A corticosteroid binding protein and endogenous ligand in C. albicans indicating a possible steroid-receptor system

Abstract

The evolution of endocrine systems has been intensively studied (for review see ref. 1). Recent findings indicate the existence of vertebrate-like hormones in primitive species including fungi and unicellular eukaryotes^2–4. Although proteins that bind vertebrate steroids have been described in at least one lower organism⁵, the presence of recognition molecules analogous to hormone receptors in higher organisms has not previously been demonstrated. We have investigated Candida albicans and now report the existence of a receptor-like protein macromolecule in the cytosol of this simple yeast which binds vertebrate corticosteroids with high affinity, selectivity and stereospecificity. In addition, a lipid-extractable material present in the yeast cells and released into the chemically defined growth medium reversibly competes for ³H-corticosterone binding sites in yeast cytosol and may represent an endogenous ligand. Of interest is the finding that the yeast extract also competes for mammalian glucocorticoid receptors. The data suggest that the evolutionary origin of hormonal steroid systems may go back at least as far as simple unicellular eukaryotes. Furthermore, the demonstration of interactions between mammalian steroids and the yeast binder and between the yeast ligand and mammalian receptors suggests the possibility of important clinical consequences.