain of PGRMC1 plays an essential role in modulating PGRMC1's capacity to both bind and respond to P4. Additional studies are required to more precisely delineate the role of each PGRMC1 domain in transducting P4's anti-apoptotic action.