LEA Proteins – Structural and Functional Diversity

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Hydrophilic proteins, which form an important part of the plant response to water shortage, were first characterized in cotton during the late stages of embryogenesis, and termed LEA proteins (Dure et al., 1989). On the basis of their predicted structure, accumulation in response to drought, salt and cold stress, and in vitro activity, LEA proteins are considered to participate in protecting cellular components from dehydration (Reyes et al., 2005). They are characterized by relatively high glycine content and hydrophilicity and low secondary structure content. Generally they remain soluble upon boiling, a property that is used as an initial step in their purification.

Several nomenclature systems have been reported to classify LEA proteins. Initially, they were classified according to their molecular weight and thus named D7, D11, D19, D29, D34, D73, D95, D113 (Hughes and Galau, 1991). In response to the growing number of newly identified LEA proteins, a new classification system, based on protein amino acid composition and sequence motifs, was introduced (Battaglia et al., 2008). LEA proteins have been classified into six groups by different nomenclature systems on the basis of conserved sequence motifs.

Group 1 LEA proteins are only found in plants. They contain a hydrophilic 20-mer motif TRKEQ[L/M]G[T/E]EGY[Q/K]EMGRKGG[L/E] (Baker et al., 1995) and exhibit a high proportion of random coil configuration in aqueous solution (Soulages et al., 2002). Group 1 LEA proteins accumulate preferentially during embryo development, and some of them are responsive to abscisic acid (ABA) and to water-limiting conditions, mainly in embryos and in vegetative tissues of young seedlings (Vicient et al., 2000).

Dehydrins belong to group 2 of the LEA proteins. All dehydrins have a conserved, lysine-rich 15-amino acid domain, EKKGIMDKIKEKLPG, named the K-segment (Close, 1997) and usually present near the C-terminus. Other typical dehydrin features are a track of Ser residues (the S-segment), a consensus motif, T/VDEYGNP (the Y-segment) located near the N-terminus, and less conserved regions, usually rich in polar amino acids (the Φ-segments). Dehydrins do not display well-defined secondary structure. The expression pattern of group 2 LEA genes is frequently associated with higher tolerance of crop plants to abiotic stresses such as cold (Zhu et al., 2000) and drought (Lopez et al., 2003; Suprunova et al., 2004, Vaseva et al., 2010).

Group 3 LEA proteins are characterized by a repeated 11-mer amino acid motif whose consensus sequence has been broadly defined as ΦΦE/QXΦKE/QKΦXE/D/Q, where Φ represents a hydrophobic residue (Dure, 1993). Proteins homologous to the group 3 LEAs have been discovered in organisms other than plants, including nematodes and prokaryotes. They are natively unfolded in solution, but appear to be more structured when the water content of the environment is lower (Tolleter et al., 2007).

Group 4 LEA proteins accumulate in mature seeds, and are also induced in leaves by plant hormones, abiotic elicitors and environmental stresses (Dure et al., 1989). The proteins of this family are conserved in their N-terminal portion, which is 70 to 80 residues long and is predicted to form amphipathic α-helices, while the less conserved C-terminal portion differs in size (Dure, 1993). A characteristic motif in the proteins in this group is motif 1, located at the N-terminal region with the consensus sequence: AQEKAEKMTA[R/H] DPXKEMAHERK[E/K][A/E][K/R]. LEA D113 gene from G. hirsutum L. is a typical representative of this group (Luo et al., 2008).

LEA 5 proteins (hydrophobic or atypical LEA proteins) contain a significantly higher proportion of hydrophobic residues and include non-homologous proteins. These proteins are not soluble after boiling, which suggests that they adopt a globular conformation (Singh et al., 2005). LEA 5 transcripts accumulate during the late stage of seed development and in response to stress conditions, such as drought, UV light, salinity, cold, and wounding (Park et al., 2003; Kim et al., 2005).

Proteins belonging to LEA 6 family have been identified in a variety of vascular plant species. Their molecular mass is relatively low (7–14 kDa) and they exhibit high sequence conservation. Four motifs distinguish this group, two of which are highly conserved (sequence LEDYK present in motif 1 and the Pro and Thr residues in motif 2). In general, these proteins are highly hydrophilic, lack Cys and Trp residues, and do not coagulate upon exposure to high temperatures. PvLEA18 protein from bean (Phaseolus vulgaris) was the first protein described from the LEA 6 group (Colmenero-Flores et al., 1997). It is present at high levels in dry seeds and pollen grains and responds to water deficit and ABA treatment. There is evidence that the molecular targets of these proteins differ from those of other LEA proteins (Reyes et al., 2005).