Phenylalanine 138 in the second intracellular loop of human thromboxane receptor is critical for receptor-G-protein coupling

Eicosanoid receptors exhibit a highly conserved ERY(C)XXV(I)XXP (L) under b ar sequence in the second intracellular loop. The carboxyl end of this moti f contains a bulky hydrophobic amino acid (L,I,V, or F). In human thromboxa ne A2 receptor (TXA(2)R), phenylalanine 138 is located at the carboxyl end of this highly conserved motif. This study examined the function of the F13 8 in Gr protein coupling. F138 was mutated to aspartic acid (D) and tyrosin e (Y), respectively. Both mutants F138D and F138Y showed similar ligand bin ding activity to that of the wild type TXA(2)R. The Kd and Bmax values of e ither mutant were comparable to those of the wild type receptor. However, b oth mutants showed significant impairment of agonist induced Ca2+ signaling and phospholipase C activation. These results suggest that the F138 plays a key role in G protein coupling. (C) 1999 Academic Press.