Unidirectional Transport Activity Mediated by the Galactose-binding Protein of Escherichia coli

Abstract

Abstract The galactose-binding protein, a necessary component of the β-methylgalactoside transport system of Escherichia coli K12 mediates the uptake of galactose against a concentration gradient. The equilibrium state of accumulation is characterized by the simultaneous entry and exit of galactose. However, in contrast to the lactose transport system of E. coli, energy coupling occurs during entry but not exit of galactose, and counterflow for the entry process could not be demonstrated in poisoned cells. Transmembranal stimulation can be observed for the exit but not for the entry of galactose and requires the presence of a functional galactose-binding protein. The galactose-binding protein mediates only the entry and not the exit of galactose. Exit of galactose is mediated either by other components of the β-methylgalactoside transport system or by an entirely different transport system specific for galactose.