CONOTOXINS AND HOMERIC NICOTINIC ACETYLCHOLINE RECEPTORS

烟碱乙酰胆碱受体（nAChRs）是五聚体配体门控离子通道复合物；在哺乳动物中，大约有16种不同的基因编码nAChR亚基。nAChR基因家族的一个独特分支编码形成同源nAChR的亚基；哺乳动物的alpha7 nAChR亚基就是一个例子。大多数其他哺乳动物nAChR亚基只有在存在一种以上亚基时才能形成功能性受体。我们最近发现了一类Conus肽，阿尔法4/3 conotoxin亚家族，似乎针对同源的nAChR亚基。

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. Alpha-conotoxin Imll appears to act through a unique site, distinct from that of the standard competitive antagonists (such as alpha-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 alpha4/3 conotoxin. 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 nAChR 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 lifferent nicotinic receptors in these organisms.