Unveiling invisible protein states with NMR spectroscopy.

Proteins interconvert between multiple conformations, including sparsely populated and transiently formed states that are difficult to characterize in structural detail using standard biophysical methods. In some cases, changes to the dynamical equilibria between conformations can lead to pathological protein aggregation and to the disruption of cellular homeostasis. The detection and characterization of lowly populated conformers is therefore crucial for understanding the basis of protein misfolding. NMR spectroscopy is exquisitely sensitive to the conformational dynamics of biomolecules and can be used to study sparsely populated states at the atomic level. Here, we review recent progress toward understanding the roles of sparsely populated, otherwise 'invisible' states present in protein folding and misfolding, where NMR has provided unique insight into folding intermediates, transiently misfolded states, and soluble oligomers that precede amyloid fibril formation.