PRAP, A PROLACTIN RECEPTOR-ASSOCIATED PROTEIN - ITS GENE-EXPRESSION AND REGULATION IN THE CORPUS-LUTEUM

We have recently identified, characterized, and cloned a luteal micros omal 32-kDa phosphoprotein that we named FRAP (for PRL-receptor associ ated protein), and we have demonstrated that PRAP binds to the intrace llular domain of the short but not the long form of the PRL receptor. In this study, we used FRAP cDNA to examine the tissue specificity, th e developmental expression, and the hormonal regulation of PRAP gene e xpression. Northern blot analysis revealed that in the corpus luteum, FRAP cDNA hybridized to multiple transcripts (5.5 kb, 4.3 kb, and 1.8 kb), with the smallest transcript (1.8 kb) corresponding to the size o f the cDNA clone. However, none of these transcripts were detected in any other tissues examined. FRAP appears to be tightly regulated by st eroids and PRL. When pregnant rats were treated with aminoglutethimide , a steroid synthesis inhibitor, all three FRAP transcripts became bar ely detectable. Similar results were obtained when all luteotropic sup port was removed by hypophysectomy and hysterectomy. Estradiol up-regu lated FRAP expression and, more specifically, the two lower transcript s. PRL had no stimulatory effect on FRAP messenger RNA (mRNA) expressi on but caused a substantial increase in the level of FRAP protein when administered to hypophysectomized pregnant rat, suggesting that PRL m ay stabilize this protein. Similar dissociation between levels of mRNA and protein were observed during luteal development. Although both FR AP mRNA and protein were barely detectable in early pregnancy, their e xpression increased abruptly from midpregnancy; however, whereas level s of PRAP mRNA declined from day 18, those of the protein remained ele vated until parturition. In summary, results of this study have define d the tissue specificity and developmental expression of FRAP mRNA dur ing pregnancy. The data have also revealed that the gene expression of this protein is up-regulated by estradiol, suggesting a pivotal role for FRAP in the synergistic action of estradiol and PRL on the functio n of the rat corpus luteum.