M. Roos, S. Link, J. Balbach, A. Krushelnitsky, K. Saalwächter.
NMR-Detected Brownian Dynamics of αB-Crystallin over a Wide Range of
Biophys. J. 108, 98-106 (2015).
Knowledge about the global translational and rotational motion of proteins under crowded conditions is highly relevant
for understanding the function of proteins in vivo. This holds in particular for human αB-crystallin, which is strongly crowded
in vivo and inter alia responsible for preventing cataracts. Quantitative information on translational and rotational diffusion is not
readily available, and we here demonstrate an approach that combines pulsed-field-gradient NMR for translational diffusion and
proton T1ρ/T2 relaxation-time measurements for rotational diffusion, thus overcoming obstacles encountered in previous studies.
The relaxation times measured at variable temperature provide a quantitative measure of the correlation function of protein tumbling,
which cannot be approximated by a single exponential, because two components are needed for a minimal and adequate
description of the data. We find that at high protein concentrations, rotational diffusion is decoupled from translational diffusion,
the latter following the macroscopic viscosity change almost quantitatively, resembling the behavior of spherical colloids. Analysis
of data reported in the literature shows that well-packed globular proteins follow a scaling relation between the hydrodynamic
radius and the molar mass, Rh ~ M1-d, with a fractal dimension of d ~ 2.5 rather than 3. Despite its oligomeric nature,
Rh of αB-crystallin as derived from both NMR methods is found to be fully consistent with this relation.