SITE-DIRECTED MUTAGENESIS OF CYSTEINYL AND SERINE RESIDUES OF HUMAN THROMBOXANE A(2) RECEPTOR IN INSECT CELLS

A thromboxane A(2) receptor cDNA was isolated from a human placenta li brary by polymerase chain reaction (PCR) and was expressed in insect ( Sf21) cells using baculovirus system. The recombinant receptor exhibit ed [H-3]-SQ29548 and [I-125]-BOP binding activities with K-d values of 1.01 +/- 0.09 nM and 1.63 +/- 0.23 nM, respectively. The receptor bin ding activity was inhibited by dithiothreitol in a time- and concentra tion-dependent manner, indicating the involvement of disulfide linkage in ligand binding. The role of the four conserved cysteinyl residues in ligand binding was further examined by site-directed mutagenesis. E ach of the four cysteinyl residues was respectively mutated to a serin e residue. C102S, C105S, and C183S mutants exhibited no ligand binding activity although successful expression was achieved as revealed by i mmunoblot analysis, whereas C257S mutant retained most of the binding activity. Homology analysis of all prostanoid receptors indicates that Cys-105 (first extracellular loop) and Cys-183 (second extracellular loop) are conserved and are presumed to form a disulfide bond for rece ptor stability as suggested by the inhibition of ligand binding by dit hiothreitol reduction. Loss of binding activity by C102S mutant reveal ed that the sulfhydryl group of Cys-102 must play an essential role in ligand binding. Molecular modeling proposed that the Ser-201 is invol ved in interacting with TXA(2) by forming hydrogen bonding. Point muta tions of both Ser-201 and a conserved Ser-255 did not affect the ligan d binding specificity and affinity for [H-3]-SQ29548, but have signifi cantly altered K-d values for [I-125]-BOP. These results indicate that various cysteinyl and serine residues of thromboxane Az receptor may play different roles in ligand binding. (C) 1996 Academic Press, Inc.