Untangling the Conformational Polymorphism of Disordered Proteins Associated With Neurodegeneration at the Single-Molecule Level.

Melissa Birol,Ana M Melo

doi:10.3389/fnmol.2019.00309

Abstract

A large fraction of the human genome encodes intrinsically disordered proteins/regions (IDPs/IDRs) that are involved in diverse cellular functions/regulation and dysfunctions. Moreover, several neurodegenerative disorders are associated with the pathological self-assembly of neuronal IDPs, including tau [Alzheimer’s disease (AD)], α-synuclein [Parkinson’s disease (PD)], and huntingtin exon 1 [Huntington’s disease (HD)]. Therefore, there is an urgent and emerging clinical interest in understanding the physical and structural features of their functional and disease states. However, their biophysical characterization is inherently challenging by traditional ensemble techniques. First, unlike globular proteins, IDPs lack stable secondary/tertiary structures under physiological conditions and may interact with multiple and distinct biological partners, subsequently folding differentially, thus contributing to the conformational polymorphism. Second, amyloidogenic IDPs display a high aggregation propensity, undergoing complex heterogeneous self-assembly mechanisms. In this review article, we discuss the advantages of employing cutting-edge single-molecule fluorescence (SMF) techniques to characterize the conformational ensemble of three selected neuronal IDPs (huntingtin exon 1, tau, and α-synuclein). Specifically, we survey the versatility of these powerful approaches to describe their monomeric conformational ensemble under functional and aggregation-prone conditions, and binding to biological partners. Together, the information gained from these studies provides unique insights into the role of gain or loss of function of these disordered proteins in neurodegeneration, which may assist the development of new therapeutic molecules to prevent and treat these devastating human disorders.

Highlights

The classical protein ‘‘structure–function’’ paradigm establishes that proteins fold into a unique ordered 3D structure determined by their amino acid sequence before acquiring a specific biological function
SmFRET revealed that the MT binding region (MTBR) responds differentially upon interaction with soluble tubulin or heparin/polyP that accounts for distinct conformational ensembles of tau in its tubulin-bound state and aggregation-prone structure, respectively
single-molecule fluorescence (SMF) methods have been recognized as powerful and versatile approaches to investigate the heterogeneous and dynamic nature of neurodegeneration-associated IDPs, including HTTex1, tau, and αS as discussed in this review article. These cutting-edge techniques have provided valuable insights into: (i) their monomeric states; (ii) the aggregation-prone structures of tau and αS; (iii) the disorder-to-order transition of αS upon membrane binding; and (iv) the formation of a ‘‘fuzzy complex’’ by tau bound to soluble tubulin

Summary

INTRODUCTION

The classical protein ‘‘structure–function’’ paradigm establishes that proteins fold into a unique ordered 3D structure determined by their amino acid sequence before acquiring a specific biological function (reviewed in Fersht, 2008). Studies over the last two decades have identified functional proteins lacking a stable secondary and/or tertiary structure, and instead adopting a dynamic ensemble of multiple conformational states (Kriwacki et al, 1996; Wright and Dyson, 1999; Mittag et al, 2010; Babu et al, 2012; Tompa, 2012; van der Lee et al, 2014) These intrinsically disordered proteins and regions (IDPs and IDRs, respectively) are widespread in the human proteome and play critical roles in diverse biological processes, including in transcription and translation, cell cycle, signaling, and transport (Iakoucheva et al, 2002; Wright and Dyson, 2015; Babu, 2016; Tsafou et al, 2018). The characterization of the conformational ensemble of HTTex under functional and aggregation-prone conditions through smFRET will provide insights into toxicity relevant conformations

TAU IN TAUOPATHIES

Findings

CONCLUSION