Structural Basis for α-Conotoxin Potency and Selectivity

Parkinson's disease is a debilitating movement disorder characterized by altered levels of alpha(6)beta(2) * ( * indicates the possible presence of additional subunits) nicotinic acetylcholine receptors (nAChRs) localized on presynaptic striatal catecholaminergic neurons. alpha-Conotoxin MII (alpha-CTx MII) is a highly useful ligand to probe alpha(6)beta(2) nAChRs structure and function, but it does not discriminate among closely related alpha(6) * nAChR subtypes. Modification of the alpha-CTx MII primary sequence led to the identification of alpha-CTx MII[E11A], an analog with 500-5300-fold discrimination between alpha(6) * subtypes found in both human and non-human primates. alpha-CTx MII[E11A] binds most strongly (femtomolar dissociation constant) to the high affinity alpha(6) nAChR, a subtype that is selectively lost in Parkinson's disease. Here, we present the three-dimensional solution structure for alpha-CTx MII[E11A] as determined by two-dimensional (1)H NMR spectroscopy to 0.13+/-0.09A backbone and 0.45+/-0.08A heavy atom root-mean-square deviation from mean structure. Structural comparisons suggest that the increased hydrophobic area of alpha-CTx MII[E11A] relative to other members of the alpha-CTx family may be responsible for its exceptionally high affinity for alpha6alpha4beta2 * nAChR as well as discrimination between alpha(6)beta(2) and alpha(3)beta(2) containing nAChRs. This finding may enable the rational design of novel peptide analogs that demonstrate enhanced specificity for alpha(6) * nAChR subunit interfaces and provide a means to better understand nAChR structural determinants that modulate brain dopamine levels and the pathophysiology of Parkinson's disease.