The substrate-binding site in the lactose permease of Escherichia coli

Site-directed N-ethylmaleimide labeling was studied with Glu-126 and/or Arg-144 mutants in lactose permease containing a single, native Cys residue at position 148 in the substrate-binding site. Replacement of either Glu-126 or Arg-144 with Ala markedly decreases Cys-148 reactivity, whereas interchanging the residues, double-Ala replacement, or replacement of Arg-144 with Lys or His does not alter reactivity, indicating that Glu-126 and Arg-144 are charge-paired. Importantly, although alkylation of Cys-148 is blocked by ligand in wild-type permease, no protection whatsoever is observed with any of the Glu-126 or Arg-144 mutants. Site-directed fluorescence with 2-(4-maleimidoanilino)-naphthalene-6-sulfonic acid (MIANS) in mutant Val-331 → Cys was also studied. In marked contrast to Val-331 → Cys permease, ligand does not alter MIANS reactivity in mutant Glu-126 → Ala/Val-331 → Cys, Arg-144 → Ala/Val-331 → Cys, or Arg-144 → Lys/Val-331 → Cys and does not cause either quenching or a shift in the emission maximum of the MIANS-labeled mutants. However, mutation Glu-126 → Ala or Arg-144 → Ala and, to a lesser extent, Arg-144 → Lys cause a red-shift in the emission spectrum and render the fluorophore more accessible to I(−). The results demonstrate that Glu-126 and Arg-144 are irreplaceable for substrate binding and suggest a model for the substrate-binding site in the permease. In addition, the findings are consistent with the notion that alterations in the substrate translocation pathway at the interface between helices IV and V are transmitted conformationally to the H(+) translocation pathway at the interface between helices IX and X.