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Abstract:

In multicellular organisms the importance of physiological regulation of water permeability in specific tissues is well understood. The reason for the presence of aquaporins in many microorganisms is much more obscure. Their absence in many others hints for another explanation than involvement in turgor regulation or osmoadaptation, processes which are important for all microorganisms. We have discovered that aquaporin expression correlates with freeze tolerance in a freeze-tolerant yeast strain (1). Subsequent work revealed that overexpression of aquaporin genes in yeast and in all other microorganisms investigated enhanced freeze tolerance and that deletion of aquaporin genes reduced freeze tolerance. These findings support a role for plasma membrane water transport activity in determination of freeze tolerance. We have suggested that rapid osmotically driven effux of water during the freezing process reduces intracellular ice crystal formation and therefore results in less cell damage. The discovery that aquaporins enhance cellular tolerance against rapid freezing suggests a possible ecological relevance. Aerial dispersal is of key importance for the survival of many micro-organisms in nature. Because of the rapid temperature drop and strong air turbulence with increasing altitude, dispersal of micro-organisms in air over long distances is likely associated with rapid and frequent freeze/thaw cycles. Other conditions of rapid freezing on a large scale of micro-organisms in nature can also be identified. Freeze-stress survival is unlikely to be the only function of microbial aquaporins. However, we suggest that in the search for a function of microbial aquaporins, the strong effect of low temperature on membrane water permeability should be given more attention. References Tanghe A., P. Van Dijck, F. Dumortier, A. Teunissen, S. Hohmann and J.M. Thevelein (2002) Aquaporin expression correlates with freeze tolerance in yeast and overexpression improves freeze tolerance in industrial yeast. Applied and Environmental Microbiology 68, 5981-5989 Tanghe A., G. Kayingo, B.A. Prior, J.M. Thevelein and P. Van Dijck (2005) Heterologous aquaporin (AQY2-1) expression strongly enhances freeze tolerance of Schizosaccharomyces pombe. Journal of Molecular Microbiology and Biotechnology 9, 52-56 Tanghe A., J.M. Carbrey, P. Agre, J.M. Thevelein and P. Van Dijck (2005) Aquaporin expression and freeze tolerance in Candida albicans. Applied and Environmental Microbiology 71, 6434- 6437 Tanghe A., P. Van Dijck and J.M. Thevelein (2006) Why do micro-organisms have aquaporins ? Trends in Microbiology 14, 78-85