Solution structure of Cn5, a crustacean toxin found in the venom of the scorpions Centruroides noxius and Centruroides suffusus suffusus.

International audienceThe crustacean toxin Cn5 from Centruroides noxius Hoffmann and peptide Css39.8 from Centruroides suffusus suffusus scorpion venoms are identical peptides, as confirmed by amino acid sequence of purified toxins and by DNA sequencing of the two respective cloned genes. Therefore in this communication they will be simply named Cn5. Cn5 is a 66 amino acid long peptide with four disulfide bridges, formed between pairs of cysteines: C1-C8, C2-C5, C3-C6, and C4-C7 (the numbers indicate the relative positions of the cysteine residues in the primary structure). This peptide is non-toxic to mammals but deadly to arthropods (LD(50) 28.5 mg/g body weight of crayfish). Its three-dimensional structure was determined by NMR using a total of 965 meaningful distance constraints derived from the volume integration of the 2D NOESY spectra. The Cn5 structure displays a mixed alpha/beta fold stabilized by four disulfide bridges, with a kink induced by a cis-proline in its C-terminal part. Cn5 electrostatic surface is compared to that of Cn2 toxin toxic to mammals. The local differences produced by additional or substituted residues that would influence toxin selectivity towards mammalian or crustacean Na(+) channels are discussed

Structural basis of the signalling through a bacterial membrane receptor HasR deciphered by an integrative approach

Bacteria use diverse signalling pathways to adapt gene expression to external stimuli. In Gram-negative bacteria, the binding of scarce nutrients to membrane transporters triggers a signalling process that up-regulates the expression of genes of various functions, from uptake of nutrient to production of virulence factors. Although proteins involved in this process have been identified, signal transduction through this family of transporters is not well understood. In the present study, using an integrative approach (EM, SAXS, X-ray crystallography and NMR), we have studied the structure of the haem transporter HasR captured in two stages of the signalling process, i.e. before and after the arrival of signalling activators (haem and its carrier protein). We show for the first time that the HasR domain responsible for signal transfer: (i) is highly flexible in two stages of signalling; (ii) extends into the periplasm at approximately 70–90 Å (1 Å=0.1 nm) from the HasR β-barrel; and (iii) exhibits local conformational changes in response to the arrival of signalling activators. These features would favour the signal transfer from HasR to its cytoplasmic membrane partners