Dual Role of the RIC-3 Protein in Trafficking of Serotonin and Nicotinic Acetylcholine Receptors

Mar Castillo,José Mulet,Luis M Gutiérrez,José A Ortiz,Francisco Castelán,Susana Gerber,Salvador Sala,Francisco Sala,Manuel Criado

doi:10.1074/jbc.m503746200

Mar Castillo, José Mulet + Show 7 more

Open Access

https://doi.org/10.1074/jbc.m503746200

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Abstract

The ric-3 gene is required for maturation of nicotinic acetylcholine receptors in Caenorhabditis elegans. The human homolog of RIC-3, hRIC-3, enhances expression of alpha7 nicotinic receptors in Xenopus laevis oocytes, whereas it totally abolishes expression of alpha4beta2 nicotinic and 5-HT3 serotonergic receptors. Both the N-terminal region of hRIC-3, which contains two transmembrane segments, and the C-terminal region are needed for these differential effects. hRIC-3 inhibits receptor expression by hindering export of mature receptors to the cell membrane. By using chimeric proteins made of alpha7 and 5-HT3 receptors, we have shown that the presence of an extracellular isoleucine close to the first transmembrane receptor fragment is responsible for the transport arrest induced by hRIC-3. Enhancement of alpha7 receptor expression occurs, at least, at two levels: by increasing the number of mature receptors and facilitating its transport to the membrane. Certain amino acids of a putative amphipathic helix present at the large cytoplasmic region of the alpha7 subunit are required for these actions. Therefore, hRIC-3 can act as a specific regulator of receptor expression at different levels.

Highlights

Receptors may be required [7,8,9]
The hRIC-3 Protein Is Present at the endoplasmic reticulum (ER) and the Golgi Apparatus—The first transmembrane segment of hRIC-3 is adjacent to the N terminus and could play the role of a leader peptide
Results were similar in the three cases, suggesting that hRIC-3 is synthesized in the ER and might be addressed to the ER and/or some other organelle along the secretory pathway, facilitating the encounter with ion channels subunits, which transit the same route

Summary

Introduction

Receptors may be required [7,8,9]. The hRIC-3 protein could be one of them, as mutations in the ric-3 gene affect maturation of nicotinic acetylcholine receptors (nAChRs) in Caenorhabditis elegans [10]. HRIC-3 enhances surface expression and currents of ␣7 nAChRs, whereas it inhibits currents produced by other nAChRs subtypes (␣4␤2 and ␣3␤4) and by 5-HT3 serotonin receptors (5-HT3Rs) These opposite effects suggested that the hRIC-3 protein might play a key role in the biogenesis of some ligand-gated receptors and prompted us to investigate how and where it performs its action. Receptors are assembled in the endoplasmic reticulum (ER) and transported to the cell surface through a process that may be of central importance in regulating the efficacy of synaptic transmission [3, 4] This process is relatively inefficient [5, 6], probably as a consequence of tight quality controls that guarantee the functional competence of the final product. Specific proteins involved in assembly and trafficking of

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