Monoclonal antibodies against putative nuclear acceptor sites of the avian oviduct progesterone receptor.

Evidence from this and other laboratories has suggested that the nuclear binding sites (acceptor sites) for steroid receptors on chromatin involves chromatin protein-DNA complexes. A saturable high affinity receptor-dependent nuclear binding to these sites by isolated steroid receptor complexes has been reported. Addition of nonradiolabelled progesterone receptor from the chicken oviduct (PRov) successfully competes for the [3H]PRov binding to these acceptor sites in isolated chromatin or in nucleoacidic protein (NAP), a partially deproteinized chromatin enriched in these binding sites. This competition does not occur with pure DNA. This laboratory has isolated and enriched the chromatin proteins (acceptor proteins) involved in the nuclear acceptor sites for the avian oviduct PRov. Monoclonal antibodies against the nuclear acceptor sites for the PRov have been prepared using highly purified hen oviduct acceptor proteins reconstituted to hen DNA. Addition of the MAbs to a cell-free assay blocks PR binding to native oviduct chromatin as well as to NAP. However, the antibodies do not block PR binding to pure DNA nor do they affect the receptor itself. A partial animal species specifically was observed with the Ab inhibition of the PR binding, whereas no tissue specificity was seen. Direct binding of the antibodies to native acceptor sites was demonstrated using an ELISA system. The antibodies showed little recognition of free acceptor protein or DNA alone, indicating specificity for the protein-DNA complex. The partial evolutionary conservation of the nuclear acceptor sites for PR, as shown by the inhibition of PRov binding, was further supported by the partial crossreactivity of the MAbs with the NAPs from the same animal species using the ELISA. These data support earlier studies using PR binding assays showing that: 1) the reconstituted PR acceptor sites resemble the native sites; 2) the sites on whole chromatin and on NAP are similar; 3) the PR binding sites of chromatin and NAP are different from those of pure DNA; and 4) the nuclear acceptor sites for PR are different from those of the estrogen receptor. These results support a receptor specificity of the PR acceptor sites as reported previously using direct receptor competition studies.