CyP40, but not Hsp70, in rabbit reticulocyte lysate causes the aryl hydrocarbon receptor–DNA complex formation

Upon ligand binding, the aryl hydrocarbon receptor (AhR) translocates into the nucleus and dimerizes with its partner aryl hydrocarbon receptor nuclear translocator (Arnt). The AhR–Arnt heterodimer binds to the dioxin response element (DRE) to regulate target gene expression. Using baculovirus expressed human AhR and Arnt, we showed that the formation of the ligand-dependent AhR–Arnt–DRE complex requires protein factors in vitro. Recently, we provided evidence that p23, an Hsp90-associated protein, is involved in the complex formation. The aim of this study was to determine whether two other Hsp90-associated proteins present in rabbit reticulocyte lysate (RRL), namely CyP40 and Hsp70, play any role in forming the AhR–Arnt–DRE complex. Fractionation and immunodepletion experiments revealed that Hsp70 is not necessary for the formation of this complex. In contrast, CyP40 is involved in forming the complex since (1) immunodepletion of CyP40 from a RRL fraction reduces the intensity of the AhR–Arnt–DRE complex by 48% and (2) recombinant human CyP40 alone causes the formation of this complex. In addition, CyP40-interacting proteins appear to be essential for the full CyP40 effect on the AhR gel shift complex.