The response of tobacco (Nicotiana tabacum) suspension-cultured cells (BY-2) to nutrient starvation was investigated. When the cells that were grown in Murashige-Skoog medium containing 3% (w/v) sucrose were transferred to the same medium without sucrose, 30 to 45% of the intracellular proteins were degraded in 2 d. An analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that proteins were degraded nonselectively. With the same treatment, protease activity in the cell, which was measured at pH 5.0 using fluorescein thiocarbamoyl-casein as a substrate, increased 3- to 7-fold after 1 d. When the cysteine protease inhibitor (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methyl-butane (10 [mu]M) was present in the starvation medium, both the protein degradation and the increase in the protease activity were effectively inhibited. Light microscopy analysis showed that many small spherical bodies accumulated in the perinuclear region of the cytosol 8 h after the start of the inhibitor treatment. These bodies were shown to be membrane-bound vesicles of 1 to 6 [mu]m in diameter that contained several particles. Quinacrine stained these vesicles and the central vacuole; thus, both organelles are acidic compartments. Cytochemical enzyme analysis using 1-naphthylphosphate and [beta]-glycerophosphate as substrates showed that these vesicles contained an acid phosphatase(s). We suggest that these vesicles contribute to cellular protein degradation stimulated under sucrose starvation conditions.