The insecticidal spider toxin SFI1 is a knottin peptide that blocks the pore of insect voltage-gated sodium channels via a large β-hairpin loop

Spider venoms contain a plethora of insecticidal peptides that act on neuronal ion channels and receptors. Because of their high specificity, potency and stability, these peptides have attracted much attention as potential environmentally friendly insecticides. Although many insecticidal spider venom peptides have been isolated, the molecular target, mode of action and structure of only a small minority have been explored. Sf1a, a 46-residue peptide isolated from the venom of the tube-web spider Segesteria florentina, is insecticidal to a wide range of insects, but nontoxic to vertebrates. In order to investigate its structure and mode of action, we developed an efficient bacterial expression system for the production of Sf1a. We determined a high-resolution solution structure of Sf1a using multidimensional 3D/4D NMR spectroscopy. This revealed that Sf1a is a knottin peptide with an unusually large β-hairpin loop that accounts for a third of the peptide length. This loop is delimited by a fourth disulfide bond that is not commonly found in knottin peptides. We showed, through mutagenesis, that this large loop is functionally critical for insecticidal activity. Sf1a was further shown to be a selective inhibitor of insect voltage-gated sodium channels, consistent with its 'depressant' paralytic phenotype in insects. However, in contrast to the majority of spider-derived sodium channel toxins that function as gating modifiers via interaction with one or more of the voltage-sensor domains, Sf1a appears to act as a pore blocker.

Identification and immunogenicity of an immunodominant mimotope of Avibacterium paragallinarum from a phage display peptide library

Avibacterium paragallinarum is the causative agent of infectious coryza. The protective antigens of this important pathogen have not yet been clearly identified. In this paper, we applied phage display technique to screen the immunodominant mimotopes of a serovar A strain of A. paragallinarum by using a random 12-peptide library, and evaluated the immunogenicity in chickens of the selected mimotope. Polyclonal antibody directed against A. paragallinarum strain 0083 (serovar A) was used as the target antibody and phage clones binding to this target were screened from the 12-mer random peptide library. More than 50% of the phage clones selected in the third round carried the consensus peptide motif sequence A-DP(M)L. The phage clones containing the peptide motif reacted with the target antibody and this interaction could be blocked, in a dose-dependent manner, by A. paragallinarum. One of the peptide sequences, YGLLAVDPLFKP, was selected and the corresponding oligonucleotide sequence was synthesized and then inserted into the expression vector pFliTrx. The recombinant plasmid was transferred into an expression host Escherichia coli GI826 by electroporation, resulting in a recombinant E. coli expressing the peptide on the bacterial surface. Intramuscular injection of the epitope-expressing recombinant bacteria into chickens induced a specific serological response to serovar A. A. paragallinarum. The chickens given the recombinant E. coli showed significant protection against challenge with A. paragallinarum 0083. These results indicated a potential for the use of the mimotope in the development of molecular vaccines for infectious coryza.