Bioenergetic Aspects of the Translocation of Macromolecules Across Bacterial Membranes

Bacteria are extremely versatile in the sense that they have gained the ability to transport all three major classes of biopolymers through their cell envelope: proteins, nucleic acids, and polysaccharides. These macromolecules are translocated across membranes in a large number of cellular processes by specific translocation systems. Members of the ABC (ATP binding cassette) superfamily of transport ATPases are involved in the translocation of all three classes of macromolecules, in addition to unique transport ATPases. An intriguing aspect of these transport processes is that the barrier function of the membrane is preserved despite the fact the dimensions of the translocated molecules by far surpasses the thickness of the membrane. This raises questions like: How are these polar compounds translocated across the hydrophobic interior of the membrane, through a proteinaceous pore or through the lipid phase; what drives these macromolecules across the membrane; which energy sources are used and how is unidirectionality achieved? It is generally believed that macromolecules are translocated in a more or less extended, most likely linear form. A recurring theme in the bioenergetics of these translocation reactions in bacteria is the joint involvement of free energy input in the form of ATP hydrolysis and via proton sym- or antiport, driven by a proton gradient. Important similarities in the bioenergetic mechanisms of the translocation of these biopolymers therefore may exist.