Regulation of the bacterial translocon SecYEG by periplasmic pH

The bacterial translocon SecYEG is a conserved heterotrimeric membrane complex located in the bacterial inner membrane. It is a protein-conducting channel, facilitating the secretion of hydrophilic protein fragments across, or in the case of hydrophobic helices, their integration into the membrane. The ATP hydrolysis by the motor protein SecA is the primary energy source for post-translational translocation. Another energy source is PMF across the bacterial inner membrane. The electrical component of PMF, the transmembrane potential, was shown to drive the translocation of segments enriched in basic residues1. However, the content of acidic and basic residues in a typical peptide chain is roughly balanced, and it is unclear how PMF may promote the protein translocation of such segments2. Here, we investigate how the chemical component of PMF affects the translocon itself. For that, we reconstituted into planar lipid bilayers the translocation intermediates consisting of the purified SecYEG channels and stalled ribosome-nascent chain complexes bearing a hydrophobic sequence3. Monitoring single-channel open probabilities of translocation intermediates electrophysiologically4, we compared the membrane partitioning kinetics of the hydrophobic helix between the translocon interior and lipid phase at different pH gradients for wild-type SecYEG and a SecYEG mutant where periplasmic histidine was substituted for neutral alanine. Our data suggest that histidine protonation affects the SecYEG mediated protein translocation by affecting the probability of the closed state. Support of the Austrian Science Fund (FWF), grant P34584 to PP, is gratefully acknowledged.