X-tox: an atypical defensin derived family of immune-related proteins specific to Lepidoptera.

We report here the isolation in Spodoptera frugiperda (Lepidoptera) of an immune-related protein (hereafter named Spod-11-tox), characterized by imperfectly conserved tandem repeats of 11 cysteine-stabilized alpha beta motifs (CS-alphabeta), the structural scaffold characteristic of invertebrate defensins and scorpion toxins. Spod-11-tox orthologs were only found in Lepidopteran species, suggesting that this new protein family (named X-tox) is specific to this insect order. Moreover, phylogenetic analysis suggests that X-tox proteins represent a new class of proteins restricted to Lepidoptera and likely derived from Lepidopteran defensins. In S. frugiperda, analysis of gene expression revealed that spod-11-tox is rapidly induced by infection. However, and conversely to what is known for most insect antimicrobial peptides (AMP), spod-11-tox is mainly expressed in blood cells. Moreover, recombinant Spod-11-tox produced in the Sf9 cell line does not show any antimicrobial activity. Altogether, these results suggest that although X-tox proteins are derived from defensins, they may play a different and still unknown role in Lepidoptera immune response.