Anhydrobiotic organisms are widespread in the plant kingdom, in fungi such as Saccharomyces cerevisiae and in small animals such as rotifers, nematodes and tardigrades. Anhydrobiosis (desiccation tolerance) is considered as a state of suspended metabolism (stasis) induced by the removal of cell water (Crowe et al. 1992). To understand the processes behind desiccation stress resistance of anhydrobiotic organisms, we must address controversial issues such as cell age, longevity, the structural and biochemical properties of anhydrous cytoplasm and metabolic stasis (Potts 2001).
The present study describes a genome-wide screen in S. cerevisiae to identify the genes that modify cell mortality after dehydration stress (Singh et al. 2005). Among the genes characterized as essential for overcoming the cell-drying/rehydration process, six belong to the group of very hydrophilic proteins known as hydrophilins. This group of proteins is defined by common physicochemical characteristics: (i) a Gly content >6%, (ii) the presence of small amino acids such as Ala and Ser and (iii) a high hydrophilicity index of >1·0 (Battaglia et al. 2008). Although the functional role of hydrophilins remains speculative, the fact that the transcripts of most genes encoding hydrophilins are induced in response to osmotic stress suggests that they represent an extensive adaptation to water deficit (Posas et al. 2000). The genome of S. cerevisiae contains 12 genes encoding proteins with the characteristics of hydrophilins. The ectopic expression of some plant hydrophilins (late embryogenesis abundant, LEA proteins) in yeast confers tolerance to water-deficit conditions (Zhang et al. 2000). On the other hand, hydrophilins protect the activities of both malate dehydrogenase and lactate dehydrogenase, which were measured in vitro dehydration tests in the presence or absence of hydrophilins from plants, bacteria and yeast. Under similar conditions, trehalose was required in a 105-fold molar excess over hydrophilins to confer the same level of protection, suggesting that they provide protection by means of different mechanisms (Reyes et al. 2005). A similar study was conducted with two hydrophilins from nematode and wheat, which were found to prevent the enzyme aggregation under desiccation and freezing stress (Goyal et al. 2005). Hydrophilins’ properties include their roles as antioxidants and as membrane and protein stabilizers during water stress, either by direct interaction or by acting as a molecular shield (Tunnacliffe and Wise 2007).
With recent advances in tissue engineering, cell transplantation and genetic technology, successful long-term storage of living cells is of critical importance. Even common requirements such as the storage of blood cells in blood banks are still a major problem. The complex regulatory network and the often contradictory results obtained with high eukaryotic cells make the application of an easier model system worth striving for. A number of advantages have made yeast cells the model of choice for anhydrobiotic engineering, including the ease of growth and modification, well-characterized cell physiology, genetics and biochemistry. Yeast promises to provide a better understanding of desiccation-tolerant genetics for potential applications in biomedicine, plant biotechnology, and beverage and bio-ethanol technology.
In this study, we performed a genetic screen of S. cerevisiae’s deletion library for mutants sensitive to dehydration stress, with which we aimed to discover cell dehydration–tolerant genes. For one of these identified genes, SIP18, we have characterized the effects of overexpression in the corresponding deletion strain, which is sensitive to stress imposition. We also show that Sip18p has an antioxidant capacity and is imported into the nucleus as a response to osmotic change during the dehydration process.