Potassium channels and epilepsy: evidence that the epileptogenic toxin, dendrotoxin, binds to potassium channel proteins.

Dendrotoxin-I, a component of the venom of the black mamba snake, Dendroaspis polylepsis, was used to affinity purify a potassium channel from bovine brain. This dendrotoxin-I binding protein was composed of several subunits with molecular weights of 35,000, 38,000, 42,000 and 74,000. Partial sequence resulting from Edman degradation of the N-terminus of the 74 kDa subunit was identical to the predicted amino acid sequence of the N-terminus of a protein encoded by a mouse/rat homologue of the Shaker gene family of potassium channels, MK2/RBK2 (RCK5). Polyclonal antibodies raised against synthetic peptides derived from the predicted amino acid sequence of another member of this family, MK1, recognized this 74 kDa subunit. Due to extensive amino acid sequence identity between MK2 and MK1, it is likely that antibodies recognized epitopes common to both. Thus, from an immunological standpoint, either MK1, MK2, or both channel proteins could have been present in this 74 kDa band on protein blots. Closely related K+ channels in bovine brain could have copurified based on their affinity for dendrotoxin-I (DTX-I). DTX-I was shown to inhibit MK1 currents in a time and voltage independent fashion. Physiological and molecular evidence indicates the existence of many types of DTX sensitive potassium channels in the mammalian brain, however, our protein sequencing of the 74 kDa subunit has detected the presence of only one unique N-terminal sequence, identical to MK2. The possible reason for the appearance of this discrepancy is discussed. This paper represents the first report identifying one dendrotoxin binding protein in bovine brain tissue (BK2) as a delayed rectifier type of potassium channel.