Structural determinants of a permeation barrier of the SecYEG translocon in the active state.

The SecYEG translocon is a channel in bacteria, which provides a passage for secretory proteins across as well as integration of membrane proteins into the plasma membrane. The molecular mechanism, by which SecYEG manages protein transport while preventing water and ion leakage through the membrane, is still controversial. We employed molecular dynamics simulations to assess the contribution of the major structural elements - the plug and the pore ring (PR) - to the sealing of SecYEG in the active state, i.e., with a signal sequence helix occupying the lateral gate. We found, that the PR alone can provide a very tight seal for the wild-type translocon in the active state for both water and ions. Simulations of the mutant I403N, in which one of the PR-defining isoleucine residues is replaced with asparagine, suggest that hydrophobic interactions within the PR and between the PR and the plug are important for maintaining a tight conformation of the wild-type channel around the PR. Disruption of these interactions results in strong fluctuations of helix TM7 and water leakage of the translocon.