Block of voltage-dependent calcium channel by the green mamba toxin calcicludine

A number of peptide toxins derived from marine snails and various spiders h ave 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 cal cicludine, L-type (alpha(1C)) calcium channels underwent a rapid, irreversi ble decrease in peak current amplitude with no change in current kinetics, or any apparent voltage-dependence. However, even at saturating toxin conce ntrations, block was always incomplete with a maximum inhibition of 58%, in dicating either partial pore block, or an effect on channel gating. Block n onetheless was of high affinity with an IC50 value of 88 nM. Three other ty pes of high voltage activated channels tested (alpha(1A), alpha(1B), and al pha(1E)) exhibited a diametrically different response to calcicludine. Firs t, the maximal inhibition observed was around 10%, furthermore, the voltage -dependence of channel activation was shifted slightly towards more negativ e potentials. Thus, at relatively hyperpolarized test potentials, calciclud ine actually upregulated current activity of (N-type) alpha(1B) channels by as much as 50%. Finally, the use of several chimeric channels combining th e major transmembrane domains of alpha(1C) and alpha(1E) revealed that calc icludine block of L-type calcium channels involves interactions with multip le structural domains. Overall, calcicludine is a potent and selective inhi bitor of neuronal L-type channels with a unique mode of action.