Rate-limiting transport of positive charges through the Sec-machinery is integral to the mechanism of protein transport

Abstract

The proton-motive force (PMF) -the electrochemical gradient of protons across energyconserving 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, promoting electrophoretic diffusion across the membrane driven by membrane-potential (∆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 transport, likely to be used by a range of other transporters of charged molecules.

Generalized Approach towards Secretion-Based Protein Production via Neutralization of Secretion-Preventing Cationic Substrate Residues

“…In fact, other transporters in the Sec transporter family exhibit a similar pattern, where cationic residues are not easily translocated [ 25 , 26 ], and among those cationic residues, arginine is much harder to secrete than lysine [ 27 , 28 , 29 ]. This could be indirect evidence that the polypeptide-secreting ABC transporters may share a similar fundamental mechanism with the Sec transporters.…”

“…Both Sec transporters and ABC transporters: (i) involve the configurational changes accompanied by the ATP hydrolysis, (ii) have to “push” the polypeptides through a narrow channel, which allows no complicated structures except for the alpha helices to pass through, and (iii) primarily secrete acidic (or negatively charged) proteins. There was an attempt to explain this behavior as a “proton ratchet” effect in Sec transporters [ 29 ], which was an application of the Brownian ratchet mechanism of secretion modes [ 30 , 31 ]. We propose that this theory might also be valid for polypeptide-secreting ABC transporters as well.…”