Dynamical coupling of intrinsically disordered proteins and their hydration water: comparison with folded soluble and membrane proteins

Abstract

Hydration water is vital for various macromolecular biological activities, such as specific ligand recognition, enzyme activity, response to receptor binding, and energy transduction. Without hydration water, proteins would not fold correctly and would lack the conformational flexibility that animates their three-dimensional structures. Motions in globular, soluble proteins are thought to be governed to a certain extent by hydration-water dynamics, yet it is not known whether this relationship holds true for other protein classes in general and whether, in turn, the structural nature of a protein also influences water motions. Here, we provide insight into the coupling between hydration-water dynamics and atomic motions in intrinsically disordered proteins (IDP), a largely unexplored class of proteins that, in contrast to folded proteins, lack a well-defined three-dimensional structure. We investigated the human IDP tau, which is involved in the pathogenic processes accompanying Alzheimer disease. Combining neutron scattering and protein perdeuteration, we found similar atomic mean-square displacements over a large temperature range for the tau protein and its hydration water, indicating intimate coupling between them. This is in contrast to the behavior of folded proteins of similar molecular weight, such as the globular, soluble maltose-binding protein and the membrane protein bacteriorhodopsin, which display moderate to weak coupling, respectively. The extracted mean square displacements also reveal a greater motional flexibility of IDP compared with globular, folded proteins and more restricted water motions on the IDP surface. The results provide evidence that protein and hydration-water motions mutually affect and shape each other, and that there is a gradient of coupling across different protein classes that may play a functional role in macromolecular activity in a cellular context.

Figure 1

(A) MSDs of side-chain motions as a function of temperature for the IDP tau (solid circles, H-tau-D₂O) and the folded MBP (open squares, H-MBP-D₂O) as determined by neutron spectroscopy on hydrogenated protein powders hydrated in D₂O to ∼0.4 g water/g protein. Effective force constants were 0.185 Nm⁻¹ (MBP) and 0.096 Nm⁻¹ (tau) as extracted from the slopes above 250 K (inset). (B) MSDs of hydration-water motions on the surfaces of tau (solid diamonds; D-tau-H₂O) and MBP (open triangles; D-MBP-H₂O) as determined on deuterated protein powders hydrated in H₂O at ∼0.4 g water/g protein. The MBP data (7) were reexamined for MSD extraction in the same Q²-ranges as for tau. Errors are shown for selected high-temperature data points and are smaller at lower temperatures (not shown). Data were measured on the IN16 spectrometer at the ILL, Grenoble, France.

Figure 2

MSDs of hydrogenated powders of three folded, globular proteins of different sizes, viz., MBP (open squares; H-MBP-D₂O), RNase (solid triangles; H-RNase-D₂O), and Mb (crosses; H-Mb-D₂O), hydrated at ∼0.4 g D₂O/g protein. Published neutron spectroscopy data for H-RNase-D₂O (38) and H-MBP-D₂O (7) as measured on IN16 were reexamined for MSD extraction in the same Q²-range (0.19–1.32 Å⁻²) used for the H-tau-D₂O sample.

Figure 3

MSDs of dry tau (H-tau-dry, open circles), hydrated tau (H-tau-D₂O, solid circles), and dry RNase (H-RNase-dry, crosses) powders. Published neutron spectroscopy data for H-RNase-dry (38), obtained on IN16, were reexamined for MSD extraction in the same Q²-range (0.19–1.32 Å⁻²) as for the H-tau-dry sample. MSDs of H-tau-D₂O are the same as those displayed in Figs. 1A and 4A.

Figure 4

Dynamical coupling of protein dynamics with hydration-water motions for different protein classes. Temperature-dependent atomic MSDs of proteins (orange data points) are compared with those of hydration water (blue data points) and are identical to the ones shown in Fig. 1. (A) IDP tau. (B) Folded, globular MBP (7). An ensemble of 100 tau structures was generated using a coil library (58) and a structure with a calculated radius of gyration (63 Å) identical to that measured by SAXS (63 Å) was chosen for the upper half of panel A. Approximately 1000 water molecules forming the first hydration shell are shown in panel A, corresponding to the experimental hydration level of 0.4 g water/g protein. Data were obtained on the IN16 spectrometer at the ILL, Grenoble, France. Arrows indicate the temperature at which water and protein MSD diverge (250 K (A) and 220 K (B) for tau and MBP, respectively). MSDs > 260 K were omitted for the protein (tau and MBP) experiments for the sake of comparison with hydration-water MSDs (tau and MBP), which could be determined only up to 260 K (see Materials and Methods section).