Molecular recognition via coupled folding and binding in a TPR domain

The majority of known tetratricopeptide repeat (TPR) domains consist of three copies of the helix-turn-helix TPR motif, together with a seventh C-terminal helix. TPR domains function as protein-protein recognition modules in intracellular signalling. This function is exemplified by the TPR domain of protein phosphatase 5 (PP5), which binds to the C terminus of the chaperone protein Hsp90. Here, we report NMR and CD spectroscopic studies that reveal that this domain is largely unfolded at physiological temperatures, and that interaction with an MEEVD pentapeptide derived from Hsp90 stabilises a folded structure. This complex, coupled folding-binding mechanism is characterised further by its observed enthalpy change on binding (determined by isothermal titration calorimetry), which displays a markedly non-linear relationship with temperature. A nested Gibbs-Helmholtz model is used in a novel combined analysis of the CD and ITC data to determine separately the thermodynamic contributions of the intrinsic folding and binding events to the overall coupled process. The analysis shows that, despite the expected large entropic opposition to the folding process, a nearly equal favourable folding enthalpy means the net effect of coupled folding on the observed affinity is small across a broad range of temperature. We hypothesise that a coupled folding-binding mechanism is common in this class of domains.