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Plant Signal Behav. Sep 2008; 3(9): 710–713.
PMCID: PMC2634567

Disordered plant LEA proteins as molecular chaperones

Abstract

Plants often respond to abiotic stresses by the increased expression of LEA (late embryogenesis abundant) proteins, so called because they also accompany seed formation. Whereas the cellular function of LEA proteins in mitigating the damage caused by stress is clear, the molecular mechanisms of their action are rather enigmatic. Several models have been developed, based on their putative activities as ion sinks, stabilizers of membrane structure, buffers of hydrate water, antioxidants and/or chaperones. Due to their known structural flexibility, this latter idea has received little experimental attention thus far. Recently, however, it has been suggested that intrinsically disordered proteins (IDPs) may exert chaperone activity by an “entropy transfer” mechanism. In our subsequent study published in the May issue of Plant Physiology, we provided evidence that two group 2 LEA proteins, ERD (early response to dehydration) 10 and 14, are potent molecular chaperones. This observation may have far-reaching implications, as it may explain how LEA proteins of ill-defined structures protect plant cells during dehydration, and it may also lead to the general experimental validation of the entropy transfer model of disordered chaperones.

Key words: abiotic stress, dehydration stress, stress tolerance, late embryogenesis abundant protein, chaperone, disordered protein, unstructured protein

Abiotic Stress and Plant Dehydration

Stressful environmental changes, such as draught, low- or high temperature, freezing or increase in salinity may cause the loss of intracellular water of plants, i.e., dehydration. Their primary line of defense is the increased expression of proteins also known to be expressed under normal conditions in seeds.1,2 These LEA proteins constitute a very divergent family, classified into three loosely defined groups by the presence of certain sequence motifs.3 Group 1 is defined to contain a hydrophilic 20-amino acid motif, members of group 2 have three distinct motifs (Y, S and K), whereas group 3 LEA proteins contain multiple copies of an 11-mer sequence.2,3 Group 2 proteins are only found in plants, they are also termed dehydrins, and are either truly late in embryogenesis (group 2a) or they are not embryogenic and/or associated with cold tolerance (group 2b). Homologues of group 1 and 3 proteins are also found in bacteria and invertebrates.

Whereas it is generally agreed that LEA proteins are associated with abiotic stress tolerance, particularly with cold stress and dehydration, their molecular functions have been rather enigmatic thus far. Based primarily on limited biochemical evidence, several molecular functions have been suggested, such as that of ion sequestration,4 membrane binding and stabilization,5 effect on redox balance as an antioxidant,6 buffering of hydrate water,7 and chaperone activity.8,9 Unequivocal evidence for any of these has been rather meager, and due to the extreme heterogeneity of this group, no useful generalizations have been reached thus far. One likely reason for the slow progress in this area of structure-function characterization of LEA proteins is that they are highly hydrophilic10 and lack well-defined structures,11,12 i.e., traditional structural clues could not be exploited for delineating their function. In all, the field had to wait for a change in the structure-function paradigm.

Intrinsically Disordered Proteins and their Functions

The deviant structural behavior of LEA proteins has only become understandable and tractable with the advent of the concept of protein disorder. A recent surge of reports has made it clear that for many proteins or regions of protein domains the functional state is intrinsically unstructured/disordered (IUPs/IDPs).1315 In structural terms, IDPs can be best described as a structural ensemble of rapidly interconverting alternative conformations. There is conclusive experimental evidence for the disorder of about 500 proteins, collected in the DisProt database,16 whereas bioinformatics predictions suggest that in eukaryotes the phenomenon of disorder is widespread, with 5–15% of proteins being fully disordered, and about 50% of proteins having at least one long disordered region.17,18 Disorder prevails in proteins involved in signal transduction and regulation of transcription, and it also occurs at high levels in proteins involved in disease. Recent evidence also points to that LEA proteins are probably also fully disordered.2,3,5,1012,19,20

Structural disorder confers advantages onto proteins, such as the increased speed of interaction, the combination of specificity with week and reversible binding, and the ability to carry out more than one function.21 The molecular functions of IDPs can be classified into six general categories:14 they either directly stem from disorder (entropic chains), or from transient (display sites, chaperones) or permanent (scavengers, effectors, assemblers) binding of partner molecule(s). Upon binding, IDPs often use short recognition elements22 in a structurally adaptive process termed disorder-to-order transition or induced folding.23

Chaperone Activity of ERD10/14

An important functional category of IDPs is chaperones, as suggested by a range of individual observations and a recent statistical analysis.24 In this, structural disorder was analyzed for a collection of RNA- and protein chaperones from the literature, and it was found that several chaperones are fully disordered (e.g., α-synuclein,25 β-synuclein,26 MAP2 protein,27 and α- and β-casein28), and RNA- and protein chaperones in general contain high levels of disorder (with 40% and 15% of their residues falling into disordered regions of >30 consecutive residues). In addition, disordered regions are directly involved in chaperone function, which has led to the formulation of the “entropy transfer” model of chaperone function.24 The key elements of this mechanism are that disorder in chaperones may play a role in rather non-specific recognition, whereas it may also provide an effect of solubilization and assist local unfolding of misfolded parts of the client protein. Some of these functional elements are similar to “molecular shield” and “space filler” functions of LEA proteins.2

Based on this premise, in our recent paper we have directly addressed whether two of group 2 LEA proteins, ERD 10 and ERD 14 (ERDs) have chaperone activity.20 These two dehydrins accumulate in response to water stress elicited by high salinity, drought and low temperature in A. thaliana, and they are also expressed in rapidly dividing tissues, such as the tips of roots and leaves. By a series of approaches, such as NMR, CD, limited proteolysis, heat-stability and bioinformatics prediction we demonstrated that both ERDs are fully disordered. By a variety of assays we have shown that they are potent chaperones in vitro. They are able to prevent the heat-induced aggregation and/or inactivation of various substrates, such as lysozyme, alcohol dehydrogenase (ADH), firefly luciferase and citrate synthase (CS), with activities commensurable with that of HSP90. ERDs are also able to bind to acidic phospholipid vesicles, without significantly affecting membrane fluidity.

These results show chaperone activities of rather wide substrate specificity, which corroborate and extend earlier observations on some other LEA proteins. Previously, prevention of aggregation of CS and lactate dehydrogenase caused by desiccation and freezing by AavLEA1, a group 3 LEA protein from the anhydrobiotic nematode A. avenae, and Em, a group 1 LEA protein from wheat8 was described. The nematode protein also abrogated desiccation-induced aggregation of the water-soluble proteomes in live mammalian cells, and of polyglutamine and polyalanine expansion proteins associated with neurodegenerative diseases.9

Our observations, however, go beyond these findings, because AavLEA1 and Em did not have “classical” chaperone activity, i.e., they could not prevent the heat-induced aggregation of CS.8 Thus, the chaperone activity of ERDs may suggest critical differences between group 2 and other LEA proteins. In addition to these findings, we have also observed chaperone activity of ERD10 in the renaturation assay of lysozyme (Fig. 1A) and either a positive (ERD 14, Fig. 1B), or negative (ERD 10, not shown) effect of ionic strength on their chaperone activity. These observations raise testable hypotheses in terms of dehydrin functions, in that their ion-sink activity may also be linked with their chaperone activity, and that they may be capable of eliciting an active role in the substrate refolding, not seen before. Whereas these latter observations require experimental verification, they suggest a variety of further experiments both in vitro and in vivo.

Figure 1
Unusual chaperone effects of ERDs. (A) The protein refolding activity of IDPs and LEA proteins have thus far not been demonstrated. Here we show one result, which demonstrates that ERD 10 is able to promote reactivation of lysozyme at 3x molar excess ...

Implications and Outlook

The observed chaperone activity of two dehydrins raises many interesting questions. First, it provides important additional evidence that IDPs in general may serve as potent chaperones. The results on ERDs extend the list of fully disordered chaperones (cf. above), and further work with these proteins may provide support for the entropy transfer model of disordered chaperones.24 Second, the chaperone effect of ERDs provides important insight into the biological action of LEA proteins under the conditions of dehydration stress. As seen above, several molecular activities have been suggested,1,2 of which chaperone activity received little attention thus far.

Third, the results might also add to the issue of establishing the functional differences between different LEA groups,3 and help address various aspects of chaperone action, such as assistance of refolding, and prevention of cold/heat-induced aggregation. Although group 1 and group 3 LEA proteins have been mostly described to be able to prevent aggregation caused by low temperature and deprivation of water, and thus termed as space fillers,8,9 group 2 LEA proteins have been shown by us as “classical” chaperones being able to prevent heat-induced aggregation and inactivation.20

As a final note, it should be pointed out that all the relevant studies cited may significantly advance the functional delineation of the LEA proteins. As pointed out,2 the situation of LEA proteins shows distinct parallels with that of the discovery and early characterization of heat-shock proteins. Their description was also phenomenological at the outset, soon to be discovered to be potent molecular chaperones functioning in protein folding and refolding.29 Recent increased interest in LEA proteins and increased activity in the field may bring about a similar breakthrough in understanding the molecular mechanisms of action of these important proteins.

Acknowledgements

This work was supported by grants OTKA K60694 and NK71582 from the Hungarian Scientific Research Fund, ETT 245/2006 from the Hungarian Ministry of Health, and International Senior Research Fellowship ISRF 067595 from the Wellcome Trust.

Footnotes

Previously published online as a Plant Signaling & Behavior E-publication: http://www.landesbioscience.com/journals/psb/article/6434

References

1. Rorat T. Plant dehydrins—tissue location, structure and function. Cell Mol Biol Lett. 2006;11:536–556. [PubMed]
2. Tunnacliffe A, Wise MJ. The continuing conundrum of the LEA proteins. Naturwissenschaften. 2007;94:791–812. [PubMed]
3. Wise MJ, Tunnacliffe A. POPP the question: what do LEA proteins do? Trends Plant Sci. 2004;9:13–17. [PubMed]
4. Alsheikh MK, Heyen BJ, Randall SK. Ion binding properties of the dehydrin ERD14 are dependent upon phosphorylation. J Biol Chem. 2003;278:40882–40889. [PubMed]
5. Koag MC, Fenton RD, Wilkens S, Close TJ. The binding of maize DHN1 to lipid vesicles. Gain of structure and lipid specificity. Plant Physiol. 2003;131:309–316. [PMC free article] [PubMed]
6. Hara M, Terashima S, Fukaya T, Kuboi T. Enhancement of cold tolerance and inhibition of lipid peroxidation by citrus dehydrin in transgenic tobacco. Planta. 2003;217:290–298. [PubMed]
7. Bokor M, Csizmok V, Kovacs D, Banki P, Friedrich P, Tompa P, Tompa K. NMR relaxation studies on the hydrate layer of intrinsically unstructured proteins. Biophys J. 2005;88:2030–2037. [PMC free article] [PubMed]
8. Goyal K, Walton LJ, Tunnacliffe A. LEA proteins prevent protein aggregation due to water stress. Biochem J. 2005;388:151–157. [PMC free article] [PubMed]
9. Chakrabortee S, Boschetti C, Walton LJ, Sarkar S, Rubinsztein DC, Tunnacliffe A. Hydrophilic protein associated with desiccation tolerance exhibits broad protein stabilization function. Proc Natl Acad Sci USA. 2007;104:18073–18078. [PMC free article] [PubMed]
10. Garay-Arroyo A, Colmenero-Flores JM, Garciarrubio A, Covarrubias AA. Highly hydrophilic proteins in prokaryotes and eukaryotes are common during conditions of water deficit. J Biol Chem. 2000;275:5668–5674. [PubMed]
11. Goyal K, Tisi L, Basran A, Browne J, Burnell A, Zurdo J, Tunnacliffe A. Transition from natively unfolded to folded state induced by desiccation in an anhydrobiotic nematode protein. J Biol Chem. 2003;278:12977–12984. [PubMed]
12. Mouillon JM, Gustafsson P, Harryson P. Structural investigation of disordered stress proteins. Comparison of full-length dehydrins with isolated peptides of their conserved segments. Plant Physiol. 2006;141:638–650. [PMC free article] [PubMed]
13. Dyson HJ, Wright PE. Intrinsically unstructured proteins and their functions. Nat Rev Mol Cell Biol. 2005;6:197–208. [PubMed]
14. Tompa P. The interplay between structure and function in intrinsically unstructured proteins. FEBS Lett. 2005;579:3346–3354. [PubMed]
15. Uversky VN, Oldfield CJ, Dunker AK. Showing your ID: intrinsic disorder as an ID for recognition, regulation and cell signaling. J Mol Recogn. 2005;18:343–384. [PubMed]
16. Sickmeier M, Hamilton JA, LeGall T, Vacic V, Cortese MS, Tantos A, Szabo B, Tompa P, Chen J, Uversky VN, Obradovic Z, Dunker AK. DisProt: the Database of Disordered Proteins. Nucl Acids Res. 2007;35:786–793. [PMC free article] [PubMed]
17. Dunker AK, Obradovic Z, Romero P, Garner EC, Brown CJ. Intrinsic protein disorder in complete genomes. Genome Inform Ser Workshop Genome Inform. 2000;11:161–171. [PubMed]
18. Ward JJ, Sodhi JS, McGuffin LJ, Buxton BF, Jones DT. Prediction and functional analysis of native disorder in proteins from the three kingdoms of life. J Mol Biol. 2004;337:635–645. [PubMed]
19. Irar S, Oliveira E, Pages M, Goday A. Towards the identification of late-embryogenic-abundant phosphoproteome in Arabidopsis by 2-DE and MS. Proteomics. 2006;6:175–185. [PubMed]
20. Kovacs D, Kalmar E, Torok Z, Tompa P. Chaperone activity of ERD10 and ERD14, two disordered stress-related plant proteins. Plant Physiol. 2008;147:381–390. [PMC free article] [PubMed]
21. Tompa P, Szasz C, Buday L. Structural disorder throws new light on moonlighting. Trends Biochem Sci. 2005;30:484–489. [PubMed]
22. Fuxreiter M, Simon I, Friedrich P, Tompa P. Preformed structural elements feature in partner recognition by intrinsically unstructured proteins. J Mol Biol. 2004;338:1015–1026. [PubMed]
23. Dyson HJ, Wright PE. Coupling of folding and binding for unstructured proteins. Curr Opin Struct Biol. 2002;12:54–60. [PubMed]
24. Tompa P, Csermely P. The role of structural disorder in the function of RNA and protein chaperones. FASEB J. 2004;18:1169–1175. [PubMed]
25. Ahn M, Kim S, Kang M, Ryu Y, Kim TD. Chaperone-like activities of alpha-synuclein: alpha-synuclein assists enzyme activities of esterases. Biochem Biophys Res Commun. 2006;346:1142–1149. [PubMed]
26. Bertoncini CW, Rasia RM, Lamberto GR, Binolfi A, Zweckstetter M, Griesinger C, Fernandez CO. Structural characterization of the intrinsically unfolded protein beta-synuclein, a natural negative regulator of alpha-synuclein aggregation. J Mol Biol. 2007;372:708–722. [PubMed]
27. Sarkar T, Mitra G, Gupta S, Manna T, Poddar A, Panda D, Das KP, Bhattacharyya B. MAP2 prevents protein aggregation and facilitates reactivation of unfolded enzymes. Eur J Biochem. 2004;271:1488–1496. [PubMed]
28. Thorn DC, Meehan S, Sunde M, Rekas A, Gras SL, Macphee CE, Dobson CM, Wilson MR, Carver JA. Amyloid fibril formation by bovine milk kappa-casein and its inhibition by the molecular chaperones alpha(S)- and beta-casein. Biochemistry. 2005;44:17027–17036. [PubMed]
29. Ellis RJ, Vandervies SM. Molecular chaperones. Annu Rev Biochem. 1991;60:321–347. [PubMed]

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