1.C.47 The Insect/Fungal Defensin (Insect/Fungal Defensin) Family
Many insect (arthropod) defensins have been sequenced and shown to form ion channels in artificial membranes. Their precursor proteins are secreted and digested to the active peptides that can be bacterocidal and toxic to specific eukaryotic cells (Kourie and Shorthouse, 2000).
The defensins with a conserved cysteine-stabilized alpha-helix and beta-sheet (CSαβ) structural motif are a group of unique antimicrobial polypeptides widely distributed in plants and animals. One defensin-like peptide (DLP), with high degrees of sequence and structural similarity to defensins from ancient arthropods and molluscs, has been identified in a saprophytic fungus (Mygind et al., 2005). This poses an important question regarding the evolutionary relationships of this class of effectors of innate immunity in three eukaryotic kingdoms.
Zhu (2008) reported 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)) were clearly assigned. Sharing of these defensin types between animals and fungi supported their closer evolutionary relationship, consistent with the Opisthokonta Hypothesis. Conservation of the PITDs across three eukaryotic kingdoms suggests an earlier origin (Zhu, 2008).
Shafee et al. 2016 have suggested that defensins and small defensin-like proteins fall into two superfamilies, which they call the cis-defensins (broadly distributed in living organisms) and the trans-defensins (narrowly distrubuted). They suggest that these two groups of proteins converged to show similar sequences, secondary and tertiary structures, and disulfide connectivities, with overlapping organismal sources and functions, in spite of their independent origins. The functions of these short proteins vary tremendously including pore formation, bacterial and fungal toxicity, lipid targeting, toxic receptor and channel interactions, fertilization, protease inhibiton and stress adaptation. However, as noted by the authors, alternative pathways involving divergent evolution from a common evolutionary source could have also occurred although they consider this possibility less likely (Shafee et al. 2017).
The generalized transport reaction catalyzed by defensins is:
ions and small molecules (in) ions and small molecules (out)