BIOENERGETIC ASPECTS OF THE TRANSLOCATION OF MACROMOLECULES ACROSS BACTERIAL-MEMBRANES

Bacteria are extremely versatile in the sense that they have gained th e ability to transport all three major classes of biopolymers through their cell envelope: proteins, nucleic acids, and polysaccharides. The se macromolecules are translocated across membranes in a large number of cellular processes by specific translocation systems. Members of th e ABC (ATP binding cassette) superfamily of transport ATPases are invo lved in the translocation of all three classes of macromolecules, in a ddition to unique transport ATPases. An intriguing aspect of these tra nsport processes is that the barrier function of the membrane is prese rved despite the fact the dimensions of the translocated molecules by far surpasses the thickness of the membrane. This raises questions lik e: How are these polar compounds translocated across the hydrophobic i nterior of the membrane, through a proteinaceous pore or through the l ipid phase; what drives these macromolecules across the membrane; whic h 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 bioen ergetics of these translocation reactions in bacteria is the joint inv olvement of free energy input in the form of ATP hydrolysis and via pr oton sym- or antiport, driven by a proton gradient. Important similari ties in the bioenergetic mechanisms of the translocation of these biop olymers therefore may exist.