NMR illuminates intrinsic disorder

Abstract

Nuclear magnetic resonance (NMR) has long been instrumental in the characterization of intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs). This method continues to offer rich insights into the nature of IDPs in solution, especially in combination with other biophysical methods such as small-angle scattering, single-molecule fluorescence, electron paramagnetic resonance (EPR), and mass spectrometry. Substantial advances have been made in recent years in studies of proteins containing both ordered and disordered domains and in the characterization of problematic sequences containing repeated tracts of a single or a few amino acids. These sequences are relevant to disease states such as Alzheimer's, Parkinson's, and Huntington's diseases, where disordered proteins misfold into harmful amyloid. Innovative applications of NMR are providing novel insights into mechanisms of protein aggregation and the complexity of IDP interactions with their targets. As a basis for understanding the solution structural ensembles, dynamic behavior, and functional mechanisms of IDPs and IDRs, NMR continues to prove invaluable.

Figure 1.

A. Schematic representation of heterogeneous interactions between an IDP and its target. Interactions with motif A are fuzzy and highly dynamic while motifs B and C interact strongly and specifically with the target such that their motions are restricted. The overall binding free energy is the sum of the free energies of interaction of each motif and the free energy associated with thermodynamic coupling (Δg) between the individual binding motifs. B. Backbone dynamics for the IDP HTF-1α in complex with TAZ1. The ribbon width of the HIF-1α transactivation domain peptide is scaled by 1-S², where S² is the order parameter describing the amplitude of N-H bond motions. The backbone color gradient ranges from blue (1-S² = 0.1) in regions where motions are restricted by strong interactions with TAZ1 to red (1- S² = 0.9) where intermolecular interactions are weak and there are large amplitude motions of the HIF-1α backbone. TAZ1 is shown in the cartoon representation in grey. Zinc atoms are shown as dark blue spheres. The secondary structural elements of HIF-1α are labeled. (Adapted from [26*] with permission).

Figure 2.

Portions of ¹H-¹⁵N HSQC (left) and ¹H-¹³C HSQC (right) spectra showing cross peaks belonging to members of a polyglutamine tract in huntingtin. The color-coded peaks were assigned by site-specific isotopic labeling [10,37**]. (adapted from reference [37**]. with permission).