Block of Voltage-dependent Calcium Channel by the Green Mamba Toxin Calcicludine
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A number of peptide toxins derived from marine snails and various spiders have been shown to potently inhibit voltage-dependent calcium channels. Here, we describe the effect of calcicludine, a 60 amino-acid peptide isolated from the venom of the green mamba (Dendroaspis angusticeps), on transiently expressed high voltage-activated calcium channels. Upon application of calcicludine, L-type (α1C) calcium channels underwent a rapid, irreversible decrease in peak current amplitude with no change in current kinetics, or any apparent voltage-dependence. However, even at saturating toxin concentrations, block was always incomplete with a maximum inhibition of 58%, indicating either partial pore block, or an effect on channel gating. Block nonetheless was of high affinity with an IC50 value of 88 nm. Three other types of high voltage activated channels tested (α1A, α1B, and α1E) exhibited a diametrically different response to calcicludine. First, the maximal inhibition observed was around 10%, furthermore, the voltage-dependence of channel activation was shifted slightly towards more negative potentials. Thus, at relatively hyperpolarized test potentials, calcicludine actually upregulated current activity of (N-type) α1B channels by as much as 50%. Finally, the use of several chimeric channels combining the major transmembrane domains of α1C and α1E revealed that calcicludine block of L-type calcium channels involves interactions with multiple structural domains. Overall, calcicludine is a potent and selective inhibitor of neuronal L-type channels with a unique mode of action.
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