cholesterol synthesis. Thus, PGRMC1 and its homologues bind and regulate P450 proteins, and it has been likened to “a helping hand for P450 proteins”. Roles in signaling and apoptosis The yeast PGRMC1 homologue is required for resistance to damage. PGRMC1 also promotes survival in human cancer cells after treatment with chemotherapy. In contrast, PGRMC1 promotes cell death in cancer cells after oxidative damage. PGRMC1 alters several known survival signaling proteins, including the Akt protein kinase and the cell death-associated protein IκB. Progesterone inhibits apoptosis in immortalized granulosa cells, and this activity requires PGRMC1 and its binding partner, PAIR-BP1 (plasminogen activator inhibitor RNA-binding protein-1). However, PAIR-BP1 is not a progesterone binding protein, and the component of the PGRMC1 complex that binds to progesterone is unknown. PGRMC1 was originally thought to represent a progesterone receptor of some sort and to bind to progesterone, but subsequently thought has moved towards PGRMC1 acting as a downstream mediator of some other progesterone-binding protein. See also PGRMC PGRMC2 Membrane progesterone receptor References