A novel human receptor involved in bitter tastant detection identified using the model organism Dictyostelium discoideum

Abstract

Detection of substances tasting bitter to humans occurs in diverse organisms including the social amoeba, Dictyostelium discoideum. To establish a molecular mechanism for bitter tastant detection in Dictyostelium, we screened a mutant library for resistance to a commonly used bitter standard, phenylthiourea. This approach identified a G-protein coupled receptor mutant, grlJ−, showing a significantly increased tolerance to phenylthiourea in growth, survival and movement. This mutant was not resistant to a structurally dissimilar potent bitter tastant, denatonium benzoate, suggesting it is not a target for at least one other bitter tastant. Analysis of the cell signalling pathway involved in the detection of phenylthiourea showed dependence upon heterotrimeric G-protein and phosphatidylinositol 3-kinase activity, suggesting this signalling pathway is responsible for phenylthiourea cellular effects. This is further supported by a phenylthiourea-dependent block in the transient cAMP-induced production of PIP3 in wild type but not grlJ− cells. Finally, we have identified an uncharacterized human protein gamma-aminobutyric acid (GABA) type B receptor subunit 1 isoform with weak homology to GrlJ that restored grlJ− sensitivity to phenylthiourea in cell movement and PIP3 regulation. Our data thus identifies a novel pathway for the detection of the standard bitter tastant, phenylthiourea, in Dictyostelium and implicates a poorly characterized human protein in phenylthiourea dependent cell responses.