COBATOXIN-1 AND COBATOXIN-2 FROM CENTRUROIDES-NOXIUS HOFFMANN CONSTITUTE A SUBFAMILY OF POTASSIUM-CHANNEL-BLOCKING SCORPION

Potassium-channel-blocking scorpion toxins (alpha-K-toxins) have been shown to be valuable tools for the study of potassium channels. Here w e report two toxins, cobatoxin 1 and 2, of 32 amino acids, containing three disulphide bridges, that were isolated from the venom of the Mex ican scorpion Centruroides noxius. Their primary sequences show less t han 40% identity to other alpha-K-toxins. It is therefore proposed tha t they belong to subfamily 9. The cDNA of cobatoxin 1 encodes a putati ve signal peptide, a putative short propeptide, the mature peptide and two amino acids that are processed to leave cobatoxin 1 amidated at t he C-terminus. In rat brain synaptosomal membranes cobatoxin 1 and cob atoxin 2 bind to a common binding site of alpha-K-toxins with K-i valu es of 109 pM and 87 pM, respectively. Moreover, they block the Shaker and K(v)1.1 K+ channels with moderate affinities, with K-d values of a round 0.7 mu M and 4.1 mu M (Shaker) and 0.5 mu M and 1.0 mu M (K(v)1. 1), respectively. A three-dimensional model of cobatoxin 1 was generat ed and used to interpret the obtained functional data on a structural basis.