IDENTIFICATION OF PROGESTERONE BINDING-SITES IN THE PLASMA-MEMBRANE OF THE FILAMENTOUS FUNGUS COCHLIOBOLUS-LUNATUS

Plasma membrane associated binding sites for progesterone have been id entified in the filamentous fungus Cochlioholus lunatus (C. lunatus). The K(d) for progesterone determined by Scatchard analysis was 13.9 +/ - 5.7 nM and the B(max) was 250-360 fmol/mg protein. A broad ligand sp ecificity of these binding sites is suggested by the observation that all tested steroids, regardless of their capability to act as inducers of the 11beta-steroid hydroxylase, competed at 250-fold excess with [ H-3]progesterone binding. A biological role of these plasma membrane a ssociated steroid binding sites is nevertheless suggested since in pro toplasts which were devoid of them, 11beta-steroid hydroxylase could n ot be induced. Progesterone binding sites were present in the plasma m embrane as well as in the cytosol and were detected in this fraction, in contrast to the plasma membrane fraction, only under special experi mental conditions in respect to redox state. K(d) and B(max) of cytoso l binding sites were of the same order of magnitude compared to the pl asma membrane progesterone binding sites. Ethisterone and 4-cholesten- 3-one which cannot induce 11beta-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, a s antagonist in the induction of 11beta-steroid hydroxylase in C. luna tus. On the basis of presented evidence we concluded that C. lunatus c ontains binding sites for steroids in the plasma membrane and in the c ytosol ard that both types of binding site are involved in the process of induction of enzymes which transform steroids in this fungus.