SELECTIVE PEPTIDE ANTAGONIST OF THE CLASS-E CALCIUM-CHANNEL FROM THE VENOM OF THE TARANTULA HYSTEROCRATES-GIGAS

We describe the first potent and selective blocker of the class E Ca2-channel. SNX-482, a novel 41 amino acid peptide present in the venom of the African tarantula, Hysterocrates gigas, was identified through its ability to inhibit human class E Ca2+ channels stably expressed in a mammalian cell line. An IC50 of 15-30 nM was obtained for block of the class E Ca2+ channel, using either patch clamp electrophysiology o r K+-evoked Ca2+ flux. At low nanomolar concentrations, SNX-482 also b locked a native resistant or R-type Ca2+ current in rat neurohypophyse al nerve terminals, but concentrations of 200-500 nM had no effect on R-type Ca2+ cut-rents in several types of rat central neurons. The pep tide has the sequence GVDKAGCRYMFGGCSVNDDCCPRLGCHSLFSYCAWDLTFSD-OH and is homologous to the spider peptides grammatoxin S1A and hanatoxin, b oth peptides with very different ion channel blocking selectivities. N o effect of SNX-482 was observed on the following ion channel activiti es: Na+ or K+ currents in several cultured cell types (up to 500 nM); K+ current through cloned potassium channels Kv1.1 and Kv1.4 expressed in Xenopus oocytes (up to 140 nM); Ca2+ flux through L- and T-type Ca 2+ channels in an anterior pituitary cell line (GH3, up to 500 nM); an d Ba2+ current through class A Ca2+ channels expressed in Xenopus oocy tes (up to 280 nM). A weal; effect was noted on Ca2+ current through c loned and stably expressed class B Ca2+ channels (IC50 > 500 nM). The unique selectivity of SNX-482 suggests its usefulness in studying the diversity, function, and pharmacology of class E and/or R-type Ca2+ ch annels.