Study of the transcortin binding to human endometrium plasma membrane

Transcortin complexed with progesterone was shown to bind specifically to the plasma membrane of human decidual endometrium. The binding reaction was characterized by a high affinity (an apparent Kd value was (1.0 +/- 0.2).10(-10) mol/l) and high selectivity: such human serum proteins as albumin, orosomucoid, transferrin, thyroxine-binding globulin and sex hormone-binding globulin did not compete with transcortin for the membrane binding sites. Transcortin binding to the membrane was steroid-dependent: transcortin-cortisol complex bound to the membranes substantially more weakly than transcortin-progesterone, and specific binding of transcortin devoid of steroid was not detected. Using a radioimmunoassay, we have measured the concentration of endogenous transcortin in highly purified membrane preparations solubilized with sodium cholate. It was found that an extensive washing of decidual strips with a physiological buffer prior to the membrane isolation resulted in a decrease of the endogenous transcortin level along with an increase of the specific membrane binding of exogenous 125I-labeled transcortin. Affinity chromatography on immobilized transcortin was used to isolate transcortin-binding components from 125I-labeled, cholate-solubilized plasma membrane of decidual endometrium. Along with lipid components, the structure of which was not investigated, a 125I-labeled transcortin-binding sialoglycoprotein with a minimal Mr of 20.0 +/- 1.5 kDa and a pI of approx. 3.3 was detected. In the presence of transcortin, this sialoglycoprotein could be precipitated with a monospecific antitranscortin antiserum. Using hydroxylapatite as a separating agent, the interaction of transcortin and the membrane sialoglycoprotein in model systems containing the two proteins and various steroid hormones was studied. It was found that the membrane sialoglycoprotein displayed a higher affinity for transcortin-progesterone than for transcortin-cortisol (the Kd values were, respectively, 2.10(-11) and 7.10(-11) mol/l) and it did not bind transcortin complexed with testosterone.