Maltose-binding protein does not modulate the activity of maltoporin as a general porin in Escherichia coli

Abstract

Maltoporin (lambda receptor) is part of the maltose transport system in Escherichia coli and is necessary for the facilitated diffusion of maltose and maltodextrins across the outer membrane. Maltoporin also allows the diffusion of nonmaltodextrin substrates, albeit with less efficiency. The preference of maltoporin for maltodextrins in vivo is thought to be the result of an interaction of maltoporin with the maltose-binding protein, the malE gene product. In a recent report Heuzenroeder and Reeves (J. Bacteriol. 144:431-435, 1980) suggested that this interaction establishes a gating mechanism which inhibits the diffusion of nonmaltodextrin substrates, such as lactose. To reinvestigate this important conclusion, we constructed ompR malTc strains carrying either the malE+ gene, the nonpolar malE444 deletion, or the malE254 allele, which specifies an interaction-deficient maltose-binding protein. Lactose uptake was measured at different concentrations below the Km of this transport system and under conditions where transport was limited by the diffusion through maltoporin. We found no difference in the kinetics of lactose uptake irrespective of the malE allele. We conclude that the maltose-binding protein does not modulate the activity of maltoporin as a general outer membrane porin.