Effect of Detergents on Galactoside Binding by Melibiose Permeases

Abstract

The effect of various detergents on the stability and function of the melibiose permeases of Escherichia coli (MelB(Ec)) and Salmonella typhimurium (MelB(st)) was studied. In n-dodecyl-beta-D-maltoside (DDM) or n-undecyl-beta-Dmaltoside (UDM), WT MelBst binds melibiose with an affinity similar to that in the membrane. However, with WT MelB(Ec), or MelB(st) mutants (Arg141 -> Cys, Arg295 -> Cys, or Arg363 -> Cys), galactoside binding is not detected in these detergents, but binding to the phosphotransferase protein IIA(Glc) is maintained. In the amphiphiles lauryl maltose neopentyl glycol (MNG-3) or glyco-diosgenin (GDN), galactoside binding with all of the MelB proteins is observed, with slightly reduced affinities. MelB(st) is more therrnostable than MelB(Ec) and the thermostability of either MelB is largely increased in MNG-3 or GDN. Therefore, the functional defect with DDM or UDM likely results from the relative instability of the sensitive MelB proteins, and stability, as well as galactoside binding, is retained in MNG-3 or GDN. Furthermore, isothermal titration calorirnetry of melibiose binding with MelB(st) shows that the favorable entropic contribution to the binding free energy is decreased in MNG-3, indicating that the conformational dynamics of MelB is restricted in this detergent.