Energy coupling in bacterial periplasmic permeases

Abstract

Bacterial active transport systems (permeases) can be broadly divided into two classes with regard to their mechanism of energy coupling: those energized by electrochemical ion gradients and those energized by substrate-level phosphorylation. The distribution of the permeases according to this classification is matched by their fundamentally different characteristics. Electrochemical ion gradient-energized permeases, as typified by the 3-galactoside (lacY) permease (25), are shock-resistant systems usually composed of a single, very hydrophobic membrane protein that acts as a symporter or an antiporter, utilizing an ion or a proton gradient. In contrast, substrate-level phosphorylation-energized permeases constitute a class including numerous shock-sensitive permeases that have a complex composition. The shock-sensitive denomination refers to the inactivation of transport through these systems upon osmotic shock treatment of the intact cell. The inactivation is due to the loss from the periplasm (the space between the outer and inner membranes of gram-negative bacteria) of one of the components of the transport machinery, the periplasmic substrate-binding protein. Thus, shock-sensitive permeases are also called periplasmic permeases. They are composed of a membrane-bound complex, usually comprising between two and four membrane-bound proteins, and a soluble periplasmic protein. They typically transport with high affinity, achieving very large concentration gradients. Numerous such permeases, acting on extremely disparate substrates (sugars, amino acids, peptides, ions, and vitamins) have been characterized, and their properties have been reviewed recently (3, 4). Their overall composition is invariably the same, irrespective of the nature of the substrate being transported. Clearly, the uniformly similar structural design of these permeases can serve to transport vastly different substrates. An understanding of their mechanism of action requires that the energy-coupling process be unravelled first. The action of periplasmic permeases involves the initial liganding of the substrate to the substrate-binding protein in the periplasm, thus resulting in the formation of the actual transport substrate, the liganded binding protein (33). The practical result of this first step is that the solute to be transported is presented in a concentrated form to the membrane-bound complex, as a consequence of the high binding affinity and very high concentration (of the order of millimolar) of the binding protein in the periplasm. The membrane-bound complex usually contains two very hydrophobic membrane-spanning proteins, which might form a heterodimer (2) or, if only one hydrophobic component is present, a homodimer. The third membrane-bound compo-