Investigation into the mechanism of action of the antimicrobial peptides Os and Os-C derived from a tick defensin

Os and Os-C are two novel antimicrobial peptides, derived from a tick defensin, which have been shown to have a larger range of antimicrobial activity than the parent peptide, OsDef2. The aim of this study was to determine whether the peptides Os and Os-C are mainly membrane acting, or if these peptides have possible additional intracellular targets in Escherichia coli and Bacillus subtilis. Transmission electron microscopy revealed that both peptides adversely affected intracellular structure of both bacteria causing different degrees of granulation of the intracellular contents. At the minimum bactericidal concentrations, permeabilization as determined with the SYTOX green assay seemed not to be the principle mode of killing when compared to melittin. However, fluorescent triple staining indicated that the peptides caused permeabilization of stationary phase bacteria and TEM indicated membrane effects. Studies using fluorescently labeled peptides revealed that the membrane penetrating activity of Os and Os-C was similar to buforin II. Os-C was found to associate with the septa of B. subtilis. Plasmid binding studies showed that Os and Os-C binds E. coli plasmid DNA at a similar charge ratio as melittin. These studies suggest membrane activity for Os and Os-C with possible intracellular targets such as DNA. The differences in permeabilization at lower concentrations and binding to DNA between Os and Os-C, suggest that the two peptides have dissimilar modes of action.

Structure, function, and evolution of biogenic amine-binding proteins in soft ticks

Two highly abundant lipocalins, monomine and monotonin, have been isolated from the salivary gland of the soft tick Argas monolakensis and shown to bind histamine and 5-hydroxytryptamine (5-HT), respectively. The crystal structures of monomine and a paralog of monotonin were determined in the presence of ligands to compare the determinants of ligand binding. Both the structures and binding measurements indicate that the proteins have a single binding site rather than the two sites previously described for the female-specific histamine-binding protein (FS-HBP), the histamine-binding lipocalin of the tick Rhipicephalus appendiculatus. The binding sites of monomine and monotonin are similar to the lower, low affinity site of FS-HBP. The interaction of the protein with the aliphatic amine group of the ligand is very similar for the all of the proteins, whereas specificity is determined by interactions with the aromatic portion of the ligand. Interestingly, protein interaction with the imidazole ring of histamine differs significantly between the low affinity binding site of FS-HBP and monomine, suggesting that histamine binding has evolved independently in the two lineages. From the conserved features of these proteins, a tick lipocalin biogenic amine-binding motif could be derived that was used to predict biogenic amine-binding function in other tick lipocalins. Heterologous expression of genes from salivary gland libraries led to the discovery of biogenic amine-binding proteins in soft (Ornithodoros) and hard (Ixodes) tick genera. The data generated were used to reconstruct the most probable evolutionary pathway for the evolution of biogenic amine-binding in tick lipocalins.

[Biochemical characteristics of ticks of the families Argasidae and Ixodidae]

Polyacrylamide gel electrophoresis was used to study homogenate supernatants from the bloodsucking ticks (Argasidae and Ixodidae) belonging to 8 different genera and species. Differences were found in the electrophoretic mobility of protein fractions in adult ticks of these two different families and between several genera of Ixodes ticks (such as Haemaphysalis, Hyalomma, Dermacentor, Rhipicephalus and Ixodes). Argasidae ticks were found to have generic and even species-specific differences at the nymphal stage of development. A. lahorensis nymph-3 contained 5 protein fractions; O. papillipes had 2 fractions, and O. moubata had 3 fractions. Electrophoretic protein separation showed the equal number of protein fractions within one genus of ticks. At the larval stage, the tick homogenates were always found to have one protein fraction. The hemolymph of O. papillipes and O. moubata ticks displayed differences between the imago and nymph-3. A new biochemical marker was proposed to establish a species affinity.