New potential antimicrobial peptides with mirror-symmetrical structure in fungi and insects

A new family of genes encoding potential antimicrobial peptides with compact and elegant structure has been found in the genomes of several Fungi and some arthropod species. Their expression products are constituted of about 85 amino acids, including a signal peptide, and are folded into two α-helical segments connected by a short unstructured coil. Three conserved disulphide bridges between cysteines located in symmetrically mirrored positions connect the two helical domains. These peptides, here named as Hairpin Loop Peptides (HLPs), have been found in the genomes of many Fungi species but only in selected clades. Orthologues have also been discovered in the genomes of some insects, notably Hemiptera, a few other arthropods and other organisms. They are not found in plants, that however express smaller peptides of similar topology with HLPs, but different amino acidic composition and physicochemical properties. They appear to have originated in Fungi and then migrated to insects through horizontal gene transfer. The antimicrobial activity of HLPs is predicted by several software programmes, but this aspect needs to be supported by experimental evidence. The occurrence of HLPs in several edible mushrooms may suggest potential uses of these peptides in food preservation and possibly also in medical applications. Their simple and nearly rigid structure can be easily modified to improve specificity, stability and solubility, thus making these peptides suitable for a variety of different applications.