Abstract
Saccharomyces cerevisiae cells are able to elongate their G1 phase upon nutrient limitation. Homology between mammalian and yeast MAPK signalling pathways led to the investigation of Slt2 activity during short and elongated G1 phases. It appeared that the activity of Slt2 was regulated during the G1 phase of different lengths.
... read more
Deletion of SLT2 led to severe cell cycle elongation, which is comparable to MAPK inhibition in mammalian cells. Trehalose accumulation is in Saccharomyces cerevisiae involved in elongation of the G1 phase. Cells that are unable to accumulate this reserve carbohydrate are less resistant to stress and when they can also not synthesize glycogen, their G1 phase cannot be elongated. The hexose transporter Hxt5 is involved in synthesis of trehalose. After glucose exhaustion in batch growth, trehalose synthesis continues and the glucose used for this is probably derived from gluconeogenesis. The localization of Hxt5p in the endoplasmic reticulum during stationary phase. The presence of a glucose transporter in the ER, which is involved in gluconeogenesis in mammalian cells, suggests a role for Hxt5p in gluconeogenesis as well. One of the components of the trehalose synthesizing complex, Tps1p, and Hxt5 are localized in the same complex upon glucose addition to starved cells. This suggested a function of this complex under these conditions and the presence of a glucose-response complex was suggested. The function of this complex is to prevent energy exhaustion due to an overflow of the glycolysis. The presence of the GRC is required for expression of hexose transporter genes on glucose. The GRC probably works via inhibition of a hexose kinase, Hxk2p, which is inhibited by trehalose-6-phosphate. Because the expression of HXT5 is regulated differently than that of the major hexose transporters, the metabolism of degradation of Hxt5p was investigated. Hxt5p levels are decreased by active degradation and dilution of Hxt5p out of the culture. Mother cells no longer incorporate Hxt5p in developing buds, while they do during stationary phase. Hxt5p contains a PEST-sequence and is transiently phosphorylated upon repression of expression, but this modification is probably not involved in the degradation of the protein. After disappearance of the phosphorylation signal, more than 60% of the Hxt5p is still present and no ubiquitination of the protein was detected. Localization of Hxt5p with electron microscopy on cryosections revealed a localization of Hxt5p at the plasma membrane, endoplasmic reticulum, vesicular structures and the vacuole. The localization at the endoplasmic reticulum represented probably newly synthesized protein. The localization at the vesicular structures and the vacuole suggested that the degradation of Hxt5p is similar to that of the major hexose transporters.
show less
