A Novel Mechanism to Control Cell/Tissue Specific Activities of Glucocorticoid Receptor

Cell/tissue‐specific differential actions of glucocorticoids are mediated by the glucocorticoid receptor (GR) splice variants that possess truncated N‐terminal domain (NTD), which exists in an intrinsically disordered (ID) conformation and consists of GR's major activation function, AF1. Human GR variants lacking AF1 sequences are virtually inactive whereas those lacking amino acids on the NT flanking sequences of AF1 are most active in regulating GR target genes. However, the underlying mechanism of these GR isoform specific effects is largely unknown. We hypothesize that the GR isoform specific activities are allosterically regulated through order‐disorder transition of ID NTD sequences upon interaction with TATA‐box binding protein (TBP). First, using surface plasmon resonance method, we determined the binding kinetics of TBP with different fragments of GR containing AF1 sequences. We found that placing AF1 immediately upstream from the DNA binding domain leads to strongest AF1‐TBP interaction (K D = 0.07 µM) whereas AF1 C (a core sub‐domain of AF1, important for AF1 activity) showed the weakest binding (K D = 3.0 µM) when compared to AF1 (K D = 0.5 µM). Interestingly, the GR fragment consisting of entire NTD showed a biphasic binding response to TBP. Our secondary/tertiary structural analyses using biophysical methods showed that upon TBP binding, each GR fragment adopts a compact/stable structure, and the stability is proportional to the TBP binding affinity. Consistent with GR isoform specific activity, TBP‐bound GR fragment lacking NT flanking sequences showed highest AF1‐mediated GR activity. Our studies for the first time have shown a mechanism for differential activities of GR isoforms that could open unique opportunities for cell/tissue‐specific endocrine‐based therapies.