Interactions between α-conotoxin MI and the Torpedo marmorata receptor α–δ interface

The muscle-type nicotinic receptor has two distinguishable acetylcholine binding sites at the α–γ and α–δ subunit interfaces; α-conotoxins can bind them selectively. Moreover, we previously reported that α-conotoxin MI can interact with Torpedo californica and Torpedo marmorata receptors showing that conotoxins can also detect receptors from different species of the same genus [L. Cortez, S.G. del Canto, F. Testai, M.B. de Jiménez Bonino, Conotoxin MI inhibits the acetylcholine binding site of the Torpedo marmorata receptor, Biochem. Biophys. Res. Commun. 295 (2002) 791–795]. Herein, to identify T. marmorata receptor regions involved in α-conotoxin MI binding, a photoactivatable reagent was used and labeled sites were mapped by enzymatic proteolysis, MALDI-TOF-MS and Edman degradation. α-Conotoxin MI binding determinants were found and studies revealed a second binding motif at the α/δ interface. A proposal for receptor–toxin interaction is discussed based on experimental results and docking studies.