Insights & Perspectives Are nicotinic acetylcholine receptors coupled to G proteins? Nadine Kabbani 1) *, Jacob C. Nordman 1) , Brian A. Corgiat 1) , Daniel P. Veltri 2) , Amarda Shehu 2) , Victoria A. Seymour 3) and David J. Adams 3) It was, until recently, accepted that the two classes of acetylcholine (ACh) receptors are distinct in an important sense: muscarinic ACh receptors signal via heterotrimeric GTP binding proteins (G proteins), whereas nicotinic ACh receptors (nAChRs) open to allow flux of Na þ , Ca 2þ , and K þ ions into the cell after activation. Here we present evidence of direct coupling between G proteins and nAChRs in neurons. Based on proteomic, biophysical, and functional evidence, we hypothesize that binding to G proteins modulates the activity and signaling of nAChRs in cells. It is important to note that while this hypothesis is new for the nAChR, it is consistent with known interactions between G proteins and structurally related ligand-gated ion channels. Therefore, it underscores an evolution- arily conserved metabotropic mechanism of G protein signaling via nAChR channels. Keywords: .acetylcholine; G protein coupling; intracellular loop; ligand-gated ion channel; loop modeling; protein interaction; signal transduction Introduction It is often said that two main types of neurotransmitter receptors exist – ionotropic ligand-gated ion channels (LGICs), which permit rapid ion flow directly across the cell membrane, and metabotropic receptors, which set in motion a slower chemical signaling cascade via the binding and activation of heterotrimeric GTP binding proteins (G proteins) following ligand activa- tion [1]. Neuronal nicotinic acetylcholine receptors (nAChRs) are a subdivision of LGICs widely distributed in nervous tissue and contribute to processes such as neurotransmitter release and synaptic plasticity [2, 3]. Mutations within nAChR genes are implicated in a number of human disorders including drug addiction and schizo- phrenia [4]. Nicotinic receptors belong to an evolutionarily conserved class of cys- loop containing receptor channels that includes GABA A , glycine, and 5HT 3 receptors as well as two newly discovered channels: a zinc-activated channel and an invertebrate GABA-gated cation chan- nel [5]. In mammals, genes encoding neuronal nAChR subunits have been identified and labeled a (a1–a10) and b (b1–b4). Functional nAChRs are derived from an arrangement of five subunits into heteromeric or homomeric receptors [6] (Fig. 1A). The activity of nAChRs also appears driven by direct protein-protein associations with molecules such as receptor kinases, scaffolds, and signaling effectors [7]. A growing list of proteins has emerged as components of the nAChR signaling network. This list includes scaffold proteins such as 1433, and the calcium sensor visinin like protein- 1 [8, 9]. In this article, we discuss findings on associations between nAChRs and G proteins. These findings support the hypothesis that nAChRs couple to G proteins at the plasma membrane. Evolutionary emergence of an intracellular protein- protein interaction domain in nicotinic receptors Nicotinic receptor subunits share a topology that consists of a large DOI 10.1002/bies.201300082 1) Department of Molecular Neuroscience, Krasnow Institute for Advanced Study, Fairfax, VA, USA 2) Department of Computer Science, George Mason University, Fairfax, VA, USA 3) Health Innovations Research Institute, RMIT University, Melbourne, VIC, Australia *Corresponding author: Nadine Kabbani E-mail: nkabbani@gmu.edu Abbreviations: ACh, acetylcholine; GPCR, G protein coupled receptor; LGIC, ligand-gated ion channel; nAChR, nicotinic acetylcholine receptor; TM, transmembrane. www.bioessays-journal.com 1025 Bioessays 35: 1025–1034, ß 2013 WILEY Periodicals, Inc. Hypotheses