Gallinacin-1 alpha

Mammalian alpha-defensins constitute a family of cysteine-rich, cationic antimicrobial peptides produced by phagocytes and intestinal Paneth cells, playing an important role in innate host defense. Following comprehensive computational searches, here we report the discovery of complete repertoires of the alpha-defensin gene family in the human, chimpanzee, rat, and mouse with new genes identified in each species. The human genome was found to encode a cluster of 10 distinct alpha-defensin genes and pseudogenes expanding 132 kb continuously on chromosome 8p23. Such alpha-defensin loci are also conserved in the syntenic chromosomal regions of chimpanzee, rat, and mouse. Phylogenetic analyses showed formation of two distinct clusters with primate alpha-defensins forming one cluster and rodent enteric alpha-defensins forming the other cluster. Species-specific clustering of genes is evident in nonprimate species but not in the primates. Phylogenetically distinct subsets of alpha-defensins also exist in each species, with most subsets containing multiple members. In addition, natural selection appears to have acted to diversify the functionally active mature defensin region but not signal or prosegment sequences. We concluded that mammalian alpha-defensin genes may have evolved from two separate ancestors originated from beta-defensins. The current repertoires of the alpha-defensin gene family in each species are primarily a result of repeated gene duplication and positive diversifying selection after divergence of mammalian species from each other, except for the primate genes, which were evolved prior to the separation of the primate species. We argue that the presence of multiple, divergent subsets of alpha-defensins in each species may help animals to better cope with different microbial challenges in the ecological niches which they inhabit.

Mammalian beta-defensins are an important family of innate host defense peptides with pleiotropic activities. As a first step to study the evolutionary relationship and biological role of the beta-defensin family, we identified their complete repertoires in the human, chimpanzee, mouse, rat, and dog following systemic, genome-wide computational searches. Although most beta-defensin genes are composed of two exons separated by an intron of variable length, some contain an additional one or two exons encoding an internal pro-sequence, a segment of carboxy-terminal mature sequences or untranslated regions. Alternatively, spliced isoforms have also been found with several beta-defensins. Furthermore, all beta-defensin genes are densely clustered in four to five syntenic chromosomal regions, with each cluster spanning <1.2 Mb across the five species. Phylogenetic analysis indicated that, although the majority of beta-defensins are evolutionarily conserved across species, subgroups of gene lineages exist that are specific in certain species, implying that some beta-defensins originated after divergence of these mammals from each other, while most others arose before the last common ancestor of mammals. Surprisingly, RT-PCR revealed that all but one rat beta-defensin transcript are preferentially expressed in the male reproductive tract, particularly in epididymis and testis, except that Defb4, a human beta-defensin-2 ortholog, is more restricted to the respiratory and upper gastrointestinal tracts. Moreover, most beta-defensins expressed in the reproductive tract are developmentally regulated, with enhanced expression during sexual maturation. Existence of such a vast array of beta-defensins in the male reproductive tract suggests that these genes may play a dual role in both fertility and host defense.

Insect defensins containing cysteine-stabilized alpha/beta motifs (Cs-alpha/beta defensin) are cationic, inducible antibacterial peptides involved in humoral defence against pathogens. To examine trends in molecular evolution of these antimicrobial peptides, sequences similar to the well-characterized Cs-alpha/beta defensin peptide of Anopheles gambiae, using six cysteine residues as landmarks, were retrieved from genomic and protein databases. These sequences were derived from different orders of insects. Genes of insect Cs-alpha/beta defensin appear to constitute a multigene family in which the copy number varies between insect species. Phylogenetic analysis of these sequences revealed two main lineages, one group comprising mainly lepidopteran insects and a second, comprising Hemiptera, Coleoptera, Diptera and Hymenoptera insects. Moreover, the topology of the phylogram indicated dipteran Cs-alpha/beta defensins are diverse, suggesting diversity in immune mechanisms in this order of insects. Overall evolutionary analysis indicated marked diversification and expansion of mature defensin isoforms within the species of mosquitoes relative to non-mosquito defensins, implying the presence of finely tuned immune responses to counter pathogens. The observed higher synonymous substitution rate relative to the nonsynonymous rate in almost all the regions of Cs-alpha/beta defensin of mosquitoes suggests that these peptides are predominately under purifying selection. The maximum-likelihood models of codon substitution indicated selective pressure at different amino acid sites in mosquito mature Cs-alpha/beta defensins is differ and are undergoing adaptive evolution in comparison to non-mosquito Cs-alpha/beta defensins, for which such selection was inconspicuous; this suggests the acquisition of selective advantage of the Cs-alpha/beta defensins in the former group. Finally, this study represents the most detailed report on the evolutionary strategies of Cs-alpha/beta defensins of mosquitoes in particular and insects in general, and indicates that insect Cs-alpha/beta defensins have evolved by duplication followed by divergence, to produce a diverse set of paralogues.