A cDNA clone for a novel isoform of prostaglandin (PG) F-2 alpha receptor (FP) was isolated from the cDNA pool of the bovine corpus luteum. The sequence analysis revealed that the new FP isoform (FPa) encodes a 295-amino acid protein carrying a specific 28-amino acid sequence from the middle of transmembrane segment VI to the carboxyl terminus. Because only one copy gene has been identified for FPa FP. was generated by alternative mRNA splicing at the middle of the Vi transmembrane region, resulting in the lack of a VII transmembrane segment and an intracellular carboxyl tail. The RT-PCR analysis for FP and FPa indicated that both mRNAs are expressed similarly during the estrous cycle and pregnancy. The PGF(2 alpha) stimulation drastically enhanced protein kinase C (PKC) activity in the COS-7 cell transfected with FPalpha whereas no PKC activation was detected in FPalpha-transfected cells. Cotransfection of an excess amount of FPa markedly reduced FP-mediated PKC activity, suggesting that the novel FP isoform might play a role as a negative regulator to attenuate normal. FP function.