Luciferase sequestration as an assay for in vitro uptake. (A) Luciferase cofractionating with vacuoles is proteinase K protected. In vitro uptake reactions of three times the standard volume were performed (60 min, 27°C) in the presence or absence of the ATP-regenerating system. The vacuoles were reisolated, washed, resuspended in 150 mM KCl in PS buffer to a concentration of 0.23 mg/ml, and split into aliquots. After addition of 1% Triton X-100 or buffer, the mixture was digested with the indicated concentrations of proteinase K (15 min, 0°C). Proteinase treatment was stopped by adding one volume of 0.5 mM PMSF in 150 mM KCl in PS buffer. Then, luciferase activity was assayed. (B) Luciferase can be released from the vacuolar lumen by freeze–thaw treatment. An uptake reaction was performed (60 min, 27°C). The vacuoles were reisolated and washed as described in the Materials and Methods section. The final pellet was resuspended in 90 μl PS buffer with 150 mM KCl and split into two aliquots. One was frozen at −80°C and thawed again slowly, whereas the other remained on ice. The samples were centrifuged (15 min, 145,000 g, 4°C). The supernatants (Sup)were recovered and the pellets were resuspended in an equal volume of PS with 150 mM KCl. Luciferase activity in both fractions was determined. The reaction tubes for this experiment had been coated with BSA (0.5 mg/ml, 15 min at room temperature) to reduce unspecific binding of luciferase to the surface.