Plasma membrane associated binding sites for progesterone have been identifiedin the filamentous fungus Cochliobolus lunatus (C. lunatus). The Kd for prosterone determined by Scatchard analysis was ...13.9 + 5.7 nM and the Bmaxwas 250-360 fmol/mg protein. A broad ligand specificity of these binding sites is suggested by the observation that all tested steroids, regardless of their capability to act as inducers of the 11á-steroid hydroxylase, competed at 250-fold excess with (3H)progesterone binding. A biological role of these plasma membrane associated steroid binding sites is nevertheless suggested since in protoplasts which were devoid of them, 11á-steroid hydroxylase could not be induced. Progesterone binding sites were present in the plasma membraneas well as in the cytosol and were detected in this fraction, in contrast to the plasma membrane fraction, only under special experimental conditions in respect to redox state. Kd and Bmax of cytosol binding sites were of the same order of magnitude compared to the plasrna membrane progesterone binding sites. Ethisterone and 4-cholesten-3one which cannot induce 11á-hydroxylase competed efficiently for plasma membrane binding sites;ethisterone, however also competed for cytosol binding sites and acted, in contrast with 4-cholesten-3-one, as antagonist in the induction of 11á-steroid hydroxylase in C. lunatus. On the basis of presented evidence we concluded that C. lunatus contains binding sites for steroids in the plasma membrane and in the cytosol and that both types of binding site are involved in the process of induction of enzymes which transform steroids in this fungus.
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