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Department of Pharmacology, University College London, London, United Kingdom
Recent studies have shown that RIC-3, originally identified in Caenorhabditis elegans as the protein encoded by the gene resistance to inhibitors of cholinesterase (ric-3), can enhance functional expression of
7 nicotinic acetylcholine receptors (nAChRs). In the present study, the influence of C. elegans and human RIC-3 upon multiple homomeric (
7,
8, and
9) and heteromeric (
3
2,
3
4,
4
2,
4
4, and
9
10) nAChR subtypes has been examined in transfected mammalian cells by radioligand binding and functional characterization. Coexpression of RIC-3 facilitates a dramatic enhancement of the ability of
7 (and the closely related
8 subunit) to generate functional nAChRs in otherwise nonpermissive mammalian cells. In contrast, coexpression of RIC-3 did not facilitate functional expression of either homomeric
9 or heteromeric
9
10 nAChRs in mammalian cell lines. It is noteworthy that whereas RIC-3 has been reported to cause a marked functional inhibition of heteromeric nAChRs such as
3
4 and
4
2 expressed in Xenopus laevis oocytes, RIC-3 significantly enhances levels of functional expression of these and other (
3
2 and
4
4) heteromeric nAChRs when expressed in mammalian cell lines. In addition, the interaction of multiple nAChR subunits (
3,
4,
7,
2, and
4) with RIC-3 has been demonstrated by coimmunoprecipitation from metabolically labeled transfected cells. It is significant that coimmunoprecipitation experiments have provided evidence that RIC-3 associates with unassembled nAChR subunits, a finding that is consistent with previous suggestions that RIC-3 may act by enhancing the maturation (subunit folding and assembly) of nAChRs. We conclude that RIC-3 is an nAChR-associated protein that can enhance functional expression of multiple nAChR subtypes in transfected mammalian cells.
Address correspondence to: Dr. Neil Millar, Department of Pharmacology, University College London, Gower St., London, WC1E 6BT United Kingdom. E-mail: n.millar{at}ucl.ac.uk
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