Identification of a novel sex steroid binding protein

We describe a cytoplasmic steroid binding protein in the chick oviduct which has intriguing characteristics. This protein binds [3H]estradiol with high affinity (Kd = 30 X 10(-9) M) and limited capacity (300 fmol/mg of cytosol protein). It sediments at 8 S in low-salt sucrose density gradients and at 4 S in high-salt gradients. Unlike the estrogen receptor, however, this protein also binds progesterone, R5020, testosterone, and 5 alpha-dihydrotestosterone with similar affinities and in a competitive manner. Moreover, it is not translocated to the nucleus by the in vivo administration of these sex steroids. The protein is only present in estrogen-responsive tissue, and like the sex steroid receptors, its synthesis appears to be regulated by estrogen.