Functional Architecture of Olfactory Ionotropic Glutamate Receptors

Abstract

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate chemical communication between neurons at synapses. A variant iGluR subfamily, the Ionotropic Receptors (IRs), was recently proposed to detect environmental volatile chemicals in olfactory cilia. Here we elucidate how these peripheral chemosensors have evolved mechanistically from their iGluR ancestors. Using a Drosophila model, we demonstrate that IRs act in combinations of up to three subunits, comprising individual odor-specific receptors and one or two broadly expressed co-receptors. Heteromeric IR complex formation is necessary and sufficient for trafficking to cilia and mediating odor-evoked electrophysiological responses in vivo and in vitro. IRs display heterogeneous ion conduction specificities related to their variable pore sequences, and divergent ligand-binding domains function in odor recognition and cilia localization. Our results provide insights into the conserved and distinct architecture of these olfactory and synaptic ion channels and offer perspectives into use of IRs as genetically encoded chemical sensors.

Mosquito odorant receptor for DEET and methyl jasmonate

“…DmelIR84a• DmelIR8a-expressing oocytes generated robust currents (253 ± 26 nA, 1 μM; 1,616 ± 294 nA, 10 μM; and 3,783 ± 159 nA, 100 μM) to phenylacetaldehyde, in agreement with a previous report (6). Although CquiIR40a coexpressed with DmelIR8a remained silent, DmelIR84a coexpressed with CquiIR8a responded to phenylacetaldehyde (717 ± 166 nA, 1 mM), thus suggesting that CquiIR8a was indeed functional.…”

Stoichiometry and specific assembly of Best ion channels

“…S1 and S2). This probability value of mEGFP being fluorescent is similar to what was observed previously for mEGFP fused to a number of other membrane proteins (28)(29)(30)(31)(32)(33). Binomial predictions based on four subunits also accounted well for the other hBest family members ( Fig.…”

“…Although earlier biochemical studies concluded that the Best1 channel is an oligomer, the estimate of the number of subunits ranged from 2 to 5 (22,27). To resolve this question, we used the single-molecule subunitcounting method (28)(29)(30)(31)(32)(33) to determine the number of subunits by counting the number of fluorescence-bleaching steps of monomeric enhanced green fluorescent protein (mEGFP)-tagged hBest1 in Xenopus oocytes [EGFP-tagged hBest1 was previously shown to be functional (34)]. The main advantages of this method are that: (i) the counting uses Total Internal Reflection Fluorescence Microscopy (TIRFM) to focus exclusively on functional channels in their native physiological environment, the plasma membrane of live cells, and (ii) because the level of expression can be easily controlled in oocytes, it is possible to obtain low enough densities so that each channel appears as an individual fluorescent spot that can be readily resolved from its neighbors.…”

Differential expression of olfactory genes in the southern house mosquito and insights into unique odorant receptor gene isoforms

“…By contrast, the other member of the family, CquiIR8a, was highly enriched in antennae (mod_lfc, 7.3). Heterologous expression in Xenopus laevis oocytes clearly demonstrated that only when coexpressed with DmelIR8a, the receptor DmelIR75a responded to propionic acid, whereas DmelIR84a·DmelIR8a was activated by phenylacetaldehyde (28). Mutations in DmelIR8a as well as DmelIR25a eliminated responses in IR-expressing OR neurons in D. melanogaster (28), thus both DmelIR8a and DmelIR25a are considered to be coreceptors, although evidence in heterologous system is still lacking for the latter.…”