Two Structures of Cyclophilin 40: Folding and Fidelity in the TPR Domains

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Background: The “large immunophilin” family consists of domains of cyclophilin or FK506 binding protein linked to a tetratricopeptide (TPR) domain. They are intimately associated with steroid receptor complexes and bind to the C-terminal domain of Hsp90 via the TPR domain. The competitive binding of specific large immunophilins and other TPR-Hsp90 proteins provides a regulatory mechanism for Hsp90 chaperone activity. Results: We have solved the X-ray structures of monoclinic and tetragonal forms of Cyp40. In the monoclinic form, the TPR domain consists of seven helices of variable length incorporating three TPR motifs, which provide a convincing binding surface for the Hsp90 C-terminal MEEVD sequence. The C-terminal residues of Cyp40 protrude out beyond the body of the TPR domain to form a charged helix—the putative calmodulin binding site. However, in the tetragonal form, two of the TPR helices have straightened out to form one extended helix, providing a dramatically different conformation of the molecule. Conclusions: The X-ray structures are consistent with the role of Cyclophilin 40 as a multifunctional signaling protein involved in a variety of protein-protein interactions. The intermolecular helix-helix interactions in the tetragonal form mimic the intramolecular interactions found in the fully folded monoclinic form. These conserved intra- and intermolecular TPR-TPR interactions are illustrative of a high-fidelity recognition mechanism. The two structures also open up the possibility that partially folded forms of TPR may be important in domain swapping and protein recognition.