The putative molybdate-stabilized progesterone receptor subunit is not a steroid-binding protein.

Several reports have appeared recently which contradict our earlier findings on the identity and characterization of chick progesterone receptor. Puri et al. (Puri, R. K., Grandics, P., Dougherty, J. J., and Toft, D. O. (1982) J. Biol. Chem. 257, 10831-10837) have isolated a protein with a molecular weight of Mr approximately 88,000 and have reported that this protein is the "nontransformed" molybdate-stabilized receptor. Because of this discrepancy from our own work, we have repeated their methods and obtained purified samples of this protein. In this paper we show that the Mr = 88,000 +/- 2,000 protein is not a receptor but is apparently an artifact of the use of particular steroid affinity resins. Evidence presented to show that this protein is not the authentic receptor include: 1) it is obtained from affinity resins in 10-fold molar excess over the mass of authentic receptor; 2) its tryptic map does not resemble either authentic receptor subunit; 3) it can be resolved from authentic hormone-binding receptors on DEAE-Sephadex A-25 chromatography; 4) the protein does not bind progesterone; and finally 5) its elution from affinity matrices is not biospecific for progesterone and can even be eluted in equivalent yields with buffer alone. We conclude that the Mr = 88,000 protein is not a progesterone-binding protein.