Steroid membrane receptor. Signals upon progestin binding, resulting in rapid activation of MAPK and down-regulation of adenylyl cyclase activity. Interacts with steroids with varying degrees of affinity, showing specificity for activation by the maturation-inducing steroid (MIS) 4-pregnen-17,20beta-diol-3-one (17,20beta-DHP). Capable of mediating progestin-induced oocyte maturation. In oocytes, expression is lowest in ovarian follicles at stages I and II but gradually increases after the onset of vitellogenic growth, peaking at vitellogenic stages (stage IIIB). Belongs to the ADIPOR family.