Fig. 1. Kinetics and reversibility of the osmotic activation of OpuA. Uptake of [¹⁴C]glycine betaine (final concentration, 76 µM) was assayed in 100 mM KPi pH 7.0 corresponding to 190 mosmol/kg. The proteoliposomes were composed of DOPC/DOPE/DOPG in a 2:1:1 mole ratio. At 105 (circles, squares and triangles) and 285 s (circles), the proteoliposomes were subjected to hyperosmotic conditions by the addition of 100 mM KCl (final osmolality corresponding to 380 mosmol/kg). Iso-osmotic conditions were restored at 180 s (circles) by dilution of the assay mixture with water (plus 76 µM [¹⁴C]glycine betaine).