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Interaction Domains of Components of the ERα/Sp1 and C/EBPβ Complex Essential for Prolactin Receptor Transcription.

Transcription of the human prolactin receptor (PRLR) is governed by an estradiol (E2)/estrogen receptor α(ERα) mechanism independent of an estrogen response element. E2/ERα, through complex formation with Sp1 and C/EBPβ that associate to cognate elements, leads to coactivator assembly, H3/4 acetylation and recruitment of TFIIB and Pol II (Dong et. al. JBC, 2006). The ligand binding domain of ERα was required for interaction with C/EBPβ or Sp1, and E2 promoted this association. The zinc finger motifs of Sp1 (2nd/ 3rd) and the leucine zipper of C/EBPβ were identified as the domain of interaction of the individual transfactors with ERα. Complex formation of ERα/Sp1/C/EBPβ enhanced by E2 was observed upon incubation of transfactors with ERα‐GST. Although C/EBPβ associated strongly with ERα, marked reduction of C/EBPβ association was observed upon its co‐incubation with ERα and Sp1. In contrast, the association of C/EBPβ was enhanced in the presence of the preformed Sp1/ERα complex (sequential addition). This is consistent with previous ChIP studies showing tethering of the C/EBPβ to the complex through association with ERα. In addition, C/EBPβ was found to associate with the Sp1 zinc finger domain. These studies provide evidence for direct association of components of the complex essential for PRLR transcription through defined interaction domains of the individual proteins. Supported by the IRP, NICHD.