Jm. Stafford et al., Accessory factors facilitate the binding of glucocorticoid receptor to thephosphoenolpyruvate carboxykinase gene promoter, J BIOL CHEM, 276(43), 2001, pp. 39885-39891
Glucocorticoid induction of the phosphoenolpyruvate carboxykinase (PEPCK) g
ene requires a glucocorticoid response unit (GRU) comprised of two non-cons
ensus glucocorticold receptor (GR) binding sites, GR1 and GR2, and at least
three accessory factor elements (gAF1-3). DNA-binding accessory proteins a
re commonly required for the regulation of genes whose products play an imp
ortant role in metabolism, development, and a variety of defense responses,
but little is known about why they are necessary. Quantitative, real time
homogenous assays of cooperative protein-DNA interactions in complex media
(e.g. nuclear extracts) have not previously been reported. Here we perform
quantitative, real time equilibrium and stopped-flow fluorescence anisotrop
y measurements of protein-DNA interactions in nuclear extracts to demonstra
te that GR binds to the GR1-GR2 elements poorly as compared with a palindro
mic or consensus glucocorticoid response element (GRE). Inclusion of either
the gAF1 or gAF2 element with GR1-GR2, however, creates a high affinity bi
nding environment for GR. GR can undergo multiple rounds of binding and dis
sociation to the palindromic GRE in less than 100 ms at nanomolar concentra
tions. The dissociation rate of GR is differentially slowed by the gAF1 or
gAF2 elements that bind two functionally distinct accessory factors, COUP-T
F/HNF4 and HNF3, respectively.