Guanine nucleotide-binding protein (G protein) alpha subunit playing a prominent role in bitter and sweet taste transduction as well as in umami (monosodium glutamate, monopotassium glutamate, and inosine monophosphate) taste transduction. Transduction by this alpha subunit involves coupling of specific cell-surface receptors with a cGMP-phosphodiesterase; Activation of phosphodiesterase lowers intracellular levels of cAMP and cGMP which may open a cyclic nucleotide-suppressible cation channel leading to influx of calcium, ultimately leading to release of neurotransmitter. Indeed, denatonium and strychnine induce transient reduction in cAMP and cGMP in taste tissue, whereas this decrease is inhibited by GNAT3 antibody. Gustducin heterotrimer transduces response to bitter and sweet compounds via regulation of phosphodiesterase for alpha subunit, as well as via activation of phospholipase C for beta and gamma subunits, with ultimate increase inositol trisphosphate and increase of intracellular Calcium. GNAT3 can functionally couple to taste receptors to transmit intracellular signal: receptor heterodimer TAS1R2/TAS1R3 senses sweetness and TAS1R1/TAS1R3 transduces umami taste, whereas the T2R family GPCRs act as bitter sensors. Functions also as lumenal sugar sensors in the gut to control the expression of the Na+-glucose transporter SGLT1 in response to dietaty sugar, as well as the secretion of Glucagon-like peptide-1, GLP-1 and glucose-dependent insulinotropic polypeptide, GIP. Thus, may modulate the gut capacity to absorb sugars, with implications for the prevention and treatment of malabsorption syndromes and diet-related disorders including diabetes and obesity. G proteins are composed of 3 units; alpha, beta and gamma, respectively GNAT3, GNB1 and GNG13 for Gustducin heterotrimer for bitter taste transduction. The alpha chain contains the guanine nucleotide binding site. Gustducin heterotrimer may also be composed of GNAT3, GNB3 and GNG13. Associated with microvilli, the presumed sites of sensory transduction in taste cells. Expressed in taste buds (sensory organs of clustered epithelial cells) of the circumvallate, foliate and fungiform papillae of the tongue, as well as in nasoincisor, palatal and epiglottal taste buds at protein level. Expressed in enteroendocrine of the gut, in the lumenal pole of a subset of brush cells lining the stomach and the intestine at protein level. Detected in solitary cells throughout the respiratory track. Expressed also in spermatozoa. Expressed in scattered solitary ovoid or bipolar cells among the oral epithelium from day 1-7, but with higher frequency in the soft palate as compared with the nasoincisor, circumvallate, and foliate papillae at day 1. During the second week, the solitary cells could no longer be recognized while cells expressing GNAT3 within the taste buds gradually increased. The onset of taste transduction accomplished by the palatal taste buds developed earlier than that achieved by taste buds in the circumvallate and foliate papillae. By bitter compounds denatonium and quinine. Potential N-myristoylation may anchor alpha-subunit to the inner surface of plasma membrane. Transgenic expression of GNAT3 restores responsiveness of GNAT3 deficient mice to both bitter and sweet compounds, whereas expression of mutated Gly-352 transgene do not. Furthermore, in wild-type mice, this mutated transgene acts as dominant-negative by inhibition of endogenous GNAT3 interactions with taste receptors. Belongs to the G-alpha family. G(i/o/t/z) subfamily.