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. 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. Expressed in epithelial cells of taste buds of the circumvallate, foliate and fungiform. Detected in various region of the respiratory track. Expressed also in spermatozoa. By bitter compounds denatonium, quinine, strychnine, nicotine, atropine, quinacrine and caffeic acid in presence of taste membranes. Belongs to the G-alpha family. G(i/o/t/z) subfamily.