Charge-driven dynamics of nascent chain movement through the SecYEG translocon

On average, every fifth residue in secretory proteins carries either a positive or a negative charge. In a bacterium such as Escherichia coli, charged residues are exposed to an electric field as they transit through the inner membrane, which should generate a fluctuating electric force on a translocating nascent chain. Here, we have used translational arrest peptides as in vivo force sensors to measure this electric force during co-translational chain translocation through the SecYEG translocon. We find that charged residues experience a biphasic electric force as they move across the membrane, including an early component with a maximum when they are 47-49 residues away from the ribosomal P-site, followed by a more slowly varying component. The early component is generated by the transmembrane electric potential while the second may reflect interactions between charged residues and the periplasmic membrane surface.