The muscle-type nicotinic receptor has two distinguishable acetylcholine binding sites at the alpha-gamma and alpha-delta subunit interfaces; alpha-conotoxins can bind them selectively. Moreover, we previously reported that alpha-conotoxin MI can interact with Torpedo california 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 Jimenez 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 alpha-conotoxin MI binding, a photoactivatable reagent was used and labeled sites were mapped by enzymatic proteolysis, MALDI-TOF-MS and Edman degradation. alpha-Conotoxin MI binding determinants were found and studies revealed a second binding motif at the alpha/delta interface. A proposal for receptor-toxin interaction is discussed based on experimental results and docking studies. (c) 2007 Elsevier Inc. All rights reserved.