Allosteric α1-Adrenoreceptor Antagonism by the Conopeptide ρ-TIA

• Published in: The Journal of biological chemistry 2003-09, Vol.278 (36), p.34451-34457
• Main Authors: Sharpe, Iain A., Thomas, Linda, Loughnan, Marion, Motin, Leonid, Palant, Elka, Croker, Daniel E., Alewood, Dianne, Chen, Songhai, Graham, Robert M., Alewood, Paul F., Adams, David J., Lewis, Richard J.
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M305410200

A peptide contained in the venom of the predatory marine snail Conus tulipa, ρ-TIA, has previously been shown to possess α1-adrenoreceptor antagonist activity. Here, we further characterize its pharmacological activity as well as its structure-activity relationships. In the isolated rat vas deferens, ρ-TIA inhibited α1-adrenoreceptor-mediated increases in cytosolic Ca2+ concentration that were triggered by norepinephrine, but did not affect presynaptic α2-adrenoreceptor-mediated responses. In radioligand binding assays using [125I]HEAT, ρ-TIA displayed slightly greater potency at the α1B than at the α1A or α1D subtypes. Moreover, although it did not affect the rate of association for [3H]prazosin binding to the α1B-adrenoreceptor, the dissociation rate was increased, indicating non-competitive antagonism by ρ-TIA. N-terminally truncated analogs of ρ-TIA were less active than the full-length peptide, with a large decline in activity observed upon removal of the fourth residue of ρ-TIA (Arg4). An alanine walk of ρ-TIA confirmed the importance of Arg4 for activity and revealed a number of other residues clustered around Arg4 that contribute to the potency of ρ-TIA. The unique allosteric antagonism of ρ-TIA resulting from its interaction with receptor residues that constitute a binding site that is distinct from that of the classical competitive α1-adrenoreceptor antagonists may allow the development of inhibitors that are highly subtype selective.