Structural Basis for α-Conotoxin Potency and Selectivity

Parkinson's disease is a debilitating movement disorder characterized by altered levels of α₆β₂* nicotinic acetylcholine receptors (nAChRs) localized on presynaptic striatal catecholaminergic neurons. α-Conotoxin MII (α-CTx MII) is a highly useful ligand to probe α₆ß₂ nAChRs structure and function, but it does not discriminate among closely related α₆* nAChR subtypes. Modification of the α-CTx MII primary sequence led to the identification of α-CTx MII[E11A], an analog with 500-5300 fold discrimination between α₆* subtypes found in both human and non-human primates. α-CTx MII[E11A] binds most strongly (femtomolar dissociation constant) to the high affinity α₆* nAChR, a subtype that is selectively lost in Parkinson's disease. Here we present the three-dimensional solution structure for α-CTx MII[E11A] as determined by two-dimensional 1H NMR spectroscopy to 0.13 +/- 0.09 Ǻ backbone and 0.45 +/- 0.08 Ǻ heavy atom root mean square deviation from mean structure. Structural comparisons suggest that the increased hydrophobic area of α-CTx MII[E11A] relative to other members of the α-CTx family may be responsible for its exceptionally high affinity for α₆α₄β₂* nAChR as well as discrimination between α₆ß₂ and α₃β₂ containing nAChRs. This finding may enable the rational design of novel peptide analogs that demonstrate enhanced specificity for α₆* 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.