Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channel complexes; in mammals, ca. 16 different genes encode nAChR subunits. A distinct branch of the nAChR gene family encodes subunits that form homomeric nAChRs; an example is the mammalian alpha7 nAChR subunit. Most other mammalian nAChR subunits form functional receptors only when more than one type of subunit is present. We recently discovered a class of Conus peptides, the alpha4/3 conotoxin subfamily, that appears to target homomeric nAChR subunits. The broad goal of the project is to investigate interactions between the alpha4/3 conotoxins and their homomeric nAChR targets; there are three general initiatives proposed. The first concerns two closely related alpha4/3 conotoxins, alpha-conotoxins Iml and Imll. Both of these functionally inhibit the alpha7 nicotinic acetylcholine receptor, but apparently at different sites. AIpha-conotoxin Imll appears to act through a unique site, distinct from that of the standard competitive antagonists (such asalpha-bungarotoxin). One goal is to provide a molecular definition of this novel binding site. A second set of experiments examines the alpha4/3 conotoxins that target molluscan acetylcholine binding proteins, which are models for nAChR ligand binding domains. The recent breakthrough in determining the structure of AChBPs provided the first detailed picture of a ligand binding site for any ligand-gated ion channel; a long-term goal is to determine whether the AChBP can be crystallized with a bound alpha4/3conotoxin. Another goal is to define the targets of various alpha4/3 conotoxins in molluscan systems. A final set of experimental objectives is to examine the effects of alpha4/3 conotoxins in model organisms such as C. elegans. Preliminary work has shown that alpha-conotoxin Iml blocks an nAOhR in this organism. In many invertebrate systems, the homomeric subunits comprise a much greater fraction of nAChR subunits than the heteromeric subunits; the proposed study of the alpha4/3 conotoxin subfamily may therefore provide the basis for an effective neuropharmacology for the spectrum of different nicotinic receptors in these organisms.
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