title: Differential subcellular localization of RIC-3 isoforms and their role in determining 5-HT3 receptor composition.
abstract: RIC-3 has been identified as a chaperone molecule involved in promoting the functional expression of nicotinic acetylcholine and 5-HT(3) receptors in mammalian cells. In this study, we examined the effects of RIC-3a (isoform a) and a truncated isoform (isoform d) on RIC-3 localization, mobility, and aggregation and its effect on 5-HT3 receptor composition in mammalian cells. Human RIC-3a possesses an amino-terminal signal sequence that targets it to the endoplasmic reticulum where it is distributed within the reticular network, often forming large diffuse "slicks" and bright "halo" structures. RIC-3a is highly mobile within and between these compartments. Despite the propensity for RIC-3a to aggregate, its expression enhances the level of surface 5-HT3A (homomeric) receptors. In contrast, RIC-3a exerts an inhibitory action on the surface expression of heteromeric 5-HT3A/B receptors. RIC-3d exhibits an altered subcellular distribution, being localized to the endoplasmic reticulum, large diffuse slicks, tubulo-vesicular structures, and the Golgi. Bidirectional trafficking between the endoplasmic reticulum and Golgi suggests that RIC-3d constitutively cycles between these two compartments. In support of the large coiled-coil domain of RIC-3a being responsible for protein aggregation, RIC-3d, lacking this cytoplasmic domain, does not aggregate or induce the formation of bright aggregates. Regardless of these differences, isoform d is still capable of enhancing homomeric, and inhibiting heteromeric, 5-HT3 receptor expression. Thus, both isoforms of RIC-3 play a role in determining 5-HT3 receptor composition.
authors.affiliation: Neurosciences Institute, Division of Pathology and Neuroscience, Ninewells Medical School, University of Dundee, Dundee DD1 9SY, Scotland.
authors.forename: Aixin
authors.initials: A
authors.lastname: Cheng
authors.forename: Karen A
authors.initials: KA
authors.lastname: Bollan
authors.forename: Sam M
authors.initials: SM
authors.lastname: Greenwood
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Irving
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Connolly
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology; D006056:Golgi Apparatus / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D044406:Receptors, Serotonin, 5-HT3 / Q000235:genetics / Q000378:metabolism*
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; D020033:Protein Isoforms; C496701:RIC3 protein, human; D044406:Receptors, Serotonin, 5-HT3