Update on LEA Proteins and Other Hydrophilins The Enigmatic LEA Proteins and Other Hydrophilins

Water limitation affects all types of organisms at some stage during their life cycle; therefore, many strategies have been selected through evolution to cope with water deficit, including changes in enzyme activities and in gene expression, among others. In plants, a group of very hydrophilic proteins, known as LATE EMBRYOGENESIS ABUNDANT (LEA) proteins, accumulate to high levels during the last stage of seed maturation (when acquisition of desiccation tolerance occurs in the embryo) and during water deficit in vegetative organs, suggesting a protective role during water limitation (Dure, 1993b; Bray, 1997; GarayArroyo et al., 2000; Hoekstra et al., 2001). LEA proteins have been grouped into various families on the basis of sequence similarity (see below; Dure et al., 1989; Ingram and Bartels, 1996; ColmeneroFlores et al., 1999; Cuming, 1999). Although significant similarity has not been detected between the members of the different families, a unifying and outstanding feature of most of them is their high hydrophilicity and high content of Gly and small amino acids like Ala and Ser (Baker et al., 1988; Dure, 1993b). Most LEA proteins are part of a more widespread group of proteins called ‘‘hydrophilins.’’ The physicochemical characteristics that define this set of proteins are a Gly content greater than 6% and a hydrophilicity index greater than 1. By database searching, it was shown that this criterion selects most LEA proteins, as well as additional proteins from different taxa (GarayArroyo et al., 2000). The genomes of Escherichia coli and Saccharomyces cerevisiae contain five and 12 genes, respectively, encoding proteins with the characteristics of hydrophilins. The fact that the transcripts of all these genes accumulate in response to osmotic stress suggests that hydrophilins represent a widespread adaptation to water deficit (Garay-Arroyo et al., 2000; Posas et al., 2000; Yale and Bohnert, 2001; Saccharomyces Genome Database project, http://www. yeastgenome.org). Remarkably, now it is known that these proteins are distributed across archeal, eubacterial, and eukaryotic domains, as will be described later in this review. Although the functional role of hydrophilins remains speculative, there is evidence supporting their participation in acclimation and/or in the adaptive response to stress. Ectopic expression of some plant hydrophilins (LEA proteins) in plants and yeast confers tolerance to water-deficit conditions (Imai et al., 1996; Xu et al., 1996; Swire-Clark and Marcotte, 1999; Zhang et al., 2000), and their presence has been associated with chilling tolerance (Danyluk et al., 1994, 1998; Ismail et al., 1999a, 1999b; Puhakainen et al., 2004a; Nakayama et al., 2007). An osmosensitive phenotype is caused by the deletion of the RMF hydrophilin gene in E. coli (Garay-Arroyo et al., 2000) and by the absence of a LEA protein in the moss Physcomitrella patens (Saavedra et al., 2006). To gain further insight into their function, in vitro assays have been established similar to those used to test the role of other protective molecules such as chaperones. Examples of these are cryoprotection assays, in which the protective role of LEA proteins is tested using freeze-labile enzymes (Lin and Thomashow, 1992). Dehydration assays, in which the activities of malate dehydrogenase and lactate dehydrogenase (LDH) were measured in the presence or absence of a putative protecting protein, showed that hydrophilins from plants, bacteria, and yeast were able to protect their enzymatic activity. Under similar conditions, trehalose was required in a 10-fold molar excess over hydrophilins to confer the same protective level to LDH, suggesting that they confer protection via different mechanisms (Reyes et al., 2005). While hydrophilin research in different organisms has provided us with significant advances to understand their biological properties, we are still far from a complete understanding of their biological functions and activities. Here, we review the structural and functional characteristics of hydrophilins to provide a reference platform to understand their role during the adaptive response to water deficit in plants and other organisms and to generate new ideas to elucidate their function. 1 This work was supported by Consejo Nacional de Ciencia y Tecnologı́a-Mexico (grant nos. 40603–Q and 50485–Q). M.B. and Y.O.-C. were supported by scholarships from Dirección General de Estudios de Posgrado-UNAM and Consejo Nacional de Ciencia y Tecnologı́a, respectively. 2 These authors contributed equally to the article. * Corresponding author; e-mail [email protected]. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Alejandra A. Covarrubias ([email protected]). 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