TRANSPORT-PROPERTIES OF ASP-51-]GLU AND ASP-120-]GLU MUTANTS OF THE MELIBIOSE CARRIER OF ESCHERICHIA-COLI

Asp-51 --> Glu and Asp-120 --> Glu mutants of the melibiose carrier of Escherichia coli were investigated for their cation/sugar cotransport properties. The carrier containing Glu-51 showed proton/ melibiose cotransport but was extremely defective in Na+ or Li+ stimulation of sugar accumulation. On the other hand, the carrier containing Glu-120 had lost the ability to couple protons with melibiose uptake while retaining considerable Na+ or Li+ cotransport with melibiose (40-fold accumulation versus 90-fold for the wild type in the presence of Na+). It is concluded that both Asp-51 and Asp-120 are important for cation recognition.