Abstract
Molecular recognition by intrinsically disordered proteins (IDPs) commonly involves specific localized contacts and target-induced disorder to order transitions. However, some IDPs remain disordered in the bound state, a phenomenon coined “fuzziness”, often characterized by IDP polyvalency, sequence-insensitivity and a dynamic ensemble of disordered bound-state conformations. Besides the above general features, specific biophysical models for fuzzy interactions are mostly lacking. The transcriptional activation domain of the Ewing's Sarcoma oncoprotein family (EAD) is an IDP that exhibits many features of fuzziness, with multiple EAD aromatic side chains driving molecular recognition. Considering the prevalent role of cation-π interactions at various protein-protein interfaces, we hypothesized that EAD-target binding involves polycation- π contacts between a disordered EAD and basic residues on the target. Herein we evaluated the polycation-π hypothesis via functional and theoretical interrogation of EAD variants. The experimental effects of a range of EAD sequence variations, including aromatic number, aromatic density and charge perturbations, all support the cation-π model. Moreover, the activity trends observed are well captured by a coarse-grained EAD chain model and a corresponding analytical model based on interaction between EAD aromatics and surface cations of a generic globular target. EAD-target binding, in the context of pathological Ewing's Sarcoma oncoproteins, is thus seen to be driven by a balance between EAD conformational entropy and favorable EAD-target cation-π contacts. Such a highly versatile mode of molecular recognition offers a general conceptual framework for promiscuous target recognition by polyvalent IDPs.
Highlights
Understanding the sequence-function relationship of a protein and how it might malfunction is central to biomedical research
While many proteins function in their folded states, recently it became clear that intrinsically disordered proteins (IDPs) play key functional roles [1,2] in transcription, translation and cell cycle regulation that, when altered, frequently lead to cancer [3]
To gain insight into ‘‘fuzzy’’ interactions, we have studied the Ewing’s Sarcoma (EWS)-activation domain (EAD) in the TET family of RNA-binding proteins [11] and Ewing’s family of oncoproteins (EFPs)
Summary
Understanding the sequence-function relationship of a protein and how it might malfunction is central to biomedical research. While many proteins function in their folded states, recently it became clear that intrinsically disordered proteins (IDPs) play key functional roles [1,2] in transcription, translation and cell cycle regulation that, when altered, frequently lead to cancer [3]. Certain IDPs engage in protein-protein interaction without coupled folding and binding [5] such that the IDP remains disordered even when bound to a globular target. This phenomenon has been termed ‘‘fuzziness’’ [6] and is characterised by IDP polyvalency, sequence-insensitivity and lack of strict geometric complementarity for binding [6]. Important examples of fuzziness include transcription factors [7], linker histones [8], prion-like proteins [9] and Sic1-Cdc in yeast [10]
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