Characterization of plasma membrane fraction from filamentous fungus Rhizopus nigricans

In the filamentous fungus Rhizopus nigricans a steroid hydroxylating multienzyme system is inducible by progesterone and by several other steroids. The biological signal carried by progesterone might be mediated by receptors, located either in the plasma membrane or inside the cell. To elucidate the first possibility, plasma membrane fraction was examined for the presence of progesterone receptors. The isolation of plasma membrane from fungal homogenate containing different other membranes is difficult because of the rigid cell wall. Three different membrane fractions were prepared by differential centrifugation of the fungal homogenate and characterized by plasma membrane and mitochondrial membrane marker enzymes, H⁺-ATPase and mit-ATPase, respectively. The same fractions were examined for the presence of specific progesterone-binding molecules. Two of these fractions comprising the highest level of plasma membrane enzyme activity contained also the highest level of specific progesterone-binding compounds: 27,6 fmol/mg protein and 18,8 fmol/mg protein. The correlation between plasma membrane marker enzyme activity and the amount of progesterone-binding proteins in plasma membrane fraction of Rhizopus nigricans might indicate the involvement of these molecules in the induction process.