In situ DETECTION OF PROGESTERONE BINDING SITES IN THE PLASMA MEMBRANE OF THE FILAMENTOUS FUNGUS Rhizopus nigricans

In situ DETECTION OF PROGESTERONE BINDING SITES IN THE PLASMA MEMBRANE OF THE FILAMENTOUS FUNGUS Rhizopus nigricans

Nataša Jeraj,^a Rok Romih,^b Helena Lenasi,^a* and Katja Breskvar^a

^aInstitute of Biochemistry, Faculty of Medicine, University of Ljubljana, Vrazov trg 2, 1000 Ljubljana, Slovenia

^b Institute of Cell Biology, Faculty of Medicine, University of Ljubljana

Abstract

Steroid hydroxylating enzymes in the fungus Rhizopus nigricans are induced by progesterone and by some other steroids. It is known that in higher organisms steroids exert their nongenomic action via steroid binding proteins located in the plasma membrane of the cells, thus our aim was to detect progesterone binding sites in R. nigricans plasma membrane. In this report membrane receptors were identified by two independent methods, analysis of progesterone binding to plasma membrane fraction by competitive binding assay using [³H]- and [¹H] progesterone (EC₅₀= 55 nM) and in situ binding of fluorescein isothiocyanate-coupled cell impermeant progesterone conjugate to fungal protoplasts, detected by fluorescence microscopy.