Two structures of cyclophilin 40: Folding and fidelity in the TPR domains

Background: The "large immunophilin" family consists of domains of cyclophi lin or FK506 binding protein linked to a tetratricopeptide (TPR) domain. Th ey are intimately associated with steroid receptor complexes and bind to th e C-terminal domain of Hsp90 via the TPR domain. The competitive binding of specific large immunophilins and other TPR-Hsp90 proteins provides a regul atory mechanism for Hsp90 chaperone activity. Results: We have solved the X-ray structures of monoclinic and tetragonal f orms of Cyp40. In the monoclinic form, the TPR domain consists of seven hel ices of variable length incorporating three TPR motifs, which provide a con vincing binding surface for the Hsp90 C-terminal MEEVD sequence. The C-term inal residues of Cyp40 protrude out beyond the body of the TPR domain to fo rm a charged helix-the putative calmodulin binding site. However, in the te tragonal form, two of the TPR helices have straightened out to form one ext ended helix, providing a dramatically different conformation of the molecul e. Conclusions: The X-ray structures are consistent with the role of Cyclophil in 40 as a multifunctional signaling protein involved in a variety of prote in-protein interactions. The intermolecular helix-helix interactions in the tetragonal form mimic the intramolecular interactions found in the fully f olded monoclinic form. These conserved intra- and intermolecular TPR-TPR in teractions are illustrative of a high-fidelity recognition mechanism. The t wo structures also open up the possibility that partially folded forms of T PR may be important in domain swapping and protein recognition.