Negative regulatory activity of a prostaglandin F-2 alpha receptor associated protein (FPRP)

The cDNA has been cloned for a protein which copurifies with and colocalizes with [H-3]PGF(2 alpha) binding activity. This cloning was based on prior purification of the [H-3]PG(2 alpha)binding complex from pregnant corpus luteum, antibody production against the protein of interest, and antibody screening of a rat ovary cDNA expression library.(1) Here I report on the activity of this prostaglandin F-2 alpha receptor (FP) associated protein (FPRP). Expression of the FPRP cDNA in COS cells results in production of a full length (approximate to 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 [H-3]PG(2 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 [H-3]PG(2 alpha)binding by FPRP occurs only when the FPRP cDNA is expressed in the same cell as the FP resides, reaches a maximum of approximate to 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.