Thomsson Elisabeth, Svensson Maria, Larsson Christer
Department of Chemistry and Bioscience, Lundberg Laboratory, Chalmers University of Technology, Box 462, SE-405 30 Gothenburg, Sweden.
Yeast. 2005 Jun;22(8):615-23. doi: 10.1002/yea.1219.
Saccharomyces cerevisiae growing exponentially in anaerobic batch cultures that are suddenly exposed to carbon starvation will rapidly lose almost all ATP. This will cause an energy deficiency and adaptation to starvation conditions is prohibited. As a result, viability and fermentative capacity will be drastically reduced during prolonged starvation. However, if the cells are incubated in the presence of rapamycin (which will inactivate the TOR pathway) before carbon starvation ATP levels, viability and fermentative capacity will be preserved to a much larger extent compared to untreated cells. The beneficial effect of rapamycin cannot be explained by induction of a stationary phase phenotype. In fact, under these anaerobic well-controlled growth conditions, rapamycin-treated cells were still metabolically active and continued to grow, albeit not exponentially and with a reduced protein content. It is hypothesized that the loss of ATP during carbon starvation occurs because protein synthesis does not make an immediate arrest at the onset of starvation. Since there are no external or internal energy sources, this will rapidly deplete the cells of ATP. Rapamycin-treated cells, on the other hand, have already downregulated the protein-synthesizing machinery and are thus better suited to cope with a sudden carbon starvation condition. This hypothesis is strengthened by the fact that treating the cells with the protein synthesis inhibitor cycloheximide also improves the carbon starvation tolerance, although not to the same extent as rapamycin. The even better effect of rapamycin is explained by accumulation of storage carbohydrates, which is not observed for cycloheximide-treated cells.
在厌氧分批培养中呈指数生长的酿酒酵母，若突然遭遇碳源饥饿，会迅速失去几乎所有的三磷酸腺苷（ATP）。这将导致能量缺乏，且无法适应饥饿条件。结果，在长期饥饿期间，细胞活力和发酵能力会大幅降低。然而，如果在碳源饥饿之前，将细胞置于雷帕霉素（会使雷帕霉素靶蛋白（TOR）信号通路失活）存在的环境中培养，与未处理的细胞相比，ATP水平、细胞活力和发酵能力将在更大程度上得以保留。雷帕霉素的有益作用无法通过诱导静止期表型来解释。事实上，在这些厌氧且控制良好的生长条件下，经雷帕霉素处理的细胞仍具有代谢活性并继续生长，尽管并非呈指数生长且蛋白质含量有所降低。据推测，碳源饥饿期间ATP的损失是因为蛋白质合成在饥饿开始时并未立即停止。由于没有外部或内部能量来源，这将迅速耗尽细胞内的ATP。另一方面，经雷帕霉素处理的细胞已经下调了蛋白质合成机制，因此更适合应对突然的碳源饥饿状况。用蛋白质合成抑制剂环己酰亚胺处理细胞也能提高对碳源饥饿的耐受性这一事实，强化了这一假说，尽管其效果不如雷帕霉素。雷帕霉素效果更佳的原因在于储存碳水化合物的积累，而经环己酰亚胺处理的细胞未观察到这种现象。
