Figure 6.

Translocation by a proton-driven engine. Folded substrate and unstructured leader sequence (gray) bind and dock to the PA channel. The α clamp nucleates and subsequently propagates helix formation. The ϕ clamp engages the compact translocating chain; deprotonated acidic residues (red) are then protonated (black). An increase in TΔS_config outside the channel favors the transition of helix to unstructured random coil; acidic residues deprotonate in the higher pH of the cytosol. The charge clamp engages, permitting the passage of protonated acidic residues while preventing the retrotranslocation of deprotonated ones. Ungating of the ϕ clamp allows the chain to translocate, while the α clamp continues to stabilize and template unfolded polypeptide into helix. The cycle repeats until the substrate is fully unfolded and translocated.