Membrane-bound progesterone receptors coupled to G proteins in the fungusRhizopus nigricans

Abstract

Steroid binding sites with high affinity for progesterone (Kd=40+/-14 nM determined by binding, and Kd=71+/-22 nM determined by displacement studies) and lower affinity for 21-hydroxyprogesterone and for testosterone, but no affinity for estradiol-17beta, onapristone and alpha-naphthoflavone were detected in the enriched plasma membrane fraction of the fungus Rhizopus nigricans. The amount of steroid binding sites is in accordance with the value of B(max)=744+/-151 fmol (mg protein)(-1). In the membrane fraction, progesterone induced about 30% activation of G proteins over basal level, as determined by GTPase activity (EC50=32+/-8 nM) and by the guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding rate (EC50=61+/-21 nM). The affinity of receptors for progesterone was substantially decreased in the presence of GTPgammaS and of cholera toxin. Our results suggest the existence of progesterone receptors in the membrane of Rhizopus nigricans and their coupling to G proteins.

Elicitation of plants and microbial cell systems

“…The factors identified in fungi which could be implicated in the signal transduction are listed here and a tentative mechanism is proposed ( Figure 1) : $ Calmodulins have been identified in many fungi and yeasts [119,120] $ Cell membrane receptors [121,122] $ K + and Cl − signalling has been established in yeast and in the filamentous fungus Neurospora crassa [123] $ Fungal vacuoles appear to be the major storage organelles for Ca 2 + and several other bivalent cations such as Mn 2 + and Zn 2 + [124] $ Signal transduction through MAPK has been identified in yeast [125] $ The cyclic nucleotides cAMP [126] and cGMP), with Ca 2 + , were the first recognized secondary messengers which modulate eukaryotic cell function [127]. These nucleotides have been detected in fungi [124,128] $ G-protein-linked receptor systems have been identified in filamentous fungi [120,129] $ ROS [130] $ Ins(3,4,5)P 3 stimulates Ca 2 + release from vacuoles of Neurospora crassa [131] $ Hormone signalling system [132] $ The existence of multiple subspecies of protein kinase C (PKC) has been demonstrated in N. crassa [133] and Saccharomyces cerevisiae [134] On the basis of the limited knowledge available so far, a preliminary picture may be drawn that could be used as a base for future investigations into the mechanism of elicitation in fungi (Figure 1) : when the elicitor comes into contact with the receptor, the first response observed is a change in the flux of ions across the cell membrane. An efflux of Cl − and K + and an intake of Ca 2 + occur.…”

“… Calmodulins have been identified in many fungi and yeasts [119, 120] Cell membrane receptors [121, 122] K + and Cl − signalling has been established in yeast and in the filamentous fungus Neurospora crassa [123] Fungal vacuoles appear to be the major storage organelles for Ca 2+ and several other bivalent cations such as Mn 2+ and Zn 2+ [124] Signal transduction through MAPK has been identified in yeast [125] The cyclic nucleotides cAMP [126] and cGMP), with Ca 2+ , were the first recognized secondary messengers which modulate eukaryotic cell function [127]. These nucleotides have been detected in fungi [124, 128] G‐protein‐linked receptor systems have been identified in filamentous fungi [120, 129] ROS [130] Ins(3,4,5) P 3 stimulates Ca 2+ release from vacuoles of Neurospora crassa [131] Hormone signalling system [132] The existence of multiple subspecies of protein kinase C (PKC) has been demonstrated in N. crassa [133] and Saccharomyces cerevisiae [134] …”

Antifungal Activity of Soft Tissue Extract from the Garden Snail Helix aspersa (Gastropoda, Mollusca)

“…Progesterone is a steroid hormone that functions as a substantial metabolic intermediate during corticosteroid and sex hormone production and also functions as a neurosteroid [ 35 , 36 ]. The antifungal effects of this compound are facilitated by the presence of high-affinity progesterone-binding sites in the plasma membrane, as confirmed by [ 37 ] in Rhizopus nigricans . A corticosteroid-binding protein and steroid receptor were identified by [ 38 ] in C. albicans [ 39 ] as well as other fungal species.…”

Characterization and ligand identification of a membrane progesterone receptor in fungi: existence of a novel PAQR in Sporothrix schenckii

“…Many studies have predicted the existence of a progesterone receptor in the membrane of filamentous fungi such as Rhizopus nigricans [27-30] but the molecular basis of steroid signalling in fungi remains unresolved [43,44]. Progesterone has been reported to bind to enriched plasma membrane fractions of R. nigricans with high affinity and this hormone has been reported to induce an activation of G proteins that decreases in the presence of cholera toxin [29]. Nevertheless, to date no progesterone receptor has been directly identified in this or any other fungi.…”