SOLUTION STRUCTURE AND PROPOSED BINDING MECHANISM OF A NOVEL POTASSIUM CHANNEL TOXIN KAPPA-CONOTOXIN PVIIA

Background: kappa-PVIIA is a 27-residue polypeptide isolated from the venom of Conus purpurascens and is the first member of a new class of conotoxins that block potassium channels. By comparison to other ion c hannels of eukaryotic cell membranes, voltage-sensitive potassium chan nels are relatively simple and methodology has been developed for mapp ing their interactions with small-peptide toxins, PVIIA, therefore, is a valuable new probe of potassium channel structure. This study of th e solution structure and mode of channel binding of PVIIA forms the ba sis for mapping the interacting residues at the conotoxin-ion channel interface. Results: The three-dimensional structure of PVIIA resembles the triple-stranded beta sheet/cystine-knot motif formed by a number of toxic and inhibitory peptides. Subtle structural differences, predo minantly in loops 2 and 4, are observed between PVIIA and other conoto xins with similar structural frameworks, however. Electrophysiological binding data suggest that PVIIA blocks channel currents by binding in a voltage-sensitive manner to the external vestibule and occluding th e pore, Comparison of the electrostatic surface of PVIIA with that of the well-characterised potassium channel blocker charybdotoxin suggest s a likely binding orientation for PVIIA, Conclusions: Although the st ructure of PVIIA is considerably different to that of the alpha K scor pion toxins, it has a similar mechanism of channel blockade. On the ba sis of a comparison of the structures of PVIIA and charybdotoxin, we s uggest that Lys19 of PVIIA is the residue which is responsible for phy sically occluding the pore of the potassium channel.