Figure 5.

Protection by P_i of ATPase activity in wild-type and mutant membranes from inactivation by NBD-Cl. Membranes were preincubated with MgP_i at 0, 2.5, 5, or 10 mM as shown, for 60 min at room temperature. Then NBD-Cl (150 μM) was added, and aliquots were withdrawn for the assay at time intervals as shown. The ATPase activity remaining is plotted vs time of incubation with NBD-Cl: (○) no P_i added, (◊) 2.5 mM P_i, (□) 5 mM P_i, and (△) 10 mM P_i. Each data point represents the average of four different experiments using two or three independent membrane preparations of each mutant.