A novel potassium channel blocking toxin from the scorpion Pandinus imperator: A H-1 NMR analysis using a nano-NMR probe

The three-dimensional solution structure of a novel peptide, Pil, purified from the venom of the scorpion Pandinus imperator and specific for potassium channels was determined by homonuclear proton NMR methods at 500 MHz from nanomole amounts of compound. P. imperator toxin is a voltage-dependent potassium channel specific peptide capable of blocking the shaker B K+ channels expressed in Sf9 cells in culture (Spodoptera frugiperda cell line no. 9) and displacing labeled noxiustoxin from rat brain synaptosomal membranes. The toxin has only 35 amino acid residues but is stabilized by four disulfide bridges (Cys4-Cys25, Cys10-Cys30, Cys14-Cys32, and Cys20-Cys35) instead of three commonly found in small potassium channel toxins. A detailed nuclear magnetic resonance structure of this protein was obtained using a nano-NMR probe and a combination of two-dimensional proton NMR experiments. The dihedral angles and distance restraints obtained from measured NMR parameters were used in structural calculations in order to determine the solution conformation of the toxin. The structure is organized around a short alpha-helix spanning residues Ser8-Thr18 and a beta-sheet. These two elements of secondary structure are stabilized by two disulfide bridges, Cys10-Cys30 and Cys14-Cys32. The antiparallel beta-sheet is composed of two strands extending from Asn22 to Cys32 with a tight turn at Arg28-Met29 in contact with the N-terminal fragment Leu1-Cys4. Comparison between the 3D structure of Pil and those of other structurally and functionally related scorpion toxins is presented.