Notably, LBD mutations in certain Kainate receptors also reduce cell-surface expression in cultured cells (Valluru et al., 2005). While IR8a (and IR25a) may associate with unknown ligands important for trafficking, we favor instead a model in which the conformation of coreceptor LBDs contributes to a scaffold for correct assembly of an IR complex to ensure only functional heteromers reach sensory cilia. VX770 In contrast to IR8a and IR25a, evolution of odor-specific IRs, such as IR84a and IR75a, was accompanied by a significant reduction in the structural complexity, as these proteins lack an ATD and bear only short, and apparently dispensable,

cytosolic C termini. Divergent LBDs and pore filters in these proteins appear to confer specificity of odor recognition and ion conduction properties of IR-receptor complexes, respectively. Traces of ancestral glutamate-binding mechanisms are detectable, however, as we show that a glutamate-conjugating arginine is conserved

and essential in IR84a for recognition of its odor ligand, phenylacetaldehyde. Odor-specific IR sequences may provide a valuable source of natural (and functional) “site-directed mutants” to understand how the ion conduction and other properties of these ligand-gated ion channels are specified at the molecular level. Our reconstitution of olfactory responses using a combination of three distinct IRs (IR25a, IR76a, and IR76b) highlights a further level of sophistication in how these proteins assemble into functional odor-sensing complexes. While IR76a is very likely to define ligand-specificity, the precise contributions of IR25a and a second putative SKI-606 in vitro coreceptor, IR76b, have not yet been resolved. It is possible that IR76b, which is more closely related to odor-specific IRs than to IR25a or IR8a, recognizes an unknown chemical ligand, whose copresence with phenylethyl amine in an odor blend could lead to synergistic or diminished neuronal responsiveness. Further variations

in IR complexes are apparent. For example, IR25a is likely to have IR76b-independent roles as a coreceptor for sacculus and aristal odor-specific IRs, as not the latter receptor is not expressed in these structures (Benton et al., 2009). Moreover, the ammonia receptor in ac1 is independent of both IR8a and IR25a. Thus, while OR-expressing neurons in vertebrates and insects encode odor stimuli through the activity of singularly expressed odor-specific receptors (Touhara and Vosshall, 2009), the IRs appear to function in “combinatorial codes” within individual OSNs. These may define unique ligand sensitivities and signaling dynamics akin to the heteromer-specific properties of iGluRs in synaptic localization and signaling (Coussen, 2009, Greger et al., 2007 and Kohr, 2006). In contrast to iGluRs (Walker et al., 2006), however, IRs do not appear to depend upon additional accessory proteins, such as TARPs (Tomita, 2010), for cell surface expression or function.