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

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.