ABSTRACT
The evolution of endo-lysosomal pH in Dictyostelium dis -coideum amoebae was examined during fluid-phase endocytosis. Pulse-chase experiments were conducted in nutritive medium or in non-nutritive medium using fluorescein labelled dextran (FITC-dextran) as fluid-phase marker and pH probe. In both conditions, efflux kinetics were characterized by an extended lag phase lasting for 45-60 min and corresponding to intracellular transit of FITC-dextran cohort. During the chase period, endosomal pH decreased during ∼20 min from extracellular pH down to pH 4.6-5.0, then, it increased within the next 20-40 min to reach pH 6.0-6.2. It was only at this stage that FITC-dextran was released back into the medium with pseudo first- order kinetics. A vacuolar H+-ATPase is involved in endosomal acidification as the acidification process was markedly reduced in mutant strain HGR8, partially defective in vacuolar H+-ATPase and in parent type strain AX2 by bafilomycin A1, a selective inhibitor of this enzyme. Our data suggest that endocytic cargo is channeled from endosomes to secondary lysosomes that are actively linked to the plasma membrane via recycling vesicles.
