Discovery of six families of fungal defensin-like peptides provides insights into origin and evolution of the CSαβ defensins

The defensins with a conserved cysteine-stabilized α-helix and β-sheet (CSαβ) structural motif are a group of unique antimicrobial polypeptides widely distributed in plants and animals. Recently, one defensin-like peptide (DLP) with high degree of sequence and structural similarity to defensins from ancient arthropods and molluscs has been identified in a saprophytic fungus [Mygind, P.H., Fischer, R.L., Schnorr, K.M., Hansen, M.T., Sönksen, C.P., Ludvigsen, S., Raventós, D., Buskov, S., Christensen, B., De Maria, L., Taboureau, O., Yaver, D., Elvig-Jørgensen, S.G., Sørensen, M.V., Christensen, B.E., Kjærulff, S.K., Frimodt-Moller, N., Lehrer, R.I., Zasloff, M., Kristensen, H.-H., 2005. Plectasin is a peptide antibiotic with therapeutic potential from a saprophytic fungus. Nature 437, 975-980], which poses an important question regarding the evolutionary relationships of this class of effectors of innate immunity in three eukaryotic kingdoms. Here, we report the computational identification of six families of fungal DLPs in which three known defensin types (antibacterial ancient invertebrate-type defensins (AITDs), antibacterial classical insect-type defensins (CITDs), and antifungal plant/insect-type defensins (PITDs)) can be clearly assigned. Sharing of these defensin types between animals and fungi supports their closer evolutionary relationship, consistent with the Opisthokonta Hypothesis. Conservation of the PITDs across three eukaryotic kingdoms suggests their earlier origin than the antibacterial defensins, probably preceded plants and Opisthokonta split. Finally, recognition of an early gene duplication event in the Aspergillus terreus genome allows us to establish a paralogous relationship between AITDs and CITDs, which highlights extensive lineage-specific defensin gene loss during evolution.