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Human progesterone receptor (PR) is expressed in two protein forms, full length PR-B and truncated PR-A lacking the first 164 amino acids of the N-terminal domain. The two PR proteins have similar steroid and DNA binding activities but differ in their ability to transactivate target genes. The PR-A isoform has unique repressor activity. PR is phosphorylated on multiple serine residues located in the N-terminal domain and in the hinge region between the DNA and steroid binding domains (1). Different kinases are involved in phosphorylating PR including cyclin A/Cdk-2, casein kinase II and MAP kinases (2,3). Groups of sites are either basally phosphorylated in the absence of progesterone and undergo a rapid (5- 10min) increase upon binding hormone, while other sites are truely hormone-dependent and require approximately 2hr of hormone treatment for maximal phosphorylation (3). The activity of PR can be regulated by phosphorylation. Individual sites such as Ser 190 enhance transcriptional activity, while others such as Ser 294 appear to be sites of convergence and regulation by crosstalk with other signaling pathways (4,5).
Monoclonal antibodies (Mabs) have been produced that recognize a basal hormone regulated phosphorylation site ( Ser 190) and a hormone-dependent phosphorylation site (Ser 294) of human PR. Both sites are located in the N-terminal domain common to PR-A and PR-B. Biochemical experiments have shown that each of the Mabs recognize the specific phosphorylated forms of PR and fail to interact with dephosphorylated PR at these sites. Additionally, the Mabs distinguish between phospho and dephospho receptors under various experimental conditions including Western blot, immunoprecipitation and within PR-DNA complexes detected by electrophoretic gel mobility shift assay (EMSA). Even though Ser294 and surrounding sequences are identical in the two forms of PR, this site is preferentially phosphorylated on PR-B suggesting that differential phosphorylation of Ser 294 is involved regulating the distinct activities of PR-A and PR-B (5).
References:
1) Knotts, TA,Orkiszewski, R.S., Cook, R.G., Edwards,D.P.,Weigel, N.L. J. Biol. Chem. 276:8475-8483, 2001.
2) Zhang, Y, Beck, C.A., Poletti, A, Edwards, D.P., Weigel, N.L. Molecular Endocrinology 9: 1029-1040, 1995.
3) Zhang,Y., Beck, C.A., Poletti, A, Clement, J.P, Prendergast, P., Edwards, D.P,Weigel, N.L.Molecular Endocrinology 11: 823-832, 1997.
4) Takimoto,G.S. Hovland, A.R., Tasset, D.M, Melville, M.Y., Tung, L, Horwitz,K.B> J.Biol.Chem. 271:13308-13316, 1996.
5) Clemm, D.L., Sherman, L., Boonyaratanakornkit, V, Schrader,W.T.,Weigel, N.L., Edwards, D.P.Molecular Endocrinology 14: 52-65, 2000.
