The Tol-Pal proteins of the Escherichia coli cell envelope: an energized system required for outer membrane integrity?

The outer membrane of Gram-negative bacteria acts as a barrier against harm ful lipophilic compounds and larger molecules unable to diffuse freely thro ugh the porins. However, outer membrane proteins together with the Tol-Pal and TonB systems have been exploited for the entry of macromolecules such a s bacteriocins and phage DNA through the Escherichia coli cell envelope. Th e TonB system is involved in the active transport of iron siderophores and vitamin B12, while no more precise physiological role of the Tol-Pal system has yet been defined than its requirement for cell envelope integrity. The se two systems, containing an energized inner membrane protein interacting with outer membrane proteins, share similarities. (C) 2001 Editions scienti fiques et medicales Elsevier SAS.