Single-molecule force spectroscopy of rapidly fluctuating, marginally stable structures in the intrinsically...

Intrinsically disordered proteins form transient, fluctuating structures that are difficult to observe directly. We used optical tweezers to apply force to single α-synuclein molecules and measure their extension, characterizing the resulting conformational transitions. Force-extension curves revealed rapid fluctuations at low force, arising from the folding of two different classes of structure that were only marginally stable. The energy landscape for these transitions was characterized via the force-dependent kinetics derived from correlation analysis of the extension trajectories. The barriers were small, only a few kBT, but the diffusion was slow, revealing a landscape that is flat but rough.