Androgen receptor binding to nuclear matrix in vitro and its inhibition by 8S androgen receptor promoting factor

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Abstract
The partially purified 4.5S [3H]dihydrotestosterone receptor binds to nuclear matrix isolated from rat Dunning prostate tumor with properties similar to those reported for androgen receptor binding in intact nuclei in that it requires Zn2+ and mercaptoethanol, is saturable and is temperature dependent and of high affinity (Ka .apprx. 1013 M-1). On 1 mg of DNA equivalent basis, the extent of matrix binding of androgen receptor (700 fmol of receptor bound/mg of matrix protein) is similar to that of intact nuclei, corresponding to .apprx. 1400 sites/nucleus. Association rate constants (ka) for 4.5S androgen recepetor binding to matrix at 0.degree., 15.degree. and 25.degree. C are 2.7 .times. 105, 1.2 .times. 106 and 2.4 .times. 106/M per min, respectively, indicating an energy of activation of 15 kcal/mol. Up to 50% of matrix-bound receptor is extractable in buffer containing 3 mM EDTA plus either 0.4 M KCl or 5 mM pyridoxal 5''-phosphate. A protein fraction designated 8S androgen receptor promoting factor that promotes conversion of the 4.5S androgen receptor to 8 S has been further purified and found to inhibit the binding of the 4.5S androgen receptor to isolated nuclei and nuclear matrix in a concentration-dependent manner. The 8S steroid receptor apparently is a complex of the activated 4.5S androgen receptor with a non-steroid binding protein that renders the receptor incapable of binding in nuclei.