RIC-3: a nicotinic acetylcholine receptor chaperone

Abstract

RIC-3 is a transmembrane protein which acts as a molecular chaperone of nicotinic acetylcholine receptors (nAChRs). For some nAChR subtypes (such as homomeric alpha7 neuronal nAChRs), RIC-3 is required for efficient receptor folding, assembly and functional expression. In contrast, for other nAChR subtypes (such as heteromeric alpha4beta2 neuronal nAChRs) there have been reports that RIC-3 can both enhance and reduce levels of functional expression. There is also evidence that RIC-3 can modulate maturation of the closely related 5-hydroxytryptamine (5-HT) receptor (5-HT(3)R). As with heteromeric nAChRs, apparently contradictory results have been reported for the influence of RIC-3 on 5-HT(3)R maturation in different expression systems. Recent evidence indicates that these differences in RIC-3 chaperone activity may be influenced by the host cell, suggesting that other proteins may play an important role in modulating the effects of RIC-3 as a chaperone. RIC-3 was originally identified in the nematode Caenorhabditis elegans as the protein encoded by the gene ric-3 (resistance to inhibitors of cholinesterase) and has subsequently been cloned and characterized from mammalian and insect species. This review provides a brief history of RIC-3; from the identification of the ric-3 gene in C. elegans in 1995 to the more recent demonstration of its activity as a nAChR chaperone.

Figure 1

Protein domains and predicted membrane topology of RIC-3. (a) The location of hydrophopic domains (blue diagonal stripes), predicted coiled-coil domains (green diagonal stripes) and proline-rich domains (red) are shown for the RIC-3 proteins of C. elegans, D. melanogaster and human (approximately to scale). Although several alternatively spliced isoforms of RIC-3 have been reported in the literature, those illustrated here correspond to the 378 amino acid C. elegans RIC-3 isoform described by Halevi et al. (2002), accession number NM_068898; the 369 amino acid human RIC-3 isoform A (Halevi et al., 2003), accession number AY326435 and the 472 amino acid Drosophila RIC-3 isoform DmRIC-3^6,7,9 (Lansdell et al., 2008), accession number AM902271. The position of the predicted signal-sequence cleavage site in human RIC-3 is indicated by a scissors symbol and arrowhead. (b) The predicted transmembrane topology of RIC-3 is illustrated. Two hydrophobic regions are present in RIC-3 proteins from all species that have been examined. In all cases, it appears that the second of these hydrophobic regions is a transmembrane domain. The first of these hydrophobic regions is also predicted to be a transmembrane domain in RIC-3 proteins from invertebrate species, such as C. elegans and Drosophila. In contrast, it appears that the first hydrophobic region may be a cleaved N-terminal signal sequence in mammalian species, such as human, rat and mouse. Therefore, in invertebrate species RIC-3 is predicted to have both its N- and C-terminus located on the cytoplasmic side of the membrane. In contrast, if the postulated N-terminal signal sequence is cleaved (as is predicted in mammalian RIC-3 proteins), this would be expected to result in a single-transmembrane protein with only the C-terminus of the mature protein on the cytoplasmic side of the membrane. The position of the predicted signal-sequence cleavage site (in human RIC-3) is indicated by a scissors symbol. Predictions of protein secondary structure (such as signal sequence cleavage sites and coiled-coil domains) and of membrane topology were based on computer programs, such as COILS (Lupas et al., 1991; Lupas, 1996), Phobius (Käll et al., 2004) and PONGO (Amico et al., 2006). RIC, resistant to inhibitors of cholinesterase.