Coupling of Conformational Transitions in the N-terminal Domain of the 51-kDa FK506-binding Protein (FKBP51) Near Its Site of Interaction with the Steroid Receptor Proteins

David M Lemaster,Sourajit M Mustafi,Matthew Brecher,Jing Zhang,Annie Héroux,Hongmin Li,Griselda Hernández

doi:10.1074/jbc.m115.650655

Abstract

Interchanging Leu-119 for Pro-119 at the tip of the β4-β5 loop in the first FK506 binding domain (FK1) of the FKBP51 and FKBP52 proteins, respectively, has been reported to largely reverse the inhibitory (FKBP51) or stimulatory (FKBP52) effects of these co-chaperones on the transcriptional activity of glucocorticoid and androgen receptor-protein complexes. Previous NMR relaxation studies have identified exchange line broadening, indicative of submillisecond conformational motion, throughout the β4-β5 loop in the FK1 domain of FKBP51, which are suppressed by the FKBP52-like L119P substitution. This substitution also attenuates exchange line broadening in the underlying β2 and β3a strands that is centered near a bifurcated main chain hydrogen bond interaction between these two strands. The present study demonstrates that these exchange line broadening effects arise from two distinct coupled conformational transitions, and the transition within the β2 and β3a strands samples a transient conformation that resembles the crystal structures of the selectively inhibited FK1 domain of FKBP51 recently reported. Although the crystal structures for their series of inhibitors were interpreted as evidence for an induced fit mechanism of association, the presence of a similar conformation being significantly populated in the unliganded FKBP51 domain is more consistent with a conformational selection binding process. The contrastingly reduced conformational plasticity of the corresponding FK1 domain of FKBP52 is consistent with the current model in which FKBP51 binds to both the apo- and hormone-bound forms of the steroid receptor to modulate its affinity for ligand, whereas FKBP52 binds selectively to the latter state.

Highlights

FK506-binding protein 51 (FKBP51) inhibits and FKBP52 stimulates transcription by various steroid receptors
The present study demonstrates that these exchange line broadening effects arise from two distinct coupled conformational transitions, and the transition within the ␤2 and ␤3a strands samples a transient conformation that resembles the crystal structures of the selectively inhibited FK1 domain of FKBP51 recently reported
An early indication of the functional significance for FKBP51 and FKBP52 of the interface between the ␤4-␤5 loop and the underlying ␤2 and ␤3a strands was the widespread use of a F67D,D68V double mutant as a presumed indicator for the participation of the peptidyl prolyl isomerase activity for these proteins in their biological functions, in particular for steroid receptor activity [15, 35,36,37,38]

Summary

Background

FK506-binding protein 51 (FKBP51) inhibits and FKBP52 stimulates transcription by various steroid receptors. In the year following our initial characterization of the conformational exchange line broadening dynamics surrounding the bifurcated main chain hydrogen bonding of the amides of Phe-67 and Asp-68 with the carbonyl oxygen of Gly-59, Hausch and colleagues [23] reported the crystal structures of two so-called iFit inhibitor-bound forms of the FK1 domain from FKBP51 in which the conformation of the ␤3a strand surrounding Phe-67 is altered so as to disrupt this bifurcated main chain hydrogen bonding interaction They interpreted these structures to demonstrate an induced-fit mechanism of binding, despite the fact that the structure of the final ligand-bound state cannot discriminate between an induced-fit or a conformational selection-based mechanism [24]. NMR relaxation analysis of systematically designed FK1 domain variants can provide a fruitful approach to characterizing such transient conformational substates [25, 26]

Experimental Procedures

Results

Discussion