GSE107584
General and sequence-specific roles for DNA in glucocorticoid receptor DNA-binding stoichiometry
The glucocorticoid receptor (GR) binds to DNA in at least three stoichiometric ratios to regulate gene expression. The dimeric binding of GR to a 15 base pair core sequence has been well studied, and is associated with activation of gene expression. Monomeric GR has been associated with repression. GR has also been shown to form tetramers on DNA in live cells. The role of DNA sequence in directing DNA-binding stoichiometry and subsequent gene regulation is not clear. We took an unbiased approach using SELEX-seq to calculate comprehensive monomeric- and dimeric-binding profiles for GR. The monomeric site is composed of a canonical half-site, but is distinguished from the dimer site by a downstream TTTg motif and the absence of a second half-site. To probe the mechanism of how this motif favors monomeric binding, we performed Spec-/Coop-seq. The TTTg increases monomeric affinity while decreasing cooperative dimeric binding. Crystal structures indicated that TTTg results in a narrowed minor groove bringing residues Y455 and K471 of GR into the proximity of DNA, increasing affinity. Interestingly, although monomeric binding is more often associated with repression of gene expression than dimeric binding, the trend is not significant. Consistent with live-cell imaging, a meta-analysis of GR:DNA structures reveals that, regardless of binding sequence, GR binds to DNA as a dimer of dimers using highly similar interfaces. This suggests that the functional stoichiometry of GR in the cell is tetrameric, and that sequence may modulate GR activity. Overall design: Two ChIP-seq samples.  GR was ChIPed from SUP-B15 cells in the presence of vehicle (0.02% ethanol) or 1µM dexamethasone.