Structural conservation of the pores of calcium-activated and voltage-gated potassium channels determined by a sea anemone toxin.
The structurally defined sea anemone peptide toxins ShK and BgK potently block the intermediate conductance, Ca(2+)-activated potassium channel IKCa1, a well recognized therapeutic target present in erythrocytes, human T-lymphocytes, and the colon. The well characterized voltage-gated Kv1.3 channel in human T-lymphocytes is also blocked by both peptides, although ShK has a approximately 1,000-fold greater affinity for Kv1.3 than IKCa1. To gain insight into the architecture of the toxin receptor in IKCa1, we used alanine-scanning in combination with mutant cycle analyses to map the ShK-IKCa1 interface, and compared it with the ShK-Kv1.3 interaction surface. ShK uses the same five core residues, all clustered around the critical Lys(22), to interact with IKCa1 and Kv1.3, although it relies on a larger number of contacts to stabilize its weaker interactions with IKCa1 than with Kv1.3. The toxin binds to IKCa1 in a region corresponding to the external vestibule of Kv1.3, and the turret and outer pore of the structurally defined bacterial potassium channel, KcsA. Based on the NMR structure of ShK, we deduce the toxin receptor in IKCa1 to have x-y dimensions of approximately 22 A, a diameter of approximately 31 A, and a depth of approximately 8 A; we estimate that the ion selectivity lies approximately 13 A below the outer lip of the toxin receptor. These dimensions are in good agreement with those of the KcsA channel determined from its crystal structure, and the inferred structure of Kv1.3 based on mapping with scorpion toxins. Thus, these distantly related channels exhibit architectural similarities in the outer pore region. This information could facilitate development of specific and potent modulators of the therapeutically important IKCa1 channel.
Department of Physiology and Biophysics, University of California, Irvine, California 92697, USA.
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Rauer
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Pennington
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Cahalan
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D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D003064:Cnidarian Venoms / Q000737:chemistry* / Q000494:pharmacology*; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D051664:Kv1.3 Potassium Channel; D008564:Membrane Potentials / Q000187:drug effects; D008958:Models, Molecular; D008969:Molecular Sequence Data; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000737:chemistry* / Q000502:physiology*; D024642:Potassium Channels, Voltage-Gated; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D012615:Sea Anemones; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013601:T-Lymphocytes / Q000502:physiology; D013816:Thermodynamics; D014162:Transfection
D003064:Cnidarian Venoms; C496973:KCNA3 protein, human; D051664:Kv1.3 Potassium Channel; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D011994:Recombinant Proteins; C095391:ShK neurotoxin; C104455:toxin BgK