Figure 5. TDF assay shows Hsp90 peptide stabilizes the TPR domain.

Apo Cyp40 has a T_m=42°C but starts to melt at 30°C (solid line). Addition of Hsp90 peptide–MEEVD delays onset of melting by ∼5°C; however, T_m only increases by ∼1°C (dashed line). Comparison of the shapes of these two melting curves suggests that the thermally unstable TPR domain is being stabilized by Hsp90 peptide at temperatures below those where the cyclophilin domain becomes destabilized. Addition of the tight binding CsA ligand (PPIase domain) raises the T_m by 4°C upto 46°C (dot line). The shape of the melting curve for addition of the MEEVD peptide along with CsA again suggests a delayed melting of the TPR (dash/dot line).