NEGATIVE REGULATORY ACTIVITY OF A PROSTAGLANDIN-F2-ALPHA RECEPTOR-ASSOCIATED PROTEIN (FPRP)

The cDNA has been cloned for a protein which copurifies with and coloc alizes 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 pre gnant corpus luteum, antibody production against the protein of intere st, 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 ce lls results in production of a full length (approximate to 130 kD) imm unoreactive molecule with an endoplasmic reticulum and Golgi network d istribution similar to that seen in granulosa lutein cells. COS cell e xpressed FPRP inhibits binding of [H-3]PG(2 alpha)to FP of COS cell or igin 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 b y second messenger perturbing agents such as phorbol ester, 8-Br-cAMP, calcium ionophore A23187, and okadaic acid. Scatchard analysis indica tes that FPRP induces a decrease in receptor number rather than affini ty constant, suggesting a non-competitive means of inhibition. Molecul ar dissection of the FPRP protein indicates that two portions of the m olecule play a role in the inhibition of FP. Whether FPRP is an FP-ass ociated regulatory molecule, an FP subunit, or a receptor for a PGF2 a lpha-antagonistic ligand is presently unknown. Physiological relevance and significance of FPRP are discussed. During the course of these ex periments it was necessary to clone the rat FP cDNA.