The overall goal of my research is to explore novel responses to hyper-osmotic stress in the eukaryotic yeast, Saccharomyces cerevisiae. We have identified two novel responses.
First, we have shown that yeast cell respond to osmotic stress by producing a low abundance osmoprotectant called glycerophosphocholine (GPC). In yeast, GPC is generated by the deacylation of the membrane lipid phosphatidylcholine (PC). Deacylation is catalyzed by the enzyme Nte1 (neuropathy targeted esterase 1). Nte1 is a conserved enzyme that is also found in higher eukaryotes (both invertebrate and vertabrate). In mammals for example, NTE catalyzes the production of GPC that is found in renal cells that are challenged by osmotic stress. In addition, studies in mammals and fruit flies have shown that NTE plays a role in embryogenesis and neural function. We are currently investigating the means by which Nte1 is regulated posttranslationally in yeast.
Second, we have also found that yeast cells accumulate glutathione when challenged by hyperosmotic stress. Glutathione is a tripeptide composed of three amino acids: glutamate, cysteine and glycine. Glutathione is known to function as a potent anti-oxidant and may play a role in aging and senescence in many organisms. In addition, it also binds to toxic, heavy metals to reduce their toxicity. Thus, we are investigating the means by which yeast cells regulate glutathione levels during hyperosmotic stress and its relationship to oxidative stress and aging in yeast.
1. Ramirez, RM, T Ishida-Schick, B Krilowicz, B Leish, and KD Atkinson (1983). Plasma membrane expansion terminates in Saccharomyces cerevisiae secretion defective mutants while phospholipid synthesis continues. J. Bact. 154:1276-1283.
2. Atkinson, KD, and RM Ramirez (1984). Secretion proceeds uncoupled from net plasma membrane expansion in inositol-starved Saccharomyces cerevisiae. J. Bact. 160:80-86.
3. Ramirez, RM, WS Prince, E Bremmer, and MR Villarejo (1989). In vitro reconstitution of osmoregulated expression of proU of Escherichia coli. P.N.A.S. (U.S.A.) 86:1153-1157.
4. Ramirez, RM, and MR Villarejo (1991). Osmotic signal transduction to proU is independent of DNA supercoiling in Escherichia coli. J. Bact. 173:879-885.
5. Esposito, MS, RM Ramirez, CV Bruschi (1994). Recombinators, recombinases and recombination genes of yeast. Current Genetics 25:1-11.
6. Esposito, MS, RM Ramirez, and CV Bruschi (1994). Non-randomly associated forward mutations and mitotic recombination yield yeast diploids homozygous for recessive mutations. Current Genetics 26:302-307.
7. Dora, EG, N Rudin, JR Martell, MS Esposito, and RM Ramirez (1999). RPD3 (REC3) mutations affect mitotic recombination in the yeast Saccharomyces cerevisiae. Current Genetics 35:68-76.
8. Soupene, E, RM Ramirez, SG Kustu (2001). Evidence that fungal MEP proteins mediate diffusion of the uncharged species NH3 across the cytoplasmic membrane. Mol. Cell. Biol. 21:5733-5741.
9. Kiewietdejonge, A, M Pitts, L Cabuhat, C Sherman, W Kladwang, G Miramontes, J Floresvillar, J Chan, and RM Ramirez (2006). Hypersaline stress induces the turnover of phosphatidylcholine and results in the synthesis of the renal osmoprotectant glycerophosphocholine in Saccharomyces cerevisiae. FEMS Yeast Research 6:205-217.