ABSTRACT
Growth of the yeast species Candida albicans and Pichia pastoris is inhibited by RsAFP2, a plant defensin isolated from radish seed (Raphanus sativus), at micromolar concentrations. In contrast, gcs-deletion mutants of both yeast species are resistant toward RsAFP2. GCS genes encode UDP-glucose:ceramide glucosyltransferases, which catalyze the final step in the biosynthesis of the membrane lipid glucosylceramide. In an enzyme-linked immunosorbent assay-based binding assay, RsAFP2 was found to interact with glucosylceramides isolated from P. pastoris but not with soybean nor human glucosylceramides. Furthermore, the P. pastoris parental strain is sensitive toward RsAFP2-induced membrane permeabilization, whereas the corresponding gcs-deletion mutant is highly resistant to RsAFP2-mediated membrane permeabilization. A model for the mode of action of RsAFP2 is presented in which all of these findings are linked. Similarly to RsAFP2, heliomicin, a defensin-like peptide from the insect Heliothis virescens, is active on C. albicans and P. pastoris parental strains but displays no activity on the gcs-deletion mutants of both yeast species. Furthermore, heliomicin interacts with glucosylceramides isolated from P. pastoris and soybean but not with human glucosylceramides. These data indicate that structurally homologous anti-fungal peptides present in species from different eukaryotic kingdoms interact with the same target in the fungal plasma membrane, namely glucosylceramides, and as such support the hypothesis that defensins from plants and insects have evolved from a single precursor.
