1.  Field of the Invention
The invention relates to newly identified mammalian chemosensory G protein-coupled receptors, to family of such receptors, and to the genes and cDNA encoding said receptors.  More particularly, the invention relates to newly identified mammalian chemosensory G protein-coupled receptors active in taste signaling which function as hetero-oligomers. 
2.  Description of the Related Art
The taste system provides sensory information about the chemical composition of the external world.  Taste transduction is one of the most sophisticated forms of chemical-triggered sensation in animals.  At present, the means by which taste sensations are elicited remains poorly understood.  See, e.g., Margolskee, BioEssays, 15:645-50 (1993); Avenet et al.  J. Membrane Biol.,  112:1-8 (1989).  Taste signaling is found throughout the animal kingdom, from simple metazoans to the most complex of vertebrates.  Taste sensation is thought to involve distinct signaling pathways.  These pathways are believed to be mediated by receptors, i.e., metabotropic or inotropic receptors.  Cells which express taste receptors, when exposed to certain chemical stimuli, elicit taste sensation by depolarizing to generate an action potential, which is believed to trigger the sensation.  This event is believed to trigger the release of neurotransmitters at gustatory afferent neuron synapses, thereby initiating signaling along neuronal pathways that mediate taste perception.  See, e.g., Roper, Ann.  Rev. Neurosci.,  12:329-53 (1989). 
As such, taste receptors specifically recognize molecules that elicit specific taste sensation.  Those molecules are also referred to herein as “tastants.”  Many taste receptors belong to the 7-transmembrane receptor superfamily (Hoon et al.,  Cell 96:451 (1999); Adler et al.,  Cell 100:693 (2000)), which are also known as G protein-coupled receptors (GPCRs).  Other tastes are believed to be mediated by channel proteins.  G protein-coupled receptors control many physiological functions, such as endocrine function, exocrine function, heart rate, lipolysis, carbohydrate metabolism, and transmembrane signaling.  The biochemical analysis and molecular cloning of a number of such receptors has revealed many basic principles regarding the function of these receptors. 
For example, U.S. Pat.  No.  5,691,188 describes how upon a ligand binding to a GPCR, the receptor presumably undergoes a conformational change leading to activation of the G protein.  G proteins are comprised of three subunits: a guanyl nucleotide binding α subunit, a β subunit, and a γ subunit.  G proteins cycle between two forms, depending on whether GDP or GTP is bound to the α subunit.  When GDP is bound, the G protein exists as a heterotrimer: the Gαβγ complex.  When GTP is bound, the α subunit dissociates from the heterotrimer, leaving a Gβγ complex.  When a Gαβγ complex operatively associates with an activated G protein-coupled receptor in a cell membrane, the rate of exchange of GTP for bound GDP is increased and the rate of dissociation of the bound Gα subunit from the Gαβγ complex increases.  The free Gα subunit and Gβγ complex are thus capable of transmitting a signal to downstream elements of a variety of signal transduction pathways.  These events form the basis for a multiplicity of different cell signaling phenomena, including for example the signaling phenomena that are identified as neurological sensory perceptions such as taste and/or smell. 
Mammals are believed to have five basic taste modalities: sweet, bitter, sour, salty, and umami (the taste of monosodium glutamate).  See, e.g., Kawamura et al.,  Introduction to Umami: A Basic Taste (1987); Kinnamon et al.,  Ann.  Rev. Physiol.,  54:715-31 (1992); Lindemann, Physiol.  Rev., 76:718-66 (1996); Stewart et al.,  Am.  J. Physiol, 272:1-26 (1997).  Numerous physiological studies in animals have shown that taste receptor cells may selectively respond to different chemical stimuli.  See, e.g., Akabas et al.,  Science, 242:1047-50 (1988); Gilbertson et al.,  J. Gen. Physiol.,  100:803-24 (1992); Bernhardt et al.,  J. Physiol.,  490:325-36 (1996); Cummings et al, J. Neurophysiol.,  75:1256-63 (1996). 
In mammals, taste receptor cells are assembled into taste buds that are distributed into different papillae in the tongue epithelium.  Circumvallate papillae, found at the very back of the tongue, contain hundreds to thousands of taste buds.  By contrast, foliate papillac, localized to the posterior lateral edge of the tongue, contain dozens to hundreds of taste buds.  Further, fungiform papillac, located at the front of the tongue, contain only a single or a few taste buds. 
Each taste bud, depending on the species, contains 50-150 cells, including precursor cells, support cells, and taste receptor cells.  See, e.g., Lindemann, Physiol.  Rev., 76:718-66 (1996).  Receptor cells are innervated at their base by afferent nerve endings that transmit information to the taste centers of the cortex through synapses in the brain stem and thalamus.  Elucidating the mechanisms of taste cell signaling and information processing is important to understanding the function, regulation, and perception of the sense of taste. 
Although much is known about the psychophysics and physiology of taste cell function, very little is known about the molecules and pathways that mediate its sensory signaling response.  The identification and isolation of novel taste receptors and taste signaling molecules could allow for new methods of chemical and genetic modulation of taste transduction pathways.  For example, the availability of receptor and channel molecules could permit the screening for high affinity agonists, antagonists, inverse agonists, and modulators of taste activity.  Such taste modulating compounds could be useful in the pharmaceutical and food industries to improve the taste of a variety of consumer products, or to block undesirable tastes, e.g., in certain pharmaceuticals. 
Complete or partial sequences of numerous human and other eukaryotic chemosensory receptors are currently known.  See, e.g. Pilpel, Y. and Lancet, D., Protein Science, 8:969-977 (1999); Mombaerts, P., Annu.  Rev. Neurosci.,  22:487-50 (1999).  See also, EP0867508A2, U.S. Pat.  No.  5,874,243, WO 92/17585, WO 95/18140, WO 97/17444, WO 99/67282.  Because of the complexity of ligand-receptor interactions, and more particularly tastant-receptor interactions, information about ligand-receptor recognition is lacking.  In part, the present invention addresses the need for better understanding of the interactions between chemosensory receptors and chemical stimuli.  The present invention also provides, among other things, novel chemosensory receptors, and methods for utilizing such receptors, and the genes a cDNAs encoding such receptors, to identify molecules that can be used to modulate chemosensory transduction, such as taste sensation.