Dimerization of the chicken progesterone receptor in vitro can occur in the absence of hormone and DNA

We have analyzed the dimerization of two forms of the chicken progesterone receptor (cPR_A and cPR_B) by nondenaturing gradient gel electrophoresis and chemical cross-linking with dimethylpimelimidate (DMP). We demonstrate by these two methods that the PRs assemble in vitro into dimers in the absence of DNA, and that dimerization does not require hormone. The cPR_A homodimer binds quantitatively to its cognate DNA response element in our nondenaturing gradient gel assay. DMP cross-linking confirms that both forms of the receptor (cPR_A and cPR_B) assemble into dimers in solution. Finally, in a standard mobility shift assay, chemically cross-linked receptors bind to the progesterone DNA response element with high affinity. We conclude that the PR contains a dimerization motif, which can promote stable subunit-subunit contacts without the presence of hormone in vitro. The complex thus formed expresses sequence-specific DNA-binding activity indistinguishable from that observed in the presence of hormone.