o-Agatoxins: Novel Calcium Channel Antagonists of Two Subtypes from Funnel Web Spider (Agelenopsis uperta) Venom*

A new series of polypeptide presynaptic antagonists (((w-agatoxins”) was purified from venom of the funnel web spider Agelenopsis aperta. Physiological data indicate that all of these peptides are antagonists of voltage-sensitive calcium channels. Although all three w-agatoxins (Aga) described here (w-Aga-IA, w-AgaIB, and w-Aga-IIA) block insect neuromuscular transmission presynaptically, biochemical data permit their subclassification as Type I and Type II toxins. Type I toxins (w-Aga-IA and -1B) are 7.5 kDa, have closely related amino acid sequences, and exhibit characteristic tryptophan-like UV absorbance spectra. Complete Edman sequencing of w-Aga-IA reveals it to be a 66amino acid polypeptide containing 9 cysteines and 5 tryptophan residues. w-Aga-IIA, a Type II toxin, is 11 kDa, shows limited amino acid sequence similarity to the Type I toxins, and exhibits mixed tryptophanand tyrosine-like absorbance. Nanomolar concentrations of w-Aga-IIA inhibit the specific binding of 1251-labeled w-conotoxin GVIA to chick synaptosomal membranes while w-Aga-IA and -1B have no effect under identical conditions. The w-agatoxins thus are defined as two subtypes of neuronal calcium channel toxins with different structural characteristics and calcium channel binding specificities.