Papiliocin is a novel 37-residue cecropin-like peptide isolated recently from the swallowtail butterfly, Papilio xuthus. With the aim of identifying a potent antimicrobial peptide, we tested papiliocin in a variety of biological and biophysical assays, demonstrating that the peptide possesses very low cytotoxicity against mammalian cells and high bacterial cell selectivity, particularly against Gram-negative bacteria as well as high anti-inflammatory activity. Using LPS-stimulated macrophage RAW264.7 cells, we found that papiliocin exerted its anti-inflammatory activities by inhibiting nitric oxide (NO) production and secretion of tumor necrosis factor (TNF)-␣ and macrophage inflammatory protein (MIP)-2, producing effects comparable with those of the antimicrobial peptide LL-37. We also showed that the innate defense response mechanisms engaged by papiliocin involve Toll-like receptor pathways that culminate in the nuclear translocation of NF-B. Fluorescent dye leakage experiments showed that papiliocin targets the bacterial cell membrane. To understand structure-activity relationships, we determined the three-dimensional structure of papiliocin in 300 mM dodecylphosphocholine micelles by NMR spectroscopy, showing that papiliocin has an ␣-helical structure from Lys 3 to Lys 21 and from Ala 25 to Val 36 , linked by a hinge region. Interactions between the papiliocin and LPS studied using tryptophan blue-shift data, and saturation transfer difference-NMR experiments revealed that Trp 2 and Phe 5 at the N-terminal helix play an important role in attracting papiliocin to the cell membrane of Gram-negative bacteria. In conclusion, we have demonstrated that papiliocin is a potent peptide antibiotic with both anti-inflammatory and antibacterial activities, and we have laid the groundwork for future studies of its mechanism of action.

Cold-shock proteins (Csps), proteins expressed when the ambient temperature drops below the growth-supporting temperature, bind to single-stranded nucleic acids and act as RNA chaperones to regulate translation. Listeria monocytogenes is a psychrophilic food-borne pathogen that is problematic for the food industry. Structures of Csps from psychrophilic bacteria have not yet been studied. Despite dramatic differences in the thermostability of Csps of various thermophilic microorganisms, these proteins share a high degree of primary sequence homology and a high degree of three-dimensional structural similarity. Here, we investigated the structural and dynamic features as well as the thermostability of L. monocytogenes CspA (Lm-CspA). Lm-CspA has a five-stranded β-barrel structure with hydrophobic core packing and two salt bridges. When heptathymidine (dT(7)) binds, values for the heteronuclear nuclear Overhauser effect and order parameters of residues in surface loop regions near nucleic acid binding sites increase dramatically. Moreover, Carr-Purcell-Meiboom-Gill experiments showed that slow motions observed for the nucleic acid binding residues K7, W8, F15, F27, and R56 disappeared in Lm-CspA-dT(7). Lm-CspA is less thermostable than mesophilic and thermophilic Csps, with a lower melting temperature (40 °C). The structural flexibility that accompanies longer surface loops and less hydrophobic core packing and a number of salt bridges and unfavorable electrostatic repulsion are likely key factors in the low thermostability of Lm-CspA. This implies that the large conformational flexibility of psychrophilic Lm-CspA, which more easily accommodates nucleic acids at low temperature, is required for RNA chaperone function under cold-shock conditions and for the cold adaptation of L. monocytogenes.

Rhamnetin (1), a commonly occurring plant O-methylated flavonoid, possesses antioxidant properties. To address the potential therapeutic efficacy of 1, its anti-inflammatory activity and mode of action in mouse macrophage-derived RAW264.7 cells stimulated with lipopolysaccharide (LPS) or interferon (IFN)-γ were investigated. Rhamnetin (1) suppressed mouse tumor necrosis factor (mTNF)-α, mouse macrophage inflammatory protein (mMIP)-1, and mMIP-2 cytokine production in LPS-stimulated macrophages. A nontoxic dose of 1 suppressed nitric oxide production. It was found that the anti-inflammatory effects of 1 are mediated by actions on the p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and cyclooxygenase (COX)-2 pathways in LPS- or IFN-γ-stimulated RAW264.7 cells. It was determined that 1 binds to human JNK1 (9.7 × 10(8) M(-1)) and p38 MAPK (2.31 × 10(7) M(-1)) with good affinity. The binding model showed interactions with the 3'- and 4'-hydroxy groups of the B-ring and the 5-hydroxy group of the A-ring of 1. Further, 1 exerted an anti-inflammatory effect, reducing the levels of pro-inflammatory cytokines and mediators.

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Piscidin 1 (Pis-1) is a novel cytotoxic peptide with a cationic alpha-helical structure isolated from the mast cells of hybrid striped bass. In our previous study, we showed that Pis-1[PG] with a substitution of Pro(8) for Gly(8) in Pis-1 had higher bacterial cell selectivity than Pis-1. We designed peptoid residue-substituted peptide, Pis-1[NkG], in which Gly(8) of Pis-1 was replaced with Nlys (Lys peptoid residue). Pis-1[NkG] had higher antibacterial activity and lower cytotoxicity against mammalian cells than Pis-1 and Pis-1[PG]. We determined the tertiary structure of Pis-1[PG] and Pis-1[NkG] in the presence of DPC micelles by NMR spectroscopy. Both peptides had a three-turn helix in the C-terminal region and a bent structure in the center. Pis-1[PG] has a rigid bent structure at Pro(8) whereas Pis-1[NkG] existed as a dynamic equilibrium of two conformers with a flexible hinge structure at Nlys(8). Depolarization of the membrane potential of Staphylococcus aureus and confocal laser-scanning microscopy study revealed that Pis-1[NkG] effectively penetrated the bacterial cell membrane and accumulated in the cytoplasm, whereas Pis-1[PG] did not penetrate the membrane but remained outside or on the cell surface. Introduction of a lysine peptoid at position 8 of Pis-1 provided conformational flexibility and increased the positive charge at the hinge region; both factors facilitated penetration of the bacterial cell membrane and conferred bacterial cell selectivity on Pis-1[NkG].

A cecropin-like peptide, papiliocin, isolated from the swallowtail butterfly Papilio xuthus, possesses high selectivity against gram-negative bacteria. Since Trp2 and Phe5 are highly conserved residues in cecropin-like peptides, we investigated the role of Trp2 and Phe5 in antibacterial activity. Substitution of Trp2 and Phe5 in papiliocin with Ala (papiliocin-2A and papiliocin-5A) revealed that Trp2 is a key residue in its antibacterial activities. In order to understand the structural requirements for papiliocin function and to design shorter, but more potent, peptide antibiotics, we designed papiliocin constructs, PapN (residues Arg1-Ala22 from the N-terminal amphipathic helix). PapN exhibited significant broad-spectrum antibacterial activities without cytotoxicity. Bactericidal kinetics of peptides against E.coli showed that papiliocin completely and rapidly killed E.coli in less than 10 minutes at 2× MIC concentration, while papiliocin-2A and papiliocin-5A killed four times more slowly than papiliocin. The PapN series peptides permeabilized bacterial membranes less effectively than papiliocin, showing no antibacterial activities in an hour. The results imply that the Trp2 and Phe5 in the amphipathic N-terminal helix are important in the rapid permeabilization of the gram-negative bacterial membrane. The hydrophobic C-terminal residues permeabilize the hydrophobic bacterial cell membrane synergistically with these aromatic residues, providing selectivity against gram-negative bacteria.

The novel 43-residue, insect defensin-like peptide coprisin, isolated from the dung beetle, Copris tripartitus, is a potent antibiotic with bacterial cell selectivity, exhibiting antimicrobial activities against Gram-positive and Gram-negative bacteria without exerting hemolytic activity against human erythrocytes. Tests against Staphylococcus aureus using fluorescent dye leakage and depolarization measurements showed that coprisin targets the bacterial cell membrane. To understand structure-activity relationships, we determined the three-dimensional structure of coprisin in aqueous solution by nuclear magnetic resonance spectroscopy, which showed that coprisin has an amphipathic α-helical structure from Ala(19) to Arg(28), and β-sheets from Gly(31) to Gln(35) and Val(38) to Arg(42). Coprisin has electropositive regions formed by Arg(28), Lys(29), Lys(30), and Arg(42) and ITC results proved that coprisin and LPS have electrostatically driven interactions. Using measurements of nitric oxide release and inflammatory cytokine production, we provide the first verification of the anti-inflammatory activity and associated mechanism of an insect defensin, demonstrating that the anti-inflammatory actions of the defensin-like peptide, coprisin, are initiated by suppressing the binding of LPS to toll-like receptor 4, and subsequently inhibiting the phosphorylation of p38 mitogen-activated protein kinase and nuclear translocation of NF-kB. In conclusion, we have demonstrated that an amphipathic helix and an electropositive surface in coprisin may play important roles in its effective interaction with bacterial cell membranes and, ultimately, in its high antibacterial activity and potent anti-inflammatory activity. In addition to elucidating the antimicrobial action of coprisin, this work may provide insight into the mechanism of action of insect defense systems.

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