Effect of proline and arginine metabolism on freezing stress of Saccharomyces cerevisiae.

In Saccharomyces cerevisiae, the PUT1-encoded proline oxidase and the PUT2-encoded delta1-pyrroline-5-carboxylate dehydrogenase are required to convert proline to glutamate. We recently showed that a put1 disruptant accumulated higher levels of proline intracellularly and conferred higher resistance to freezing stress. Here, we determined the effect of put2 disruption on yeast cell viability under freezing stress. When grown on arginine as the sole nitrogen source, the put2 disruptant showed a significant decrease in cell viability after freezing despite the high proline and arginine contents. This result suggests that delta1-pyrroline-5-carboxylate or glutamate-gamma-semialdehyde, a proline catabolism intermediate, is toxic to yeast cells under freezing stress. In contrast, the survival rate of the wild-type and the put1-disruptant strains was found to increase after freezing in proportion to their arginine contents. This indicates that arginine has a cryoprotective function in yeast. Furthermore, the yeast cells accumulated proline as well as arginine in the vacuole, suggesting that there is a system for the transport of excess proline to the vacuole and that this vacuolar accumulation may be important in the freezing resistance of yeast cells.