Functions of the Hsp90-Binding FKBP Immunophilins.

The Hsp90 chaperone is known to interact with a diverse array of client proteins. However, in every case examined, Hsp90 is also accompanied by a single or several co-chaperone proteins. One class of co-chaperone contains a tetratricopeptide repeat (TPR) domain that targets the co-chaperone to the C-terminal region of Hsp90. Within this class are Hsp90-binding peptidylprolyl isomerases, most of which belong to the FK506-binding protein (FKBP) family. Despite the common association of FKBP co-chaperones with Hsp90, it is abundantly clear that the client protein influences, and is often influenced by, the particular FKBP bound to Hsp90. Examples include Xap2 in aryl hydrocarbon receptor complexes and FKBP52 in steroid receptor complexes. In this chapter, we discuss the known functional roles played by FKBP co-chaperones and, where possible, relate distinctive functions to structural differences between FKBP members.

Management of Hsp90-Dependent Protein Folding by Small Molecules Targeting the Aha1 Co-Chaperone.

Hsp90 plays an important role in health and is a therapeutic target for managing misfolding disease. Compounds that disrupt co-chaperone delivery of clients to Hsp90 target a subset of Hsp90 activities, thereby minimizing the toxicity of pan-Hsp90 inhibitors. Here, we have identified SEW04784 as a first-in-class inhibitor of the Aha1-stimulated Hsp90 ATPase activity without inhibiting basal Hsp90 ATPase. Nuclear magnetic resonance analysis reveals that SEW84 binds to the C-terminal domain of Aha1 to weaken its asymmetric binding to Hsp90. Consistent with this observation, SEW84 blocks Aha1-dependent Hsp90 chaperoning activities, including the in vitro and in vivo refolding of firefly luciferase, and the transcriptional activity of the androgen receptor in cell-based models of prostate cancer and promotes the clearance of phosphorylated tau in cellular and tissue models of neurodegenerative tauopathy. We propose that SEW84 provides a novel lead scaffold for developing therapeutic approaches to treat proteostatic disease.

Functional Comparison of Human and Zebra Fish FKBP52 Confirms the Importance of the Proline-Rich Loop for Regulation of Steroid Hormone Receptor Activity.

Previous studies demonstrated that the 52-kDa FK506-binding protein (FKBP52) proline-rich loop is functionally relevant in the regulation of steroid hormone receptor activity. While zebra fish (Danio rerio; Dr) FKBP52 contains all of the analogous domains and residues previously identified as critical for FKBP52 potentiation of receptor activity, it fails to potentiate activity. Thus, we used a cross-species comparative approach to assess the residues that are functionally critical for FKBP52 function. Random selection of gain-of-function DrFKBP52 mutants in Saccharomyces cerevisiae identified two critical residues, alanine 111 (A111) and threonine 157 (T157), for activation of receptor potentiation by DrFKBP52. In silico homology modeling suggests that alanine to valine substitution at position 111 in DrFKBP52 induces an open conformation of the proline-rich loop surface similar to that observed on human FKBP52, which may allow for sufficient surface area and increased hydrophobicity for interactions within the receptor-chaperone complex. A second mutation in the FKBP12-like domain 2 (FK2), threonine 157 to arginine (T157R), also enhanced potentiation, and the DrFKBP52-A111V/T157R double mutant potentiated receptor activity similar to human FKBP52. Collectively, these results confirm the functional importance of the FKBP52 proline-rich loop, suggest that an open conformation on the proline-rich loop surface is a predictor of activity, and highlight the importance of an additional residue within the FK2 domain.