Table I. ATPase activity of membrane-reconstituted OpuA^a as a function of ionic osmolyte concentration.

Osmolytes	Glycine betaine (0.5 mM)	Osmolality (mosmol/kg)	ATPase activities (nmol/min*mg OpuA)
KPi 50 mM	+	90	18 ± 7 [10%]
KPi 100 mM	+	180	23 ± 7 [13%]
KPi 150 mM	+	270	117 ± 16 [65%]
KPi 200 mM	+	360	180 ± 20 [100%]
KPi 200 mM	–	360	5 ± 6 [3%]
KPi 250 mM	+	450	146 ± 15 [81%]
KPi 350 mM	+	630	124 ± 15 [69%]
KPi (50 mM) + K2SO4 (160 mM)	+	450	148 ± 16 [82%]
KPi (50 mM) + KCl (200 mM)	+	450	99 ± 12 [55%]
KPi (50 mM) + NaCl (200 mM)	+	450	95 ± 14 [53%]
KPi (50 mM) + sucrose (290 mM)	+	450	9 ± 4 [5%]

^aProteoliposomes were composed of 50 mol% DOPC, 25 mol% DOPE and 25 mol% DOPG. The osmolality of the assay medium was varied by the addition of salt or sugar as indicated under osmolytes.