The binding of L-glutamate or L-aspartate to the umami receptor (TAS1R1:TAS1R3) causes a conformational change in the receptor that is transmitted primarily through the TAS1R1 subunit to the associated gustducin G-protein heterotrimer (GNAT3:GNB1,3:GNG13) and causes the alpha subunit, GNAT3 (gustducin), to exchange GDP for GTP (Sainz et al. 2007). In mouse taste cells, gustducin alpha (GNAT3) and Ggamma 13 (GNG13) colocalize with TAS1R1:TAS1R3 bitter receptors and are required to activate downstream IP3 signaling (Huang et al. 1999). In rat taste cells of circumvallate papillae, both GNB1 and GNB3 are expressed and participate in activation of downstream signaling in response to bitter tastants (Rossler et al. 2000).