Fig. 4. The effect of anionic lipids on the osmotic activation of OpuA. (A) Uptake of [¹⁴C]glycine betaine (final concentration, 76 µM) was assayed in 100 mM KPi pH 7.0. The proteoliposomes were composed of 50 mol% DOPE and 0 (circles), 13 (upright triangles), 25 (inverted triangles), 38 (diamonds) or 50 mol% DOPG (squares) plus 50, 37, 25, 12 or 0 mol% DOPC, respectively. (B) The proteoliposomes were composed of 50 mol% DOPE and 13 (upright triangles), 25 (inverted triangles) or 38 mol% DOPG (diamonds) plus 37, 25 or 12 mol% DOPC, respectively, and were obtained via fusion of proteoliposomes (DOPE/DOPG, 1:1 mole ratio) with liposomes of the appropriate composition in a 1:1 mole ratio. The osmolality of the medium was varied with KCl. Δ Osmolality refers to the difference in external and internal osmolality.