Solution structure of ADO1, a toxin extracted from the saliva of the assassin bug, Agriosphodrus dohrni

ADO1 is a toxin purified from the saliva of the assassin bug, Agriosphodrus dohrni. Because of its similarity in sequence to Ptul from another assassin bug, we did not assess its pharmacologic target. Here, we demonstrate by electrophysiologic means that ADO1 targets the P/Q-type voltage-sensitive calcium channel. We also determine the solution structure of ADO1 using two-dimensional NMR techniques, followed by distance geometry and molecular dynamics. The structure of ADO1 belongs to the inhibitory cystine knot (ICK) structural family (i.e., a compact disulfide-bonded core from which four loops emerge). ADO1 contains a two-stranded, antiparallel beta-sheet structure. We compare the structure of ADO1 with other voltage-sensitive calcium-channel blockers, analyze the topologic juxtaposition of key functional residues, and conclude that the recognition of voltage-sensitive calcium channels by toxins belonging to the ICK structural family requires residues located on two distinct areas of the molecular surface of the toxins.