Abstract

The proton-motive force (PMF) – the electrochemical gradient of protons across energy-conserving membranes – powers protein transport in bacteria, mitochondria and chloroplasts. Here, we propose a ‘proton ratchet’ mechanism for this process. In the Sec system of bacteria, protons are stripped from lysine side chains of the pre-protein at the cytosolic face of the plasma membrane, then replaced on the exterior, aided by the pH component of the PMF (∆pH; acidic outside). This gives the translocating region of pre-protein a net negative charge, allowing it to diffuse down the electrical component (∆ψ; positive outside). For mitochondrial import (through the TIM23 complex) the proton ratchet acts in the opposite direction, with negatively charged residues protonated for passage across the inner membrane into the negative matrix. The proton ratchet is an elegant solution for coupling the PMF to membrane transport, likely to be used by a range of other transporters of charged molecules.