Suppression of Conformation-Compromised Mutants of Salmonella enterica Serovar Typhimurium MelB

The crystal structure of the Na⁺-coupled melibiose permease of Salmonella enterica serovar Typhimurium (MelB_St) demonstrates that MelB is a member of the major facilitator superfamily of transporters. Arg residues at positions 295, 141, and 363 are involved in interdomain interactions at the cytoplasmic side by governing three clusters of electrostatic/polar interactions. Insertion of (one at a time) Glu, Leu, Gln, or Cys at positions R295, R141, and R363, or Lys at position R295, inhibits active transport of melibiose to a level of 2 to 20% of the value for wild-type (WT) MelB_St, with little effect on binding affinities for both sugar and Na⁺. Interestingly, a spontaneous suppressor, D35E (periplasmic end of helix I), was isolated from the R363Q MelB_St mutant. Introduction of the D35E mutation in each of the mutants at R295, R141 (except R141E), or R363 rescues melibiose transport to up to 91% of the WT value. Single-site mutations for the pair of D35 and R175 (periplasmic end of helix VI) were constructed by replacing Asp with Glu, Gln, or Cys and R175 with Gln, Asn, or Cys. All mutants with mutations at R175 are active, indicating that a positive charge at R175 is not necessary. Mutant D35E shows reduced transport; D35Q and D35C are nearly inactivated. Surprisingly, the D35Q mutation partially rescues both R141C and R295Q mutations. The data support the idea that Arg at position 295 and a positive charge at positions 141 and 363 are required for melibiose transport catalyzed by MelB_St, and their mutation inhibits conformational cycling, which is suppressed by a minor modification at the opposite side of the membrane.