Tian et al. BMC Genomics 2010, 11:187 http://www.biomedcentral.com/1471-2164/11/187 Page 8 of 19 Figure 5 Abaecin-like peptides. A. Multiple sequence alignment. The propeptide sequence of navitripenicin is italized and underlined once. [0] indicates a phase 0 intron between the two residues and [?] represents unknown intron information due to the lack of genomic sequence. B. Phylogenetic tree constructed with the NJ method of MEGA 4.0. Solid circle represents gene duplication. Bi: Bombus ignites; Pp: Pteromalus puparum. composed of an antiparallel b-sheet well fitted with tachystatin-B1. Linear a-helical peptides Linear peptides with an a-helical conformation are a large class of AMPs involved in immune response of arthropods, amphibians and vertebrates. Since cecropin, the first insect-derived linear AMP, was isolated from Cecropia moth [71], the number of such peptides dra- matically increases in recent decades [72,73]. Several lin- ear a-helical AMPs have also been characterized in social wasps [74-76]. From the N. vitripennis genome, we identified one AMP-like peptide (called nahelixin) belonging to this class (Figure 7C). The precursor of nahelixin is 118 residues in length with an amino-term- inal signal peptide and a putative carboxyl-terminal pro- peptide. The predicted mature peptide of 22 amino acids shares about 40% sequence identity with four known antibacterial peptides from frogs [77]. Secondary structure prediction indicated it could adopt an a-heli- cal structure (data not shown), which was further con- firmed by ab initio structural prediction. An amphiphilic surface, with hydrophilic and hydrophobic residues separately arranged at two directions of the helical axis, is present in the model, as seen at the helical wheel pro- jection (Figure 7C). Such structural feature represents a prerequisite for antibacterial activity of linear AMPs [6].