Isolation and characterization of oat globulin messenger RNA

Impact of in vitro gastrointestinal digestion on peptide profile and bioactivity of cooked and non-cooked oat protein concentrates

Oat (Avena sativa) is one of the most cultivated and consumed cereals worldwide. Recognized among cereals for its high protein content (12%-24%), it makes it an excellent source of bioactive peptides, which could be modified during processes such as heating and gastrointestinal digestion (GID). This work aims to evaluate the impact of heat treatment on the proteolysis of oat proteins and on the evolution of antioxidant peptide released during in vitro static GID, in terms of comparative analysis between cooked oat protein concentrate (COPC) and non-heated oat protein concentrate (OPC) samples. The protein extraction method and cooking procedure used showed no detrimental effects on protein quality. After GID, the proportion of free amino acids/dipeptides (<0.2 ​kDa) reached >40% for both samples (OPC and COPC), thus producing peptides with low molecular weight and enhanced bioactivity. Furthermore, during GID, the amino acid profile showed an increase in essential, positively-charged, hydrophobic and aromatic amino acids. At the end of GID, the reducing power of OPC and COPC increased >0.3 and 8-fold, respectively, in comparison to the non-digested samples; while ABTS•+ and DPPH• showed a >20-fold increase. Fe2+ chelating capacity of OPC and COPC was enhanced >4 times; similarly, Cu2+ chelation showed a >19-fold enhancement for OPC and >10 for COPC. β-carotene bleaching activity was improved 0.8 times in OPC and >9 times in COPC; the oxygen radical antioxidant capacity assay increased 2 times in OPC and >4.7 times in COPC, respectively. This study suggests that OPC after cooking and GID positively influenced the nutritional and bioactive properties of oat peptides. Thus, COPC could be used as a functional food ingredient with health-promoting effects, as hydrothermal treatment is frequently used for this type of cereals.

Proteins in oats; their synthesis and changes during germination: a review

Oats (Avena sativa L.) are distinct among cereals due to their considerably higher protein concentration. At the same time oats possess a protein quality of high nutritional value and a special protein composition. Most cereals like wheat, barley, and rye have a high percentage of prolamins, the alcohol-soluble fraction, which usually contains most of the storage proteins, but oats are an exception. Their major storage proteins belong to the salt-water soluble globulin fraction, whereas oats prolamins are a minor component. During oats groat development, most obvious is the fairly linear increase in the globulin fraction. Oats globulins share structural features with the 11 S globulins of legumes and other dicots. Amino acid composition of oats is superior to that of other cereals due to the higher amount of limiting amino acids like lysine and threonine. During germination, total amino acid analysis revealed an increase in essential amino acids like lysine and tryptophan, which leads to an increased nutritional value of germinated oats. Oats protein products including globulin have been chemically modified by various methods to improve their functional properties.

Storage Protein Synthesis during Oat (Avena sativa L.) Seed Development

Oat (Avena sativa L.) seeds harvested at 2-day intervals from anthesis to maturity were tested for their ability to incorporate [(35)S]sulfate into protein. Incorporation of [(35)S]sulfate into TCA-insoluble material began 2 to 4 days postanthesis (DPA), reached a peak 14 to 16 DPA, and was barely detectable by 24 DPA. Incorporation of label into globulin was parallel to total protein accumulation, and averaged about 85% of the total protein synthesis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of total protein extracted from developing seeds indicated that some polypeptides coinciding with the alpha and beta globulin subunits were present 2 to 4 DPA, but the full complement of globulin polypeptides was not present until 10 DPA. Immunoprecipitation of in vivo labeled seed extracts showed that globulin polypeptides and the 59 kilodalton precursor were present at early stages of development (4 DPA). Quantitation of dot blot analysis, using an oat globulin cDNA clone as a probe, indicated that one species of oat globulin mRNA was most abundant 15 DPA, which is during the peak time of storage protein synthesis.

Isolation and characterization of cDNAs encoding oat 12S globulin mRNAs

A cDNA library was made from poly(A(+)) RNA isolated from developing oat seeds, and oat globulin cDNA clones were identified by hybridization with synthetic oligonucleotides. Globulin clones were characterized by restriction enzyme mapping and cross-hybridization analysis. Based on these comparisons, four classes of globulin clones were distinguished. These clones hybridized to multiple DNA fragments in restriction enzyme digests of oat genomic DNA, indicating that the genes exist in a multigene family. The nucleotide sequence of one of the globulin cDNA clones was determined. The amino acid sequence derived from the DNA sequence verified its identity as an oat globulin and confirmed that the protein is synthesized as a precursor similar to legume 11S storage globulins. The basic polypeptide encoded at the 3' end of the mRNA was found to be homologous to the basic polypeptides of other 11S seed globulins.

Characterization of developing oat seed mRNA: evidence for many globulin mRNAs

Polyadenylated mRNA from developing oat (Avena sativa L.) seeds was isolated and analyzed. Prominent mRNA species of 18S, 15S and 12S were observed; the 18S mRNA was judged to be esentially free of ribosomal RNA by hybridization analysis. Size fractionation andin vitro translation of this mRNA was performed. SDS, IEF-SDS gel electrophoresis and immunoprecipitation were used to analyze the translation products. It is shown that globulin mRNA (18S) accounts for roughly 30% of the total mRNA in developing seeds, the 12S and 15S mRNAs accounting for the remainder. The 18S mRNA directs the synthesis of a series of distinct but related polypeptides, suggesting that some of the heterogeneity seen in the oat globulins is at the amino acid sequence level.

Evidence for translational control of storage protein biosynthesis during embryogenesis ofAvena sativa L. (oat endosperm)

Oat polysomes direct the synthesisin vitro of a large number of products, the majority of which are the salt-soluble globulins (1,3,10,11,21). Total RNA or poly A(+) RNA isolated from these polysomes directs the synthesis of the same number and types of products; however, the amount of globulins synthesized no longer represents the major products; rather, there is a decreased level of globulins and an increased amount of the other products synthesizedin vitro (6, 18). These results imply that the translational control can dictate final product levels. Reconstruction experiments using oat poly A(+) mRNA and polysomal factors that are made free of endogenous RNA by nuclease digestion demonstrate that these factors do influence the translational specificity of oat globulin mRNA relative to other mRNAs. It is suggested that translational control is partially responsible for the levels of globulin in the mature grain.

Biochemical characterization of rice glutelin

The two major subunits of rice glutelin, the acidic (alpha) and basic (beta) polypeptides were purified by chromatofocusing and cation exchange chromatography, respectively. The molecular weight range of the alpha polypeptides was 28.5 to 30.8 kilodaltons and the molecular weight range of the beta polypeptides was 20.6 to 21.6 kilodaltons. Electrofocusing in polyacrylamide gels showed that the isoelectric points of the alpha and beta polypeptides were 6.5 to 7.5 and 9.4 to 10.3, respectively. At least 12 polypeptides of the alpha-group and nine polypeptides of the beta-group could be separated by electrofocusing. The amino acid compositions of whole glutelin, and the purified alpha and beta subunits were analyzed. The alpha subunit contained more glutamic acid/glutamine, serine, and glycine, and less alanine, lysine, aspartic acid/asparagine, and isoleucine than the beta subunit. A comparison of the amino acid composition of rice glutelin subunits with those of the 11S proteins from eight other plant species indicated that there is more similarity between the beta subunits than the alpha subunits of several diverse plant species.

Intracellular transport and posttranslational cleavage of oat globulin precursors

The synthesis, transport, and posttranslational processing of reserve globulin in Avena sativa L. seeds were studied by pulse-chase labeling. Developing oat seeds were labeled with radioactive sulfate and tissue homogenates were used for globulin extraction.Two globulin precursors (58-62 kilodaltons) were labeled after 1 hour pulse. The alpha and beta globulin subunits appeared between 2 and 10 hours later, while simultaneously the 58 to 62 kilodaltons polypeptides gradually disappeared. This confirmed a precursor-product relationship. In a second pulse-chase experiment, the tissue extracts were fractionated on a sucrose gradient. The major portion of radioactivity was initially (1 hour pulse) associated with the endoplasmic reticulum. However, a significant amount of radioactivity shifted from the endoplasmic reticulum to protein bodies after 20 hours chase, suggesting the transport of the newly synthesized proteins. Protein bodies isolated from pulse-chased seeds were analyzed for the arrival of the newly synthesized globulin. Labeled precursors were detected after 2 hours chase and gradually disappeared. The alpha and beta subunits appeared during the same chase period and assembled into a 12S oligomer.The data indicated that oat globulin was synthesized as two large precursors which were transported from endoplasmic reticulum into protein bodies where they were processed to the alpha and beta subunits forming a 12S oligomer.

Quantitation of oat globulin by radioimmunoassay

A radioimmunoassay was used to determine the globulin content of developing oat (Avena sativa L. var Coker 6622) seeds. The globulin content increased from 0.32 to 2.37 milligrams per seed from 2 to 21 days post anthesis. The amount of globulin did not increase from 21 to 30 days post anthesis. When the total protein of seeds harvested 21, 24, and 30 days post anthesis was measured by micro-Kjeldahl analysis, it was determined to be 3.15 milligrams per seed for each sampling date. When the amount of globulin per seed was compared to this value, the relative proportion of globulin to total seed protein was approximately 75%.

Role of endoplasmic reticulum in biosynthesis of oat globulin precursors

Oat (Avena sativa L.) groats were labeled with radioactive leucine and salt-soluble proteins were extracted and analyzed. Polyacrylamide gel electrophoresis followed by fluorography indicated two radioactive polypeptides with molecular weight 58 to 62 kilodaltons which were similar in size to unreduced globulin alpha-beta dimers. The role of endoplasmic reticulum in the synthesis of these globulin polypeptides was investigated by in vivo and in vitro protein synthesis studies. Labeled tissue was fractionated by centrifugation and rough endoplasmic reticulum was isolated. Two polypeptides which had molecular weights of 58 to 62 kilodaltons and were immunoprecipitable with antiglobulin immunoglobulin G were found to be transiently associated with the endoplasmic reticulum. Rough endoplasmic reticulum, as well as membrane-bound polysomes, directed the in vitro synthesis of two polypeptides with molecular weight 58 to 62 kilodaltons corresponding in size to unreduced alpha-beta dimers and could be immunoprecipitated with antiglobulin immunoglobulin G. The translation products of free polysomes did not show this. In pulse-labeling, globulin polypeptides with molecular weight 58 to 62 kilodaltons, as well as the alpha + beta subunits, were labeled in protein bodies.The data suggest that oat globulin polypeptides are synthesized as higher molecular weight precursors on ER-associated polysomes. These precursors are probably transported into protein bodies and cleaved into smaller alpha and beta subunits.