AN AMINO-TERMINAL TRUNCATED PROGESTERONE-RECEPTOR ISOFORM, PR(C) ENHANCES PROGESTIN-INDUCED TRANSCRIPTIONAL ACTIVITY

Previously we reported the identification of two unique progesterone r eceptor (PR) messenger RNA transcripts that encode a smaller PR isofor m, termed the C-receptor (PR(C)). These two PR transcripts encode a pr otein that is N-terminally truncated, so that it lacks the first zinc finger of the DNA binding domain, but still contains a complete hormon e binding region with sequences for dimerization and nuclear localizat ion. We also have demonstrated the existence of a 60-kDa progestin-spe cific binding protein in progestin target cells using a monoclonal ant ibody directed to the C-terminus of PRs, suggesting that these two nov el transcripts generate a truncated form of PR. In this paper, we addr ess the hypothesis that the C-receptor arises from the initiation of t ranslation of a methionine C-terminal to the methionine start sites th at generate the larger 94-kDa A and 116-kDa B human PR isoforms. The s tudies shown here support the postulate that another downstream in-fra me methionine within the PR-coding region can serve as a translation i nitiation site for the generation of a third PR protein. A partial PR complementary DNA, lacking the translation start sites for B- and A-re ceptors was translated in vitro. The synthetic protein product bound [ H-3]progestins and unlabeled progestins. The antiprogestin RU486 also competed for this binding. Transfection of this partial PR complementa ry DNA into PR-negative HeLa cells resulted in progestin-specific bind ing activity. Because the third PR isoform lacks the first zinc finger of the DNA binding domain, but contains sequences for dimerization, w e reasoned that the C-receptor isoform would be transcriptionally inac tive and not bind DNA directly. Surprisingly, however, in the presence of A- and/or B-receptors, we found that C-receptors can modulate the transcriptional activity of A- and/or B-receptors using a reporter gen e. These studies emphasize that multiple receptor isoforms may have di stinct biological properties, and that the truncated C-receptor may pl ay a role in explaining some of the pleiotropic effects of progestins.