AcKNOwledgemeNTSWe are indebted to Prof. Mori M. (Kumamoto University, Japan) for generously providing pCAGGS-pOTC-GFP. We thank Ms. Nagami Yamashita for her supports. Prof. Ohkuma died on November 5, 2006 by myocardial infarction at his home. We greatly miss him as a scientist and a friend. We offer sincere thanks to all the friends, colleagues and former collaborators of Prof. Ohkuma who showed him kindness during his lifetime. AbSTrAcTAutophagy is the bulk degradation of cytoplasmic constituents in response to starvation and other environmental or intracellular cues. During this process, most of the cytoplasm is sequestered into autophagosomes, which then fuse with lysosomes where the degradation of the sequestered material proceeds. We investigated the relationship between autophagosome-lysosome fusion and the pH in acidic compartments by visualizing the fusion process using fluorescence in CHO cells. In this experiment, mitochondria were labeled with GFP by transfecting CHO cells with the presequence of ornithine transcarbamylase, and lysosomes were labeled with Texas Red Dextran; any fusion was identified by the colocalization of mitochondria (in autophagosomes) and lysosomes using fluorescence microscopy. When CHO cells were treated with rapamycin or starvation medium to induce autophagy, the colocalization of fluorescence was observed. Whereas when they were treated with 3-MA, an inhibitor of autophagy, the colocalization disappeared. We conclude that the colocalization reflects the fusion of autophagosomes and lysosomes. Moreover, when the CHO cells were treated with drugs that increase the pH of acidic compartments, the colocalization disappeared. This suggests that the autophagosome-lysosome fusion is inhibited by increasing pH in acidic compartments independently of V-ATPase activity in CHO cells.

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