CLONING OF A CARBOXYL-TERMINAL ISOFORM OF THE PROSTANOID FP RECEPTOR

An FP prostanoid receptor isoform, which appears to arise from alterna tive mRNA splicing, has been cloned from a mid cycle ovine large cell corpus luteum library. The isoform, named the FPB receptor, is identic al to the original isoform, the FPA, throughout the seven transmembran e domains, but diverges nine amino acids into the carboxyl terminus. I n contrast to FPA, whose carboxyl terminus continues for another 46 am ino acids beyond the nine shared residues, the FPB terminates after on ly one amino acid. The FPA isoform appears to arise by the failure to utilize a potential splice site, while a 3.2-kilobase pair intron is s pliced out from the FP gene to generate the FPB isoform mRNA. The two isoforms have indistinguishable radioligand binding properties, but se em to differ in functional coupling to phosphatidylinositol hydrolysis . Thus, in COS-7 cells transiently transfected with either the FPA or the FPB receptor cDNAs, prostaglandin F-2 alpha stimulates inositol ph osphate accumulation to the same absolute maximum, but the basal level of inositol phosphate accumulation is approximately 1.3-fold higher i n cells transfected with the FPB as compared with cells transfected wi th the FPA isoform. Using the polymerase chain reaction, mRNA encoding the FPB isoform was identified in the ovine corpus luteum.