Temperature-induced extended helix/random coil transitions in a group 1 late embryogenesis-abundant protein from soybean

Group 1 late embryogenesis-abundant (LEA) proteins are a subset of hydrophilins that are postulated to play important roles in protecting plant macromolecules from damage during freezing, desiccation, or osmotic stress. To better understand the putative functional roles of group 1 LEA proteins, we analyzed the structure of a group 1 LEA protein from soybean (Glycine max). Differential scanning calorimetry of the purified, recombinant protein demonstrated that the protein assumed a largely unstructured state in solution. In the presence of trifluoroethanol (50% [w/v]), the protein acquired a 30% alpha-helical content, indicating that the polypeptide is highly restricted to adopt alpha-helical structures. In the presence of sodium dodecyl sulfate (1% [w/v]), 8% of the polypeptide chain adopted an alpha-helical structure. However, incubation with phospholipids showed no effect on the protein structure. Ultraviolet absorption and circular dichroism spectroscopy revealed that the protein existed in equilibrium between two conformational states. Ultraviolet absorption spectroscopy studies also showed that the protein became more hydrated upon heating. Furthermore, circular dichroism spectral measurements indicated that a minimum of 14% of amino acid residues existed in a solvent-exposed, left-handed extended helical or poly (L-proline)-type (PII) conformation at 20 degrees C with the remainder of the protein being unstructured. The content of PII-like structure increased as temperature was lowered. We hypothesize that by favoring the adoption of PII structure, instead of the formation of alpha-helical or beta-sheet structures, group 1 LEA proteins retain a high content of surface area available for interaction with the solvent. This feature could constitute the basis of a potential role of LEA proteins in preventing freezing, desiccation, or osmotic stress damage.     

Two New Group 3 LEA Genes of Wheat and Their Functional Analysis in Yeast

The group 3 late embryogenesis abundant （LEA） proteins are thought to protect cells from stresses associated with dehydration during periods of water deficit. To investigate the functions of different members of the group 3 LEA genes, we isolated and characterized two new group 3 LEA genes, namely TaLEA2 and TaLEA3, from wheat （Triticum aestivum L.） and introduced TaLEA2 and TaLEA3 into Saccharmyces cerevisiae to examine the effect of these genes on yeast cell tolerance to osmotic, salt, and cold stresses. The TaLEA2 gene encoded a protein of 211 amino acids and possessed five repeats of 11-mer amino acid motifs. The TaLEA3 gene encoded a polypeptide of 211 amino acids with nine repeated units. Overexpression of TaLEA2 and TaLEA3 improved stress tolerance in transgenic yeast cells when cultured in medium containing sorbitol, salt and-20℃ freezing treatments respectively. However, the yeast transformants with TaLEA2 seemed to be more tolerant to hyperosmotic and freezing stress than transformants with TaLEA3. This implies that a close relationship exists between function and the number of repeats of the 11- mer amino acid motif in the group 3 LEA protein.     

Functional characterization of selected LEA proteins from Arabidopsis thaliana in yeast and in vitro

Main conclusion

Expression of eight LEA genes enhanced desiccation tolerance in yeast, including two LEA_2 genes encoding atypical, stably folded proteins. The recombinant proteins showed enzyme, but not membrane protection during drying. To screen for possible functions of late embryogenesis abundant (LEA) proteins in cellular stress tolerance, 15 candidate genes from six Arabidopsis thaliana LEA protein families were expressed in Saccharomyces cerevisiae as a genetically amenable eukaryotic model organism. Desiccation stress experiments showed that eight of the 15 LEA proteins significantly enhanced yeast survival. While none of the proteins belonging to the LEA_1, LEA_5 or AtM families provided protection to yeast cells, two of three LEA_2 proteins, all three LEA_4 proteins and three of four dehydrins were effective. However, no significantly enhanced tolerance toward freezing, salt, osmotic or oxidative stress was observed. While most LEA proteins are highly hydrophilic and intrinsically disordered, LEA_2 proteins are “atypical”, since they are more hydrophobic and possess a stable folded structure in solution. Because nothing was known about the functional properties of LEA_2 proteins, we expressed the three Arabidopsis proteins LEA1, LEA26 and LEA27 in Escherichia coli. The bacteria expressed all three proteins in inclusion bodies from which they could be purified and refolded. Correct folding was ascertained by Fourier transform Infrared (FTIR) spectroscopy. None of the proteins was able to stabilize liposomes during freezing or drying, but they were all able to protect the enzyme lactate dehydrogenase (LDH) from inactivation during freezing. Significantly, only LEA1 and LEA27, which also protected yeast cells during drying, were able to stabilize LDH during desiccation and subsequent rehydration.     

A predicted N-terminal helical domain of a Group 1 LEA protein is required for protection of enzyme activity from drying.

Late embryogenesis abundant (LEA) proteins have been repeatedly implicated in the acquisition of desiccation tolerance in angiosperm seed embryos. However, the mechanism(s) by which protection occurs is not well understood. While the Group 1 LEA proteins are predicted to be largely unordered in solution, there is strong evidence that upon drying these proteins undergo a structural transition that leads to an increase in alpha-helical content. Several studies also suggest there is a direct interaction between Group 1 LEA proteins and other molecules in the cytoplasm that may be critical for the establishment of desiccation tolerance during embryo maturation. We have produced a recombinant Group 1 LEA protein and show that it is capable of protecting the enzyme lactate dehydrogenase from the deleterious effects of drying. We have also evaluated the ability of various altered recombinant Group 1 LEA proteins to protect in the same assay. Our results suggest that the highly conserved 20 amino acid Group 1 LEA signature motif is not required for protection in our in vitro assay. However, introduction of two juxtaposed proline residues into an N-terminal helical domain predicted to exist in the hydrated structure significantly compromises the ability of the recombinant protein to provide protection from drying. These results suggest that the N-terminal domain of Group 1 LEA proteins may be important for proper folding during dehydration.     

Polyproline II structure is critical for the enzyme protective function of soybean Em (LEA1) conserved domains

Group 1 late embryogenesis-abundant (LEA1) proteins protect enzyme activity from dehydration and are structurally conserved with three different 20 amino acid motifs in the N-terminal, middle and C-terminal domains. Three soybean Em (LEA1) domain peptides (Em-N, Em-2M and Em-C) covering these respective motifs were constructed and had differential protective ability on lactate dehydrogenase against freeze–thaw: Em-C > Em-2M > Em-N. CD spectroscopy revealed that Em-2M and Em-C contained both polyproline II (PII) helical structure and α-helix, while Em-N had a high potential to form α-helix but did not contain PII structure. The PII helical structure between the third and fifth glycine in the middle motif was shown, through site mutation, to be critical for the enzyme protective function of soybean Em (LEA1) conserved domain under freezing stress.     

The effect of salt and temperature on the conformational changes of P1LEA‐22, a repeat unit of plant Late Embryogenesis Abundant proteins

The effect of choline chloride on the conformational dynamics of the 11‐mer repeat unit P1LEA‐22 of group 3 Late Embryogenesis Abundant (G3LEA) proteins was studied. Circular dichroism data of aqueous solutions of P1LEA‐22 revealed that the peptide favors a polyproline II (PPII) helix structure at low temperature, with increasing temperature promoting a gain of unstructured conformations. Furthermore, increases in sample FeCl₃ or choline chloride concentrations causes a gain in PPII helical structure at low temperature. The potential role of PPII structure in intrinsically disordered and G3LEA proteins is discussed, including its ability to easily access other secondary structural conformations such as α‐helix and β‐sheet, which have been observed for dehydrated G3LEA proteins. The observed effect of FeCl₃ and choline chloride salts on P1LEA‐22 suggests favorable cation interactions with the PPII helix, supporting ion sequestration as a G3LEA protein function. As choline chloride is suggested to improve salt tolerance and protect cell membrane in plants at low temperature, our results support adoption of the PPII structure as a possible damage‐preventing measure of Late Embryogenesis Abundant proteins.     

Group 1 LEA proteins contribute to the desiccation and freeze tolerance of Artemia franciscana embryos during diapause

Water loss either by desiccation or freezing causes multiple forms of cellular damage. The encysted embryos (cysts) of the crustacean Artemia franciscana have several molecular mechanisms to enable anhydrobiosis—life without water—during diapause. To better understand how cysts survive reduced hydration, group 1 late embryogenesis abundant (LEA) proteins, hydrophilic unstructured proteins that accumulate in the stress-tolerant cysts of A. franciscana, were knocked down using RNA interference (RNAi). Embryos lacking group 1 LEA proteins showed significantly lower survival than control embryos after desiccation and freezing, or freezing alone, demonstrating a role for group 1 LEA proteins in A. franciscana tolerance of low water conditions. In contrast, regardless of group 1 LEA protein presence, cysts responded similarly to hydrogen peroxide (H₂O₂) exposure, indicating little to no function for these proteins in diapause termination. This is the first in vivo study of group 1 LEA proteins in an animal and it contributes to the fundamental understanding of these proteins. Knowing how LEA proteins protect A. franciscana cysts from desiccation and freezing may have applied significance in aquaculture, where Artemia is an important feed source, and in the cryopreservation of cells for therapeutic applications.     

第三组胚胎晚期丰富蛋白lea3基因研究概述

晚期胚胎富集蛋白(late embryogenesis abundant protein,LEA蛋白)是在高等植物胚胎发育晚期大量积累的一类蛋白,根据其结构特点LEA蛋白一般分为6组,其中第3组LEA蛋白(LEA3)含有11个氨基酸串联重复的基元序列,可以形成α-螺旋结构,能在干旱胁迫的环境中保护生物大分子,减轻水份胁迫对植物造成的伤害,与植物抗逆性密切相关。该文就lea3基因及其蛋白的结构、功能、基因表达和应用等进行简要的综述,并对lea3基因及其蛋白今后的研究方向和应用前景进行了展望。     

Polyproline II helical structure in protein unfolded states: lysine peptides revisited

The left-handed polyproline II (PPII) helix gives rise to a circular dichroism spectrum that is remarkably similar to that of unfolded proteins. This similarity has been used as the basis for the hypothesis that unfolded proteins possess considerable PPII helical content. It has long been known that homopolymers of lysine adopt the PPII helical conformation at neutral pH, presumably a result of electrostatic repulsion between side chains. It is shown here that a seven-residue lysine peptide also adopts the PPII conformation. In contrast with homopolymers of lysine, this short peptide is shown to retain PPII helical character under conditions in which side-chain charges are heavily screened or even neutralized. The most plausible explanation for these observations is that the peptide backbone favors the PPII conformation to maximize favorable interactions with solvent. These data are evidence that unfolded proteins do indeed possess PPII content, indicating that the ensemble of unfolded states is significantly smaller than is commonly assumed.     

Dehydration-specific induction of hydrophilic protein genes in the anhydrobiotic nematode Aphelenchus avenae

Some organisms can survive exposure to extreme desiccation by entering a state of suspended animation known as anhydrobiosis. The free-living nematode Aphelenchus avenae can be induced to enter the anhydrobiotic state by exposure to a moderate reduction in relative humidity. During this preconditioning period, the nematode accumulates large amounts of the disaccharide trehalose, which is thought to be necessary, but not sufficient, for successful anhydrobiosis. To identify other adaptations that are required for anhydrobiosis, we developed a novel SL1-based mRNA differential display technique to clone genes that are upregulated by dehydration in A. avenae. Three such genes, Aav-lea-1, Aav-ahn-1, and Aav-glx-1, encode, respectively, a late embryogenesis abundant (LEA) group 3 protein, a novel protein that we named anhydrin, and the antioxidant enzyme glutaredoxin. Strikingly, the predicted LEA and anhydrin proteins are highly hydrophilic and lack significant secondary structure in the hydrated state. The dehydration-induced upregulation of Aav-lea-1 and Aav-ahn-1 was confirmed by Northern hybridization and quantitative PCR experiments. Both genes were also upregulated by an osmotic upshift, but not by cold, heat, or oxidative stress. Experiments to investigate the relationship between mRNA levels and protein expression for these genes are in progress. LEA proteins occur commonly in plants, accumulating during seed maturation and desiccation stress; the presence of a gene encoding an LEA protein in an anhydrobiotic nematode suggests that some mechanisms of coping with water loss are conserved between plants and animals.     

Chirality responsive helical poly(phenylacetylene) bearing L-proline pendants

A novel, optically active, cis-transoidal poly(phenylacetylene) bearing an L-proline residue as the pendant group (poly-1) was prepared by the polymerization of the corresponding monomer using a rhodium catalyst in water, and its chiroptical property was investigated using circular dichroism spectroscopy. Poly-1 showed intense Cotton effects in the UV-visible region of the polymer backbone in water, resulting from the prevailing one-handed helical conformation induced by the covalent-bonded chiral L-proline pendants and exhibited a unique helix-sense inversion in response to external, achiral, and chiral stimuli, such as the solvent and interactions with chiral small molecules. We found that poly-1 could enantioselectively trap 1,1'-2-binaphthol within its hydrophobic helical cavity inside the polymer in aqueous media and underwent an inversion of its helical sense in the presence of one of the enantiomers. The effect of the optical purity of 1,1'-2-binaphthol on the chiroptical properties of poly-1 was also investigated.     

The intrinsic helical propensities of the helical fragments in prion protein under neutral and low pH conditions: a replica exchange molecular dynamics study

Replica exchange molecular dynamics simulations in neutral and acidic aqueous solutions were employed to study the intrinsic helical propensities of three helices in both Syrian hamster (syPrP) and human (huPrP) prion proteins. The helical propensities of syPrP HA and huPrP HA are very high under both pH conditions, which implies that HA is barely involved in the helix-to-β transition. The SyPrP HB chain has a strong tendency to adopt an extended conformation, which is possibly involved in the mechanism of infectious prion diseases in Syrian hamster. HuPrP HC has more of a preference for the extended conformation than huPrP HA and huPrP HB do, which leads to the conjecture that it is more likely to be the source of β-rich structure for human prion protein. We also noticed that the presence of salt bridges is not correlated with helical propensity, indicating that salt bridges do not stabilize helices.     

Conformational and biological properties of partial sequences of salmon calcitonin

According to the Chou-Fasman rules for predicting the secondary structures of proteins, the 12-20 portion of salmon calcitonin should adopt an alpha helical conformation. These residues would form an amphipathic helix and contribute to the solubilization of certain phospholipids by the peptide. Circular dichroism was used to assess the extent that peptide segments of salmon calcitonin fold into structures of higher helical content in the presence of dimyristoylphosphatidylglycerol, lysolecithin or sodium dodecyl sulfate. All of the segments studied are carboxyl terminal amides as is the native, intact, salmon calcitonin. Salmon calcitonin segments 11-23 or 12-23 form no more helical structure in the presence of lipids or detergents than does a segment comprising the hydrophilic carboxyl terminal residues 22-32 which is not predicted to adopt a helical conformation. Even a larger segment containing residues 12-32 does not exhibit a great increase in helical content in the presence of lipids or detergents, and it causes only a small broadening of the phase transition of dimyristoylphosphatidylglycerol. In contrast, a preparation with an equivalent molar ratio of dimyristoylphosphatidylglycerol to the salmon calcitonin segment 1-23 exhibits a very marked broadening of the phase transition, similar to what is found with the 32 amino acid native hormone. This amino terminal segment also adopts a conformation of higher helical content than even the intact hormone. This 1-23 segment is the only one studied that showed significant interaction with lipids, and it is also the only one which exhibited any hypocalcemic activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Wheat Dehydrin DHN-5 Exerts a Heat-Protective Effect on β-Glucosidase and Glucose Oxidase Activities

Group-2 late embryogenesis abundant (LEA) proteins, also known as dehydrins, are claimed to stabilize macromolecules against damage caused by freezing, dehydration, ionic or osmotic stresses. However, their precise function remains unknown. Here, we investigated the effect of wheat dehydrin (DHN-5) protein on the activity and thermostability of two distinct enzymes, β-glucosidase (bglG) and glucose oxidase/peroxidase (GOD/POD) in vitro. The purified DHN-5 protein had the capacity to preserve and stabilize the activity of bglG subjected to heat treatment. In addition, DHN-5 stabilized oxidizing enzymes, as it improved reliability in measuring glucose concentrations with a glucose oxidase/peroxidase (GOD/POD) kit while the temperature increased from 37 to 70 °C. All together the data presented provide evidence that DHN-5 is a dehydrin able to preserve enzyme activities in vitro from adverse effects induced by heating.     

Isolation and characterization of a D-7 LEA protein from pollen that stabilizes glasses in vitro

A heat-soluble protein present in substantial quantities in Typha latifolia pollen was purified to homogeneity. The protein was subjected to cyanogen bromide cleavage, and the peptides produced were separated by HPLC chromatography and sequenced. The two sequences determined were found to be related to the putative D76 LEA protein from Brassica napus seeds and one of them to the D-7 LEA protein from upland cotton. This suggests the pollen protein to be a member of the LEA group III family of proteins. The secondary structure of the protein in solution and in the dry state was investigated using Fourier transform IR spectroscopy. Whereas the protein in solution was highly unordered, being largely in a random coil conformation, the conformation was largely alpha-helical after fast drying. Slow drying reversibly led to both alpha-helical and intermolecular extended beta-sheet structures. When dried in the presence of sucrose, the protein adopted alpha-helical conformation, irrespective of drying rate. The effect of the protein on the stability of sucrose glasses was also investigated. The dehydrated mixture of sucrose and the LEA protein had higher glass transition temperatures and average strength of hydrogen bonding than dehydrated sucrose alone. We suggest that LEA proteins may play a role together with sugars in the formation of a tight hydrogen bonding network in the dehydrating cytoplasm, thus conferring long-term stability.     

Late Embryogenesis Abundant (LEA) proteins confer water stress tolerance to mammalian somatic cells

Late Embryogenesis Abundant (LEA) proteins are commonly found in plants and other organisms capable of undergoing severe and reversible dehydration, a phenomenon termed “anhydrobiosis”. Here, we have produced a tagged version for three different LEA proteins: pTag-RAB17-GFP-N, Zea mays dehydrin-1dhn, expressed in the nucleo-cytoplasm; pTag-WCOR410-RFP, Tricum aestivum cold acclimation protein WCOR410, binds to cellular membranes, and pTag-LEA-BFP, Artemia franciscana LEA protein group 3 that targets the mitochondria. Sheep fibroblasts transfected with single or all three LEA proteins were subjected to air drying under controlled conditions. After rehydration, cell viability and functionality of the membrane/mitochondria were assessed. After 4 h of air drying, cells from the un-transfected control group were almost completely nonviable (1% cell alive), while cells expressing LEA proteins showed high viability (more than 30%), with the highest viability (58%) observed in fibroblasts expressing all three LEA proteins. Growth rate was markedly compromised in control cells, while LEA-expressing cells proliferated at a rate comparable to non-air-dried cells. Plasmalemma, cytoskeleton and mitochondria appeared unaffected in LEA-expressing cells, confirming the protection conferred by LEA proteins on these organelles during dehydration stress. This is likely to be an effective strategy when aiming to confer desiccation tolerance to mammalian cells.     

Plant dehydrins and stress tolerance: Versatile proteins for complex mechanisms

Dehydrins (DHNs), or group 2 LEA (Late Embryogenesis Abundant) proteins, play a fundamental role in plant response and adaptation to abiotic stresses. They accumulate typically in maturing seeds or are induced in vegetative tissues following salinity, dehydration, cold and freezing stress. The generally accepted classification of dehydrins is based on their structural features, such as the presence of conserved sequences, designated as Y, S and K segments. The K segment representing a highly conserved 15 amino acid motif forming amphiphilic a-helix is especially important since it has been found in all dehydrins. Since more than 20 y, they are thought to play an important protective role during cellular dehydration but their precise function remains unclear. This review outlines the current status of the progress made toward the structural, physico-chemical and functional characterization of plant dehydrins and how these features could be exploited in improving stress tolerance in plants.Key words: abiotic stress, dehydration stress, drought, cold acclimation, freezing tolerance, LEA proteins, dehydrins