The Helix 1-3 Loop in the Glucocorticoid Receptor LBD Is a Regulatory Element for FKBP Cochaperones

Abstract

The heat-shock protein 90 (Hsp90) cochaperone FK506-binding protein 52 (FKBP52) upregulates, whereas FKBP51 inhibits, hormone binding and nuclear targeting of the glucocorticoid receptor (GR). Decreased cortisol sensitivity in the guinea pig is attributed to changes within the helix 1 to helix 3 (H1-H3) loop of the guinea pig GR (gpGR) ligand-binding domain. It has been proposed that this loop serves as a contact point for FKBP52 and/or FKBP51 with receptor. We examined the role of the H1-H3 loop in GR activation by FKBP52 using a Saccharomyces cerevisiae model. The activity of rat GR (rGR) containing the gpGR H1-H3 loop substitutions was still potentiated by FKBP52, confirming the loop is not involved in primary FKBP52 interactions. Additional assays also excluded a role for other intervening loops between ligand-binding domain helices in direct interactions with FKBP52 associated with enhanced receptor activity. Complementary studies in FKBP51-deficient mouse embryo fibroblasts and HEK293 cells demonstrated that substitution of the gpGR H1-H3 loop residues into rGR dramatically increased receptor repression by FKBP51 without enhancing receptor-FKBP51 interaction and did not alter recruitment of endogenous Hsp90 and the p23 cochaperone to receptor complexes. FKBP51 suppression of the mutated rGR did not require FKBP51 peptidylprolyl cis-trans isomerase activity and was not disrupted by mutation of the FK1 proline-rich loop thought to mediate reciprocal FKBP influences on receptor activity. We conclude that the gpGR-specific mutations within the H1-H3 loop confer global changes within the GR-Hsp90 complex that favor FKBP51 repression over FKBP52 potentiation, thus identifying the loop as an important target for GR regulation by the FKBP cochaperones.

NF-κB Transcriptional Activity Is Modulated by FK506-binding Proteins FKBP51 and FKBP52

“…On the other hand, the PPIase activity of FKBP52 is very important for its stimulatory action, although this enzymatic activity is not required for the FKBP51 inhibitory action. A similar independent effect on the PPIase activity was described for the regulation of the GR by FKBP51 (19,54), whereas a PPIase-dependent mechanism is implicated for FKBP52 (19,51).…”

Ligand Binding Mechanism in Steroid Receptors: From Conserved Plasticity to Differential Evolutionary Constraints

“…Mutation and peptide competition studies suggest that Hsp90 interacts at the AF-2 surface (Ricketson et al, 2007;Fang et al, 2006). In addition, co-chaperones have been mapped to interact with regions surrounding the C-terminal end of H1 and the N-terminal end of H3 (Caamañ o et al, 1998), and with the loop that connects them (Cluning et al, 2013). Neither of these areas overlap with the entry site proposed here.…”

The FKBP52 Cochaperone Acts in Synergy with β-Catenin to Potentiate Androgen Receptor Signaling

“…This could also explain why MJC13, which is thought to target the AR BF3 surface, shows specificity for AR as compared to GR. In support of this idea, recent studies suggest that the Helix 1–3 (H1-H3) loop in the GR LBD is important for FKBP regulation and may represent an FKBP52 regulatory site [ 32 ]. Thus, FKBP52 likely regulates distinct sites on AR at distinct steps within the AR signaling pathway.…”