The chicken progesterone receptor: sequence, expression and functional analysis.

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Abstract
The complete mRNA sequence of the chicken progesterone receptor (cPR) has been determined. Expression of the cloned cDNA both in vivo and in vitro produces a protein that has the same apparent mol. wt on SDS–polyacrylamide gels as the ‘natural’ cPR form B (109 kd) as determined by immunoblotting and photoaffinity labelling. When expressed in HeLa or in Cos‐1 cells the ‘cloned’ cPR displays hormone binding characteristics indistinguishable from the ‘natural’ receptor and, in the presence of progestins, exhibits ‘tight nuclear binding’. A protein corresponding in size to the cPR form A (79 kd) could be detected by expressing in vivo and in vitro an N‐terminally truncated cPR starting at methionine 128. A protein of the same apparent mol. wt results from internal initiation during in vitro translation. In contrast, such a protein was barely detectable after in vivo expression of the cPR cDNA in Cos‐1 cells. These results suggest that form A is generated by an oviduct cell specific process involving either internal initiation of translation and/or proteolysis in the vicinity of methionine‐128. The cPR contains two highly conserved regions C and E, a characteristic of the steroid/thyroid hormone receptor supergene family. By expression of a series of cPR deletion mutants, region E could be defined as the hormone binding domain whereas region C is indispensable for the tight nuclear association of the progestin‐receptor complex. In the presence of progestins, the cloned cPR efficiently trans‐activates transcription from the long terminal repeat region (LTR) of the mouse mammary tumor virus (MMTV). Deletion of the entire N‐terminal region A/B or of the hormone binding domain E results in a 100‐fold reduction of transcriptional activation. No stimulation of transcription can be detected when the C‐terminal deletion extends into region C, indicating that this region is involved in the recognition of the hormone responsive element (HRE) of the MMTV LTR.