Bacterial Translocation Ratchets: Shared Physical Principles with Different Molecular Implementations

Abstract

Secretion systems enable bacteria to import and secrete large macromolecules including DNA and proteins. While most components of these systems have been identified, the molecular mechanisms of macromolecular transport remain poorly understood. Recent findings suggest that various bacterial secretion systems make use of the translocation ratchet mechanism for transporting polymers across the cell envelope. Translocation ratchets are powered by chemical potential differences generated by concentration gradients of ions or molecules that are specific to the respective secretion systems. Bacteria employ these potential differences for biasing Brownian motion of the macromolecules within the conduits of the secretion systems. Candidates for this mechanism include DNA import by the type II secretion/ type IV pilus system, DNA export by the type IV secretion system, and protein export by the type I secretion system. Here, we propose that these three secretion systems employ different molecular implementations of the translocation ratchet mechanism.

A Mechanistic Perspective on PEX1 and PEX6, Two AAA+ Proteins of the Peroxisomal Protein Import Machinery

“…In addition, some type of energy is used to provide directionality to the protein translocation step. Frequently, energy from ATP or GTP hydrolysis is used to trigger conformational alterations in components of the translocation machinery which in turn either push the protein substrate through the membrane (power stroke mechanisms) or bind and retain the substrate at the trans side of that membrane (molecular ratchet mechanisms) [8,9,10]. In at least one case—protein import into mitochondria—the electric component of the membrane potential is explored to induce electrophoretic movement of the translocation substrate [11].…”

Force-dependent diffusion coefficient of molecular Brownian ratchets

“…(14) leads to the same upper bound on the binding energy as in Eq. (12). For the lower bound, however, the resulting bound on the binding energy…”

“…Since chaperones are too large to pass through the pore, the movement of the chain gets biases towards the side with the higher concentration of chaperones. Translocation ratchets have been discussed in the context of various different translocation processes in biological systems [12,13]. Recent experiments showed that the uptake of DNA molecules by Neiseria gonorrhoeae bacteria [14] can be described by a rather simple translocation ratchet model [15].…”