ISOLATION AND CHARACTERIZATION OF AMARANTIN, THE 11S AMARANTH SEED GLOBULIN

Structural Properties of Quinoa Protein Isolate: Impact of Neutral to High Alkaline Extraction pH

In this work, we extracted proteins from white quinoa cultivated in the northeast of Qinghai-Tibet plateau using the method of alkaline solubilization and acid precipitation, aiming to decipher how extraction pH (7-11) influenced extractability, purity and recovery rate, composition, multi-length scale structure, and gelling properties of quinoa protein isolate (QPI). The results showed that protein extractability increased from 39 to 58% with the increment of pH from 7 to 11 whereas protein purity decreased from 89 to 82%. At pH 7-11, extraction suspensions and QPI showed the similar major bands in SDS-PAGE with more minor ones (e.g., protein fractions at > 55 or 25-37 kDa) in suspensions. Extraction pH had limited effect on the secondary structure of QPI. In contrast, the higher-order structures of QPI were significantly affected, e.g., (1) emission maximum wavelength of intrinsic fluorescence increased with extraction pH; (2) surface hydrophobicity and the absolute value of zeta-potential increased with increasing extraction pH from 7 to 9, and then markedly decreased; (3) the particle size decreased to the lowest value at pH 9 and then increased to the highest value at pH 11; and (4) denaturation temperature of QPI had a large decrease with increasing extraction pH from 7/8 to 9/10. Besides, heat-set QPI gels were formed by loosely-connected protein aggregates, which were strengthened with increasing extraction pH. This study would provide fundamental data for industrial production of quinoa protein with desired quality.

A peptide derived from enzymatic digestion of globulins from amaranth shows strong affinity binding to the replication origin of Tomato yellow leaf curl virus reducing viral replication in Nicotiana benthamiana

Tomato yellow leaf curl virus (TYLCV; genus Begomovirus; family Geminiviridae) infects mainly plants of the family Solanaceae, and the infection induces curling and chlorosis of leaves, dwarfing of the whole plant, and reduced fruit production. Alternatives for direct control of TYLCV and other geminiviruses have been reported, for example, the use of esterified whey proteins, peptide aptamer libraries or artificial zinc finger proteins. The two latter alternatives affect directly the replication of TYLCV as well as of other geminiviruses because the replication structures and sequences are highly conserved within this virus family. Because peptides and proteins offer a potential solution for virus replication control, in this study we show the isolation, biochemical characterization and antiviral activity of a peptide derived from globulins of amaranth seeds (Amaranthus hypochondriacus) that binds to the replication origin sequence (OriRep) of TYLCV and affects viral replication with a consequent reduction of disease symptoms in Nicotiana benthamiana. Aromatic peptides obtained from papain digests of extracted globulins and albumins of amaranth were tested by intrinsic fluorescent titration and localized surface resonance plasmon to analyze their binding affinity to OriRep of TYLCV. The peptide AmPep1 (molecular weight 2.076 KDa) showed the highest affinity value (Kd = 1.8 nM) for OriRep. This peptide shares a high amino acid similarity with a part of an amaranth 11S globulin, and the strong affinity of AmPep1 could be explained by the presence of tryptophan and lysine facilitating interaction with the secondary structure of OriRep. In order to evaluate the effect of the peptide on in vitro DNA synthesis, rolling circle amplification (RCA) was performed using as template DNA from plants infected with TYLCV or another begomovirus, pepper huasteco yellow vein virus (PHYVV), and adding AmPep1 peptide at different concentrations. The results showed a decrease in DNA synthesis of both viruses at increasing concentrations of AmPep1. To further confirm the antiviral activity of the peptide in vivo, AmPep1 was infiltrated into leaves of N. benthamiana plants previously infected with TYLCV. Plants treated with AmPep1 showed a significant decrease in virus titer compared with untreated N. benthamiana plants as well as reduced symptom progression due to the effect of AmPep1 curtailing TYLCV replication in the plant. The peptide also showed antiviral activity in plants infected with PHYVV. This is the first report, in which a peptide is directly used for DNA virus control in plants, supplied as exogenous application and without generation of transgenic lines.

Isolation and characterization of proteins from chia seeds (Salvia hispanica L.)

Chia ( Salvia hispanica L.) is a plant that produces seeds rich in some nutraceutical compounds with high protein content, but little is known about them; for this reason the aim of this study was to characterize the seed storage proteins. Protein fractions were extracted from chia seed flour. The main protein fraction corresponded to globulins (52%). Sedimentation coefficient studies showed that the globulin fraction contains mostly 11S and 7S proteins. The molecular sizes of all the reduced fractions were about 15-50 kDa. Electrophoretic experiments under native conditions exhibited four bands of globulins in the range of 104-628 kDa. The denaturation temperatures of crude albumins, globulins, prolamins, and glutelins were 103, 105, 85.6, and 91 °C, respectively; albumins and globulins had relatively good thermal stability. Selected globulin peptides by mass spectrometry showed homology to sesame proteins. A good balance of essential amino acids was found in the seed flour and globulins, especially of methionine+cysteine.

Expression and characterization of the acidic subunit from 11S Amaranth seed protein

Amarantin acidic subunit has the potential to be employed as a functional and a nutraceutical protein. To evaluate both possibilities this protein was produced in recombinant Escherichia coli Origami (DE3) harboring the expression plasmid pET-AC6His. Three different expression factors were assayed: inductor concentration, temperature and time of the amarantin acidic subunit accumulation. The results indicated that a 0.3 mmol/L concentration of isopropyl-beta-D-thiogalactoside, at 37 degrees C and 6 h after induction were favorable for high expression of amarantin acidic subunit, mostly in the form of inclusion bodies. The protein was purified from soluble fraction by immobilized metal affinity chromatography, up to 30 mg amarantin acidic subunit/L Terrific broth culture were obtained. Sucrose density gradient ultracentrifugation analysis of the expressed soluble amarantin acidic subunit revealed that it was assembled in monomers. The expression of the amarantin acidic subunit, together with the one-step purification will facilitate further investigation of this storage protein through site-directed mutagenesis.

Amaranth Globulin Polypeptide Heterogeneity

The polypeptides integrating amaranth globulin-p and 11S-globulin were characterized by two-dimensional electrophoresis, ion-exchange chromatography and RP-HPLC. All polypeptides exhibited charge and hydrophobic heterogeneity. Almost all acid (A, pI 5-7) and basic (B, pI 9-10) polypeptides were present in both globulins, and the same happened with the unprocessed M polypeptides with pI in the range of 7-7.5 which fits well with a sequence containing both the A and B polypeptides. There were other polypeptides only present in 11S-globulin, like some of 41 and 16 kDa, which might come from another precursor or be the products of a different processing of the propolypeptide. These results suggested that, although amaranth subunits from different subfamilies are interchangeable in different oligomers, some structural differences between them might affect the assembly of globulin molecules. Structural differences arising from this behavior could account for the different physicochemical properties of globulin molecules.

Endoplasmic reticulum-retention C-terminal sequence enhances production of an 11S seed globulin fromAmaranthus hypochondriacus inPichia pastoris

The methylotrophic yeast Pichia pastoris was used to express an 11S seed globulin from Amaranthus hypochondriacus. Three different plasmids were tested for expression of amarantin. One of them, which included the untranslated regions (UTR) of the full cDNA, failed to express the amarantin under tested conditions, whereas the other plasmids, one without UTR and the other similar but including the endoplasmic reticulum-retention signal KDEL, were able to express the proamarantin in P. pastoris. After 48 h of induction, KDEL-proamarantin had accumulated quite significantly compared to unmodified proamarantin. Different solubilization patterns were also obtained from both proamarantin versions; only soluble protein was obtained from the system that included the KDEL retrieval signal. Protein fractionation was carried out by differential precipitation with ammonium sulfate, and proamarantin purification was performed using an HPLC ion exchange column. The endoplasmic reticulum-retention C-terminal sequence (KDEL retrieval signal), not commonly employed in this heterologous expression system, can therefore be used to enhance accumulation of recalcitrant protein in P. pastoris. The results obtained here also suggest that this expression system is suitable for expression and evaluation of engineered seed globulin proteins.

One-step purification and structural characterization of a recombinant His-tag 11S globulin expressed in transgenic tobacco

Amarantin, an 11S globulin, is one of the most important storage proteins of amaranth seeds, with relevant nutritional-functional and nutraceutical characteristics. Its cDNA was cloned in-frame with a sequence encoding a polyhistidine tag and expressed under the direction of a 35S promoter in transgenic tobacco seeds. The presence of a (His)(6) tag on the polypeptide permitted a high-yield single-step purification using immobilized metal-ion affinity chromatography and rapid characterization. Purified His-tag amarantin accounted for up to 5% of total soluble seed protein. Biochemical characterization indicated that purified His-tag amarantin migrated with the expected molecular weight (53 kDa) and was correctly processed into an acidic polypeptide (32 kDa) with isoelectric point (pI) of 5.58 and a basic polypeptide (21 kDa) with pI of 9.24, linked by a disulfide bridge. Moreover, His-tag amarantin was assembled into both homo- and hetero-hexameric 11S structures. These results show that the His tag did not change the biochemical and physicochemical properties of amarantin. The strategy presented here for rapid and high-yield expression and purification procedure should facilitate structure-function studies for this nutritional protein.