Charge Neutralization During Peptide Transport in the Bacterial SecYEG Translocon

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Protein translocation across cell membranes is crucial for cellular function but its driving force remains unclear.
• In bacteria, the electrochemical membrane potential can influence charged residues during protein transport.
• The SecYEG channel is a key component in bacterial protein translocation.

Purpose of the Study:

• To investigate the protonation states of charged residues (lysine and glutamate) within the SecYEG channel during peptide translocation.
• To understand how the channel environment affects the charge of translocating substrates.

Main Methods:

• Utilized Poisson-Boltzmann continuum electrostatic free energy calculations.
• Employed Monte Carlo titrations on molecular dynamics (MD) simulation snapshots.
• Analyzed protonation states of lysine and glutamate test residues within the SecYEG channel.

Main Results:

• Observed a shift towards the uncharged state for both lysine and glutamate as they moved deeper into the SecYEG channel.
• Demonstrated charge neutralization irrespective of the test residue's initial charge (positive or negative).
• Identified the channel's dielectric properties and interactions with non-titrating charges as primary determinants of protonation state.

Conclusions:

• Charge neutralization of translocating peptides occurs within the SecYEG channel, influenced by its low dielectric constant.
• Electrostatic interactions within the channel environment play a significant role in modulating substrate charge.
• Results highlight the importance of considering environmental dielectric properties in protein translocation mechanisms.