Downstream Signaling Effectors for Umami Taste

Monosodium glutamate (MSG) elicits a unique taste called umami. A characteristic feature of umami taste is its potentiation by 5′-ribonucleotides (primarily GMP and IMP), which also have an umami taste of their own. Based on recent molecular studies, two putative umami receptors have been identified: a truncated variant of the metabotropic glutamate receptor mGluR4 (taste-mGluR4) (Chaudhari et al., 1996, 2000) and the heterodimer, T1R1 + T1R3 (Li et al., 2002; Zhao et al., 2003). Both of these receptors are expressed in taste cells and both receptors have been expressed in heterologous cells, where they respond to glutamate at taste effective concentrations. The T1R1 + T1R3 heterodimer is potenti-ated strongly by IMP, when MSG is presented together with the nucleotide. Targeted gene deletion of T1R1 or T1R3 abolishes the synergistic responses of MSG and IMP (Damak et al., 2003; Zhao et al., 2003), suggesting that T1R1 + T1R3 is required for th