alpha-Conotoxin PIA is a novel nicotinic acetylcholine receptor (nAChR) antagonist isolated from Conus purpurascens that targets nAChR subtypes containing alpha 6 and alpha 3 subunits. alpha-conotoxin PIA displays 75-fold higher affinity for rat alpha 6/alpha 3 beta 2 beta 3 nAChRs than for rat alpha 3 beta 2 nAChRs. We have determined the three-dimensional structure of alpha-conotoxin PIA by nuclear magnetic resonance spectroscopy. The alpha-conotoxin PlA has an w-shaped" overall topology as other alpha 4/7 subfamily conotoxins. Yet, unlike other neuronally targeted alpha 4/7-conotoxins, its N-terminal tail Arg(1)-ASP(2) -Pro(3) protrudes out of its main molecular body because Asp(2)-Pro(3)-CYS4-CyS5 forms a stable type I beta-turn. In addition, a kink introduced by Pro 15 in the second loop of this toxin provides a distinct steric and electrostatic environment from those in alpha-conotoxins MII and GIC. By comparing the structure of alpha-conotoxin PIA with other functionally related alpha-conotoxins we suggest structural features in alpha-conotoxin PIA that may be associated with its unique receptor recognition profile. (c) 2005 Elsevier Inc. All rights reserved.