The Dynamic Association of an IDP with a Folded Protein Without Localized Binding Sites or Persistent Contacts

Abstract

In the traditional paradigm, protein interactions are encoded in the complementary shapes and non-covalent forces between partners. However, it has become clear that structural disorder may be present in protein complexes to varying degrees. An extreme example of this is the complex between linker histone H1 (H1) and its chaperone prothymosin-α (ProTα), which associate with picomolar affinity, while retaining their intrinsically disordered nature and without involvement of defined binding sites. In the ProTα-H1 complex, the highest-affinity association occurs between the disordered C-terminal of H1 and the fully intrinsically disordered ProTα. In isolation, the small folded domain of H1, the globular domain (GD), associates with ProTα with a KD of 2 μM, but the molecular details of this association are unknown. Using primarily NMR spectroscopy in combination with molecular dynamics simulations, we have characterized the complex between the GD and ProTα, which entails no induction of structure in ProTα. We have solved the structure in solution of the GD in its free form and characterized structural and dynamical effects of binding at different stoichiometries. Furthermore, to deduce the importance of charge localization versus net charge, we have investigated the effect of charge variants of GD on affinity and contact localization. Our results suggest that there are no localized binding sites for ProTα on the GD surface, and that complex formation does not entail persistent contacts between specific residues. Rather, ProTα dynamically associate with GD through an electrostatic mean-field-type interaction between all involved charges. Our findings support that mean-field-type interactions are not limited to fully disordered complexes but may be extended to IDP-folded protein interactions.