SYNTHESIS AND CHARACTERIZATION OF KALIOTOXIN - IS THE 26-32 SEQUENCE ESSENTIAL FOR POTASSIUM CHANNEL RECOGNITION

Kaliotoxin (KTX), a scorpion toxin characterized as a 37-residue inhib itor of the neuronal high conductance Ca2+-activated K+ channels (KCa channels), has been chemically synthesized. Differences were observed between natural toxin and the two peptides, KTX(1-37) and KTX(1-37)-am ide. Re-examination of the KTX sequence showed that an extra lysine re sidue was present at the C-terminal end. The 38-residue synthetic pept ide was found identical with natural toxin. All three peptides had com parable activities, with LD50 Values of 6-9 pmol/mouse after intracere broventricular injection, and K(d) = 2-8 nM for blockage of the whole cell and unitary molluscan KCa currents. Pairing of the disulfide bond s in synthetic KTX corresponded to that in charybdotoxin and iberiotox in. A competition assay between I-125-KTX(1-37) and different toxins ( KTX, dendrotoxin, charybdotoxin, MCD peptide, and iberiotoxin) for bin ding to rat brain synaptosomal membranes suggested that KTX interacts also with voltage-gated K+ channels. Shorter peptides, KTX(25-35)-amid e and KTX(26-32)amide, expressed no KTX activity, but were able to com pete in binding. They were further shown to antagonize KTX in both its toxicity and blocking activity. The (26-32) sequence of KTX, which is a highly conserved region, may contain a low affinity binding subsite essential for potassium channel recognition.