3D structure of human FK506-binding protein 52: implications for the assembly of the glucocorticoid receptor/Hsp90/immunophilin heterocomplex

Abstract

FK506-binding protein 52 (FKBP52), which binds FK506 and possesses peptidylprolyl isomerase activity, is an important immunophilin involved in the heterocomplex of steroid receptors with heat-shock protein 90. Here we report the crystal structures of two overlapped fragments [N(1-260) and C(145-459)] of FKBP52 and the complex with a C-terminal pentapeptide from heat-shock protein 90. Based on the structures of these two overlapped fragments, the complete putative structure of FKBP52 can be defined. The structure of FKBP52 is composed of two consecutive FKBP domains, a tetratricopeptide repeat domain and a short helical domain beyond the final tetratricopeptide repeat motif. Key structural differences between FKBP52 and FKBP51, including the relative orientations of the four domains and some important residue substitutions, could account for the differential functions of FKBPs.

Fig. 1.

(a) Sequence alignment of human FKBP52 (hFKBP52) and human FKBP51 (hFKBP51). Amino acids with high consensus are shown in red. Human FKBP52 shares 60% amino acid sequence identity and 75% similarity with human FKBP51. Four domains are indicated by different underlines: single underline, FK1 domain; double underline, FK2 domain; thick underline, TPR domain; dashed underline, calmodulin-binding domain. (b) The final N(1–260) model contains residues 21–257. (c) The final C(145–459) model contains residues 145–427. (d) The overall structure of FKBP52 has been defined based on the superposition of overlapped regions of N(1–260) and C(145–459). (e) Stereo view of the structural comparison between FKBP51 (blue) and FKBP52 (yellow) shows their similar structural architectures but the different orientations of their corresponding domains.

Fig. 2.

Stereo view of the superposition of FK and TPR domains. (a) Two FKBP domains of FKBP51 and FKBP52 were superimposed onto FKBP12. FKBP12 (green), 51-FK1 (blue), and 52-FK1 (red) are similar. The structures of 51-FK2 (cyan) and 52-FK2 (yellow) are more closed than the others. (b) TPR domains are superimposed onto the TPR domains of FKBP52. FKBP52 is shown in yellow, FKBP51 is shown in cyan, Hop is shown in green, Cyp40 is shown in purple, and PP5 is shown in pink. The conformations of all the TPR domains are similar, containing six α-helices (α1–α6). The orientations of the extra α-helix (α7) are different. (c) Superposition of the structures of TPR domains and the α7-helixes of FKBP51 (blue) and FKBP52 (yellow). Gln-333, Phe-335, and Ala-365 of FKBP52 are replaced by Arg-331, Tyr-333, and Leu-363 in FKBP51, which may be responsible for the differential binding pattern of FKBPs to Hsp90. The side chain of Ile-400 of FKBP52, corresponding to Ala-398 of FKBP51, will clash with Phe-369, and this may cause the different orientations of the α7-helix.

Fig. 3.

(a) Stereo view of the hydrogen bonds between FK1 and FK2 of FKBP52. Hydrogen bonds at the interface of FK1 and FK2 form a complicated network, which stabilizes the conformation. Residues in FK1 are shown in red, residues in FK2 are shown in yellow, and residues in the loop are shown in white. (b and c) Stereo view of the MEEVD peptide bound to molecules A (b)and B(c) of C(145–459). The omit electron-density map is contoured at 0.7 σ above the mean. Residues of the peptide are shown in white, and residues of the TPR domain are shown in yellow. Residues involved in important interactions are shown in ball-and-stick representation. Hydrogen bonds are shown as dotted lines.