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Summary
Single-molecule force spectroscopy (SMFS) (1) measures the extension of a molecule when subjected to force. The folding of a protein can be explored by pulling on one terminus to unfold it; upon relaxation, it may refold toward its native states (2, 3). Transmembrane proteins typically unfold stepwise as structural segments (which can consist of parts of single or multiple secondary structures) are extracted from the membrane (see the figure, top panel) (4). Once extracted, the unfolded segment can insert back into the membrane and fold toward the native protein structure (5, 6). Complex proteins extracted from the membrane may refold back into the membrane (5, 6) but tend to misfold and require the assistance of chaperones, insertases, or both (7, 8). Incremental folding steps of secondary structures are theorized to happen in the order of milliseconds (9) but have been undetectable because of the limited time resolution of SMFS-related techniques. On page 945 of this issue, Yu et al. (10) address this problem by applying a newly developed SMFS technique based on atomic force microscopy (AFM) that records the response of individual membrane proteins (bacteriorhodopsin) subjected to mechanical forces at microsecond resolution.
