Constitutively active mutant gives novel insights into the mechanism of bitter taste receptor activation

J. Neurochem. (2012) 122, 537544. Abstract The human bitter taste receptors (T2Rs) belong to the G-protein coupled receptor (GPCR) superfamily. T2Rs share little homology with the large subfamily of Class A G-protein coupled receptors, and their mechanisms of activation are poorly understood. Guided by biochemical and molecular approaches, we identified two conserved amino acids Gly281.46 and Ser2857.47 present on transmembrane (TM) helices, TM1 and TM7, which might play important roles in T2R activation. Previously, it was shown that naturally occurring Gly511.46 mutations in the dim light receptor, rhodopsin, cause autosomal dominant retinitis pigmentosa, with the mutants severely defective in signal transduction. We mutated Gly281.46 and Ser2857.47 in T2R4 to G28A, G28L, S285A, S285T, and S285P, and carried out pharmacological characterization of the mutants. No major changes in signaling were observed upon mutation of Gly281.46 in T2R4. Interestingly, S285A mutant displayed agonist-independent activity (approximately threefold over basal wild-type T2R4 or S285T or S285P). We propose that Ser2857.47 stabilizes the inactive state of T2R4 by a network of hydrogen-bonds connecting important residues on TM1-TM2-TM7. We compare and contrast this hydrogen-bond network with that present in rhodopsin. Thus far, S285A is the first constitutively active T2R mutant reported, and gives novel insights into T2R activation.