Highly hydrophilic proteins in prokaryotes and eukaryotes are common during conditions of water deficit

The late embryogenesis abundant (LEA) proteins are plant proteins that are synthesized at the onset of desiccation in maturing seeds and in vegetative organs exposed to water deficit. Here, we show that most LEA proteins are comprised in a more widespread group, which we call "hydrophilins," The def ining characteristics of hydrophilins are high glycine content (>6%) and a high hydrophilicity index (>1.0). By data base searching, we show that this criterion selectively differentiates most known LEA proteins as well as ad ditional proteins from different taxons. We found that within the genomes o f Escherichia coli and Saccharomyces cerevisiae, only 5 and 12 proteins, re spectively, meet our criterion. Despite their deceivingly loose definition, hydrophilins usually represent <0.2% of the proteins of a genome. Addition ally, we demonstrate that the criterion that defines hydrophilins seems to be an excellent predictor of responsiveness to hyperosmosis since most of t he genes encoding these proteins in E. coli and S. cerevisiae are induced b y osmotic stress. Evidence for the participation of one of the E. coli hydr ophilins in the adaptive response to hyperosmotic conditions is presented. Apparently, hydrophilins represent analogous adaptations to a common proble m in such diverse taxons as prokaryotes and eukaryotes.