ISOPRENOID-MEDIATED CHANGES IN THE GLYCEROPHOSPHOLIPID MOLECULAR-SPECIES OF THE STEROL AUXOTROPHIC FUNGUS LAGENIDIUM-GIGANTEUM

The mosquito pathogenic fungus Lagenidium giganteum (Oomycetes: Lageni diales) is a sterol auxotroph that can grow vegetatively in the absenc e of these compounds, but requires an exogenous source of sterols to e nter its sexual and asexual reproductive cycles. Electrospray mass spe ctrometry (MS) and electrospray MS/MS were used to examine three major glycerophospholipid molecular species - glycerophosphocholine (GPC), glycerophosphoethanolamine (GPE) and glycerophosphoinositol (GPI) - fr om fungal mycelium and nuclei grown in defined medium with and without isoprenoids which induce (cholesterol and ergosterol) or do not induc e (squalene, cholestane) reproduction. Testosterone supplementation of defined media inhibited growth of L. giganteum, so the effect of this steroid on phospholipid metabolism could not be assessed. Mycelium gr own in defined media supplemented with these isoprenoids produced sign ificantly different quantities of total phospholipid relative to unsup plemented media and to each other, ranging from a mean of 292 mu g pho sphate per g wet weight for cholesterol-supplemented media to 56 mu g phosphate per g wet weight for mycelium grown in the presence of squal ene. A very large percentage of the GPC (69-80 mol%) and GPI (74-79 mo l%) molecular species from mycelia and nuclei contained ether linkages . GPE molecular species had 13-20 mol% ether-containing moieties. The elevated levels of ether lipids may be related to the sterol auxotroph ic nature of the fungus. Isoprenoid supplementation of defined growth media resulted in many significant changes in molecular species for al l three lipid classes. Significant differences (P < 0.05) in the perce ntage of total cell ether lipids in GPC and GPE were generated by isop renoid supplements to culture media. Mycelium grown in the presence of the two sterols which induce asexual and sexual reproduction in L. gi ganteum, cholesterol and ergosterol, had a significantly greater perce ntage of ether-containing GPE moieties. The glycerolipid species from nuclei isolated from cultures grown with cholesterol and ergosterol we re similar to the composition of nuclei isolated from fungus cultured in defined medium without any supplement or supplemented with squalene . The nuclear membrane from mycelia grown in cholestane-supplemented m edia, however, had a very different glycerophospholipid composition re lative to either whole cells or nuclei from cells grown on other media . It appears that one of the reasons that cyclic isoprenoids such as c holestane do not induce fungal reproduction is that they drastically a lter the nuclear membrane glyceraphospholipid composition.