Endocytosis in Dictyostelium discoideum amoebae: Inhibition by cyclohexlmide and other inhibitors of protein synthesis

Cycloheximide, an inhibitor of protein synthesis, inhibited endocytosis pathways in the vegetative amoebae of the cellular slime mold Dictyostelium discoideum. Half-maximal inhibition of fluid-phase pinocytosis in nutrient medium or in buffer occurred at 0.08 mM or 0.35 mM cycloheximide, respectively. Other inhibitors of eucaryotic protein synthesis inhibited fluid-phase pinocytosis, with the following IC50 in nutrient medium: 0.15 mM anisomycin, ImM emetine, 0.25 mM puromycin. Kinetic studies showed that the inhibitory effects of cycloheximide occurred rapidly, with a half-time of less than 10 min, and were fully reversible upon removal of the drug. Both fluid-phase pinocytic influx and efflux were inhibited by cycloheximide, which furthermore prevented the vanadate-induced efflux of fluorescein-labeled dextran, the fluid-phase marker used in these experiments. The massive secretion of lysosomal enzymes: acid phosphatase, N-acetylhexosaminidase or α-mannosidase, which follows overloading of the endosomal compartment by sucrose, was also inhibited by cycloheximide. All these phenomena were unimpaired by cycloheximide in the cycloheximide-resistant (cycAl) mutant strain X6.