Surface Solvation Tunes Molecular-Recognition Plasticity in IDPs

Abstract

Surface water dynamics is key to molecular-recognition in biomolecules. The role of such dynamics becomes more prominent for Intrinsically Disordered Proteins (IDPs); as molecular-recognition of IDPs is not the fallout of classical ‘structure-function’ paradigm and thus dynamics is what determines their binding characteristics. There exists great diversity in terms of binding modalities of IDPs, ranging from ultrafast transient interactions to very stable recognition, attributing multifarious functionality to IDPs. While recent experiments have linked IDP biophysics and hydration dynamics, the exact mechanism by which solvation dynamics govern such diverse molecular-recognition in IDPs remains largely unanswered.In this work, we combine site-selective fluorescent labelling with picosecond-resolved fluorescence and single-molecule FRET, to study the interplay between solvation dynamics and binding modes of IDPs. We probed dynamics of hydration water on IDP surfaces and perturbation of the same when IDPs bind their partners. We focused on two IDPs from the nucleocytoplasmic transport pathway, Nup153FG and IBB. Both these IDPs bind a nuclear transport receptor (NTR) Importinβ, albeit in very different ways. Nup153FG binds Importinβ through a set of ultrafast transient multivalent interactions without any major conformational change, retaining its disorder. We observed solvation fluctuations in the dynamic Nup153FG. Importinβ complex to be unperturbed relative to unbound Nup153FG. IBB forms a stable helix upon binding Importinβ. We detected a ∼2 fold relative slowdown of water dynamics in the IBB. Importinβ complex. Our results provide direct evidence showing interfacial water dynamics retardation to be an activation barrier, instrumental in tuning the plasticity of the complex from stable long lived to transient and dynamic. In the context of nucleocytoplasmic transport such differential behavior of these two IDPs can be linked to their functions; dynamic Nup153. NTR complex facilitate fast nucleocytoplasmic transport while stable IBB. Importinβ ensures integrity of the import-complex during a transport event.