Negative regulatory activity of a prostaglandin F2 alpha receptor associated protein (FPRP)

The cDNA has been cloned for a protein which copurifies with and colocalizes with [3H]PGF2 alpha binding activity. This cloning was based on prior purification of the [3H]PGF2 alpha binding complex from pregnant corpus luteum, antibody production against the protein of interest, and antibody screening of a rat ovary cDNA expression library. Here I report on the activity of this prostaglandin F2 alpha receptor (FP) associated protein (FPRP). Expression of the FPRP cDNA in COS cells results in production of a full length (approximately 130 kD) immunoreactive molecule with an endoplasmic reticulum and Golgi network distribution similar to that seen in granulosa lutein cells. COS cell expressed FPRP inhibits binding of [3H]PGF2 alpha to FP of COS cell origin or FP expressed from cotransfected rat or mouse FP cDNA in a dose-dependent manner. This inhibition of [3H]PGF2 alpha binding by FPRP occurs only when the FPRP cDNA is expressed in the same cell as the FP resides, reaches a maximum of approximately 80%, and is unaffected by second messenger perturbing agents such as phorbol ester, 8-Br-cAMP, calcium ionophore A23187, and okadaic acid. Scatchard analysis indicates that FPRP induces a decrease in receptor number rather than affinity constant, suggesting a non-competitive means of inhibition. Molecular dissection of the FPRP protein indicates that two portions of the molecule play a role in the inhibition of FP. Whether FPRP is an FP-associated regulatory molecule, an FP subunit, or a receptor for a PGF2 alpha-antagonistic ligand is presently unknown. Physiological relevance and significance of FPRP are discussed. During the course of these experiments it was necessary to clone the rat FP cDNA.