THE LEUCINE ZIPPERS OF C-FOS AND C-JUN FOR PROGESTERONE-RECEPTOR DIMERIZATION - A-DOMINANCE IN THE A B HETERODIMER/

Human progesterone receptors (hPR) exist as two isoforms: 120 kDa B-re ceptors (hPR(B)) and N-terminally truncated 94 kDa A-receptors (hPR(A) ). When transfected separately, each isoform exhibits different transc riptional properties that are ligand- and promoter-specific. In human target tissues, both receptor isoforms are present, so that a mixture of three dimeric species, A/A, A/B, and B/B, bind to DNA at progestero ne response elements (PRE), and regulate transcription. To study the t ranscriptional phenotype of pure A/B heterodimers uncontaminated by A/ A or B/B homodimers, we exploited the property of the leucine zipper ( zip) domains of fos and jun, to form pure heterodimers. Chimeric const ructs were made linking the zip of either c-fos or c-jun to the C-term inus of hPR(B) or hPR(A) (hPR-zip) to produce A-fos, B-fos, A-jun or B -jun. To determine whether the A- or B-isoform is functionally dominan t in the A/B heterodimer, cells expressing hPR-zip chimeras were treat ed with the progestin antagonist RU486, which produces opposite transc riptional effects with the two isoforms. Gel mobility shift and immune co-precipitation assays show that in the presence of RU486 only pure heterodimers form between A-fos/B-jun or A-jun/B-fos, and bind DNA at PREs. Thus, in these pairs, interactions between the extrinsic fos/jun zipper domains override interactions between the intrinsic hPR dimeri zation domains. We find that under these conditions, antagonist-occupi ed B-zip homodimers stimulate transcription, while antagonist-occupied A-zip homodimers are inhibitory, and that pure A/B zip heterodimers h ave the inhibitory transcriptional phenotype of the A-zip homodimers. We conclude that, in pure heterodimers, A-receptors are dominant negat ive inhibitors of B-receptors. Additionally, the pure PR-zip heterodim ers, unlike wild-type receptors, bind a PRE in the absence of hormone but do not activate transcription. Thus, PR dimerization and PRE bindi ng are necessary but, without hormone, not sufficient to activate tran scription.