Structural dynamics of membrane-protein folding from single-molecule FRET

Membrane proteins fulfil a plethora of vital functions, are major drug targets, and are implicated in many diseases. Their importance, however, is in no way paralleled by our current understanding of the dynamic processes by which these proteins fold into and function within cellular membranes. This is mainly due to fundamental challenges in resolving the structural dynamics of proteins embedded within lipid-bilayer membranes or membrane-mimetic environments. Single-molecule spectroscopy bears great potential for dissecting this complexity. Particularly, single-molecule Förster resonance energy transfer (smFRET), owing to its sensitivity and versatility, has emerged as a new tool for accessing the spatial, temporal, and energetic features of membrane-protein folding reactions, providing unique insights into protein subpopulations and their associated dynamics on timescales ranging from nanoseconds to hours. Here, we review recent advances in the application of smFRET to the structural dynamics of membrane-protein folding and discuss the benefits that this new toolset affords to provide a molecular-level description of the dynamics governing this physiologically and therapeutically eminent class of proteins.