The FK506‐binding protein 51 (FKBP51) is a high‐molecular‐weight immunophilin that emerged as an important drug target for stress‐related disorders, chronic pain, and obesity. It has been implicated in a plethora of molecular pathways but remains best characterized as a co‐chaperone of Hsp90 in the steroid hormone receptor (SHR) maturation cycle. However, the mechanistic and structural basis for the regulation of SHRs by FKBP51 and the usually antagonistic function compared with its closest homolog FKBP52 remains enigmatic. Here we review recent structural and biochemical studies of FKBPs as regulators in the Hsp90 machinery. These advances provide important insights into the roles of FKBP51 and FKBP52 in SHR regulation.

The large immunophilins FKBP51 and FKBP52 play key roles in the Hsp90-mediated maturation of steroid hormone receptors, which is crucial for stress-related disorders and correct sexual embryonic development, respectively. A prominent regulatory target is the glucocorticoid receptor (GR), whose activation is repressed by FKBP51 and facilitated by FKBP52. Despite their vital roles, the molecular modes of action of FKBP51 and FKBP52 are poorly understood since the transient key states of FKBP-mediated GR-regulation have remained experimentally elusive. Here we present the architecture and functional annotation of FKBP51-, FKBP52- and p23-containing Hsp90-apoGR preactivation complexes, trapped by systematic incorporation of photoreactive amino acids inside human cells. The identified crosslinking sites depended on a functional Hsp90 chaperone cycle, were disrupted by GR activation, and clustered in characteristic patterns, defining the relative orientation and contact surfaces within the FKBP/p23-apoGR complexes. Strikingly, GR binding to the FKBPFK1 but not the FKBPFK2 domains were modulated by FKBP ligands, explaining the lack of FKBP51-mediated GR derepression by certain classes of FKBP ligands. These findings show how FKBP51 and FKBP52 differentially interact with the apoGR ligand binding domain, they explain the differentiated pharmacology of FKBP51 ligands, and provide a structural basis for the development of FKBP ligands with higher efficacy.