A Spring-Loaded Mechanism Governs the Clamp-like Dynamics of the Skp Chaperone.

The trimeric periplasmic holdase chaperone Skp binds and stabilizes unfolded outer membrane proteins (OMPs) as part of bacterial OMP biogenesis. Skp binds client proteins in its central cavity, thereby reducing its backbone dynamics, but the molecular mechanisms that govern Skp dynamics and adaptation to differently sized clients remains unknown. Here, we employ a combination of microsecond timescale molecular dynamics simulation, small-angle X-ray scattering, and nuclear magnetic resonance spectroscopy to reveal that Skp is remarkably flexible, and features a molecular spring-loaded mechanism in its "tentacle" arms that enables switching between two distinct conformations on sub-millisecond timescales. The conformational switch is executed around a conserved pivot element within the coiled-coil structures of the tentacles, allowing expansion of the cavity and thus accommodation of differently sized clients. The spring-loaded mechanism shows how a chaperone can efficiently modulate its structure and function in an ATP-independent manner.