Novel GTP-binding proteins in plasma membranes of the fungus Metarhizium anisopliae

We report the existence of several families of GTP-binding proteins in plasma membranes of Metarhizium anisopliae. Two proteins (18.4 and 24 kDa) resemble mammalian Gn-proteins in their being toxin insensitive, binding [alpha-32P]GTP on nitrocellulose blots of sodium dodecyl sulfate (SDS)-polyacrylamide gels, and also in their immunological properties. Four other proteins (31-38.2 kDa) were similar except that they did not bind [alpha-32P]GTP after treatment with sodium dodecyl sulfate. An 18.2 kDa cholera toxin substrate and three toxin insensitive bands (18.6, 18.8, and 24 kDa) are novel proteins antigenically related both to mammalian G-proteins and ras gene products. An additional 23 kDa pertussis toxin substrate (the major G-protein in a crude mycelial extract) reacted strongly with antisera to G-proteins but not with anti-ras serum. Other substrates ADP ribosylated by cholera toxin or botulinum D toxin were immunologically unreactive. Analysis of the structural and functional characteristics of these multiple GTP-binding proteins will promote a better understanding of signal transduction in fungi.