Assembly and trafficking of nicotinic and 5HT₃ receptors
Nicotinic acetylcholine receptors (nAChRs) and 5-hydroxytryptamine type 3 receptors (5HT3R.S) are pentameric ligand-gated ion channels which form both heteromeric and homomeric complexes. Aspects of the assembly and pharmacological properties of the a 7 nAChR and 5HT3R were examined through three independent studies. The first study examined the mechanism by which the 5HT3B subunit, when expressed alone, is retained within the endoplasmic reticulum (ER). The 5HT3R forms both homomeric (composed of 5HT3A subunits) and heteromeric (composed of 5HT3A and 5HT3B subunits) complexes. In contrast to 5HT3A, the 5HT3B subunit cannot form a functional homomeric receptor. An ER retention motif (RAR) was identified in the 5HT3B subunit, which appears to be masked by the 5HT3A subunit. Evidence to support this conclusion was obtained from co-expression of the subunits, which resulted in the presence of 5HT3B on the cell surface. The a 7 nAChR and 5HT3R have similar N-terminal ligand binding domains and cross-reactivity of some ligands is observed. Both mouse 5HT3A and a 7 are potentiated by the aromatic moiety of 5-HT, 5-hydroxyindole (5-HI), whereas human 5HT3A is not. In an attempt to define the 5-HI binding site, human/mouse 5HT3A subunit chimeras were constructed. Studies using the chimeras suggest that the action of 5-HI may be mediated by both the N- and C-terminal domains of 5HT3A. In the final study, the effects of the putative chaperone protein, RIC3, on ct7 receptor expression were examined. The efficient functional expression of the cc7 nAChR has been shown to be critically dependent on host-cell type, unlike the 5HT3R. RIC3 was shown to facilitate the efficient cell-surface expression of al in a mammalian cell line, where functional expression was not previously observed. The RIC3 protein has been identified as an a7-interacting protein which promotes the efficient assembly and folding of the subunit. RIC3 was also shown to promote 5HT3aR assembly.