In close collaboration with Philip Anfinrud, novel hardware was designed and developed that makes it possible, for the first time, to obtain demonstrate that it is readily possible to monitor the folding of the protein chain in a residue-specific manner upon jumping the applied pressure. Pressure changes of up to 2.5 kbar, requiring 1-2 ms, are shown to feasible and compatible with the recording of high quality NMR data. For proteins with a substantial volume difference between the folded and unfolded states, their thermodynamic equilibrium can be altered by varying the hydrostatic pressure. Using a pressure-sensitized mutant of ubiquitin, we have demonstrated that rapidly switching the pressure within an NMR sample cell enables study of the unfolded protein under native conditions and, vice versa, study of the native protein under denaturing conditions. This approach makes it possible to record two- and three-dimensional NMR spectra of the unfolded protein at atmospheric pressure, providing new, residue specific information on the folding process. 15N and 13C chemical shifts measured immediately after dropping the pressure from 2.5 kbar (favoring unfolding) to 1 bar (native) are close to the random coil chemical shifts observed for a large, disordered peptide fragment of the protein. However, 15N relaxation data show evidence for rapid exchange, on a ca 100 us time scale, between the unfolded state and unstable, structured states which can be considered as failed folding events. The NMR data also provide direct evidence for parallel folding pathways, with ca half the protein molecules efficiently folding through an on-pathway kinetic intermediate, whereas the other half fold in a single step. At protein concentrations above ca 300 uM, oligomeric off-pathway intermediates compete with folding of the native state. For a two-state downhill folding protein, the change in resonance frequency will occur nearly instantaneously when the protein clears the transition state barrier, resulting in a mono-exponential change of the ensemble-averaged chemical shift. However, protein folding pathways can be more complex and contain meta-stable intermediates. With a pseudo-3D NMR experiment that utilizes stroboscopic observation, we demonstrate that it is possible to measure the ensemble-averaged chemical shifts, including those of exchange-broadened intermediates, during the folding process. Such measurements for a pressure-sensitized mutant of ubiquitin show an on-pathway kinetic intermediate whose 15N chemical shifts differ most from the natively folded protein for residues in strands beta5, its preceding turn, and the two strands that pair with beta5 in the native structure.
| Charlier, Cyril; Alderson, T Reid; Courtney, Joseph M et al. (2018) Study of protein folding under native conditions by rapidly switching the hydrostatic pressure inside an NMR sample cell. Proc Natl Acad Sci U S A 115:E4169-E4178 |
| Alderson, T Reid; Lee, Jung Ho; Charlier, Cyril et al. (2018) Propensity for cis-Proline Formation in Unfolded Proteins. Chembiochem 19:37-42 |
| Charlier, Cyril; Courtney, Joseph M; Alderson, T Reid et al. (2018) Monitoring 15N Chemical Shifts During Protein Folding by Pressure-Jump NMR. J Am Chem Soc 140:8096-8099 |
