Fig. 7. The effect of ionic strength on the osmotic activation of OpuA. Uptake of [¹⁴C]glycine betaine (final concentration, 76 µM) was assayed in 100 (172 mosmol/kg) or 150 mM (258 mosmol/kg) KPi pH 7.0. Proteoliposomes were composed of DOPC/DOPE/DOPG in a 58:25:17 (A) and 50:25:25 (B) mole ratio. The standard components (circles, 172 mosmol/kg) plus additional KPi (upright triangles), KCl (squares) or sucrose (inverted triangles) (total of 258 mosmol/kg) were enclosed in the proteoliposomal lumen. (C) Uptake of [¹⁴C]glycine betaine (final concentration, 76 µM) was assayed in KPi pH 7.0 (equi-osmolal to the intravesicular osmolality). Proteoliposomes were composed of DOPC/DOPE/DOPG in a 50:25:25 mole ratio. Potassium phosphate pH 7.0 was enclosed in the proteoliposomal lumen at the concentrations indicated.