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A new type of scorpion Na+-channel-toxin-like polypeptide active on K+ channels.

ABSTRACT: We have purified and characterized two peptides, named KAaH1 and KAaH2 (AaH polypeptides 1 and 2 active on K+ channels, where AaH stands for Androctonus australis Hector), from the venom of A. australis Hector scorpions. Their sequences contain 58 amino acids including six half-cysteines and differ only at positions 26 (Phe/Ser) and 29 (Lys/Gln). Although KAaH1 and KAaH2 show important sequence similarity with anti-mammal beta toxins specific for voltage-gated Na+ channels, only weak beta-like effects were observed when KAaH1 or KAaH2 (1 microM) were tested on brain Nav1.2 channels. In contrast, KAaH1 blocks Kv1.1 and Kv1.3 channels expressed in Xenopus oocytes with IC50 values of 5 and 50 nM respectively, whereas KAaH2 blocks only 20% of the current on Kv1.1 and is not active on Kv1.3 channels at a 100 nM concentration. KAaH1 is thus the first member of a new subfamily of long-chain toxins mainly active on voltage-gated K+ channels. NMR spectra of KAaH1 and KAaH2 show good dispersion of signals but broad lines and poor quality. Self-diffusion NMR experiments indicate that lines are broadened due to a conformational exchange on the millisecond time scale. NMR and CD indicate that both polypeptides adopt a similar fold with alpha-helical and b-sheet structures. Homology-based molecular models generated for KAaH1 and KAaH2 are in accordance with CD and NMR data. In the model of KAaH1, the functionally important residues Phe26 and Lys29 are close to each other and are located in the alpha-helix. These residues may constitute the so-called functional dyad observed for short alpha-KTx scorpion toxins in the beta-sheet.

SUBMITTER: Srairi-Abid N

PROVIDER: S-EPMC1138952 | biostudies-literature | 2005 Jun

REPOSITORIES: biostudies-literature

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A new type of scorpion Na+-channel-toxin-like polypeptide active on K+ channels.

Srairi-Abid Najet N Guijarro Joseba Iñaki JI Benkhalifa Rym R Mantegazza Massimo M Cheikh Amani A Ben Aissa Manel M Haumont Pierre-Yves PY Delepierre Muriel M El Ayeb Mohamed M

The Biochemical journal 20050601 Pt 2

We have purified and characterized two peptides, named KAaH1 and KAaH2 (AaH polypeptides 1 and 2 active on K+ channels, where AaH stands for Androctonus australis Hector), from the venom of A. australis Hector scorpions. Their sequences contain 58 amino acids including six half-cysteines and differ only at positions 26 (Phe/Ser) and 29 (Lys/Gln). Although KAaH1 and KAaH2 show important sequence similarity with anti-mammal beta toxins specific for voltage-gated Na+ channels, only weak beta-like e ...[more]

PMID: 15656785

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Project description:Two types of voltage-dependent Ca(2+) channels have been identified in heart: high (I(CaL)) and low (I(CaT)) voltage-activated Ca(2+) channels. In guinea pig ventricular myocytes, low voltage-activated inward current consists of I(CaT) and a tetrodotoxin (TTX)-sensitive I(Ca) component (I(Ca(TTX))). In this study, we reexamined the nature of low-threshold I(Ca) in dog atrium, as well as whether it is affected by Na(+) channel toxins. Ca(2+) currents were recorded using the whole-cell patch clamp technique. In the absence of external Na(+), a transient inward current activated near -50 mV, peaked at -30 mV, and reversed around +40 mV (HP = -90 mV). It was unaffected by 30 microM TTX or micromolar concentrations of external Na(+), but was inhibited by 50 microM Ni(2+) (by approximately 90%) or 5 microM mibefradil (by approximately 50%), consistent with the reported properties of I(CaT). Addition of 30 microM TTX in the presence of Ni(2+) increased the current approximately fourfold (41% of control), and shifted the dose-response curve of Ni(2+) block to the right (IC(50) from 7.6 to 30 microM). Saxitoxin (STX) at 1 microM abolished the current left in 50 microM Ni(2+). In the absence of Ni(2+), STX potently blocked I(CaT) (EC(50) = 185 nM) and modestly reduced I(CaL) (EC(50) = 1.6 microM). While TTX produced no direct effect on I(CaT) elicited by expression of hCa(V)3.1 and hCa(V)3.2 in HEK-293 cells, it significantly attenuated the block of this current by Ni(2+) (IC(50) increased to 550 microM Ni(2+) for Ca(V)3.1 and 15 microM Ni(2+) for Ca(V)3.2); in contrast, 30 microM TTX directly inhibited hCa(V)3.3-induced I(CaT) and the addition of 750 microM Ni(2+) to the TTX-containing medium led to greater block of the current that was not significantly different than that produced by Ni(2+) alone. 1 microM STX directly inhibited Ca(V)3.1-, Ca(V)3.2-, and Ca(V)3.3-mediated I(CaT) but did not enhance the ability of Ni(2+) to block these currents. These findings provide important new implications for our understanding of structure-function relationships of I(CaT) in heart, and further extend the hypothesis of a parallel evolution of Na(+) and Ca(2+) channels from an ancestor with common structural motifs.

Dataset Information

A new type of scorpion Na+-channel-toxin-like polypeptide active on K+ channels.

Publications

A new type of scorpion Na+-channel-toxin-like polypeptide active on K+ channels.

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