Pro and Contra Carrier Proteins; Sugar Transport via the Periplasmic Galactose-Binding Protein

Abstract

Studies with bacterial transport systems have revealed a surprisingly complex variety of different transport systems in a simple organism such as Escherichia coli. With respect to the transport mechanism or the characteristic properties of their components, the majority of bacterial systems are divided in three different classes: (1) systems that are similar to the E. coli lactose transport system, (2) the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) and other group translocation systems, and (3) transport systems mediated by periplasmic substrate-binding proteins. This chapter discusses the role of substrate-binding proteins in transport, using the galactose-binding protein and the β-methylgalactoside (MeGal) transport system as a model. It describes similar phenomena observed in other binding protein-mediated systems and compression is made to transport systems not mediated by periplasmic protein components. The chapter discusses the question of energy coupling and the arguments concerning a possible membrane carrier function of the substrate-binding proteins, as well as the function of the galactose-binding protein in galactose chemotaxis.