Heterodimerization between the glucocorticoid receptor and the unrelated DNA-binding protein, Xenopus glucocorticoid receptor accessory factor.

The adrenal steroid hormones, glucocorticoids, control many physiological responses to trauma, including elevated synthesis of fibrinogen, a major blood-clotting protein. Glucocorticoid regulation of the gamma-fibrinogen subunit gene in Xenopus laevis is mediated by a binding site for Xenopus glucocorticoid receptor accessory factor (XGRAF) and a contiguous glucocorticoid response element (GRE) half-site. Here, we characterize the protein:DNA complex formed by a cooperative interaction between XGRAF, GR, and the DNA. We demonstrate that the complex contains XGRAF by competition in a gel shift assay. The presence of GR is established by two criteria: 1) size dependence of the XGRAF:GR:DNA complex on the size of the GR component and 2) interference with complex formation by GR antibody. Cooperative binding of XGRAF and GR to the DNA was quantitated, showing that GR favors binding to XGRAF:DNA compared with free DNA by a factor of 30. The cooperative interaction between XGRAF and GR can occur on nicked DNA but is disrupted when 1 bp is inserted between the XGRAF binding site and half-GRE. Significantly, this loss of physical association in vitro correlates with loss of XGRAF amplification of GR activity in transiently transfected primary Xenopus hepatocytes. The simplest explanation for cooperativity between XGRAF and GR is formation of a DNA-bound heterodimer of these two proteins. This mechanism represents a new mode of transcriptional regulation in which GR and a nonreceptor protein form a heterodimer, with both partners contacting their specific DNA sites simultaneously.