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Seed Storage Proteins

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Seed Storage Proteins SEED STORAGE PROTEINS PRESENTED BY: DIVYA KAUSHIK M.Sc BIOTECHNOLOGY 2nd SEMESTER INTRODUCTION The plant seed is not only an organ of propagation and dispersal but also the major plant tissue harvested by humankind. The amount of protein present in seeds varies from 10% (in cereals) to 40% (in certain legumes and oilseeds) of the dry weight, forming a major source of dietary protein. Although the vast majority of the individual proteins present in mature seeds have either metabolic or structural roles, all seeds also contain one or more groups of proteins that are present in high amounts and that serve to provide a store of amino acids for use during germination and seedling growth. These storage proteins are of particular importance because they determine not only the total protein content of the seed but also its quality for various end uses. CHARACTERISTICS OF SEED STORAGE PROTEINS Seed storage proteins (SSPs) are specifically synthesized during seed maturation and accumulate in the endosperm of monocots or in the cotyledons and embryos of dicots. Their presence in mature seeds in discrete deposit are called protein bodies. One of the earliest and first isolated proteins is wheat gluten and Brazil nut globulin. They are synthesized at high levels in specific tissues and at certain stages of development. their synthesis is regulated by nutrition, and they act as a sink for surplus nitrogen. CLASSIFICATION Classification of proteins in to their groups is based on their solubility. Three protein groups have been categorized during classification (Osborne’s classical definitions). Albumin • Water soluble Globulin • Dilute salt soluble • Alcohol/water micture Prolamins soluble • Dilute acid or alkali Glutelins • CLASSIFICATION OF PROTEINS BASED ON THEIR SOLUBILITY 2S GLOBULIN PROLAMIN ALBUMIN • 11S globulin • Gliadin (S-rich) • napin • C hordein (S- • 7S globulin poor) • Conglotin • HMW subunit • Sunflower • Avenin SFAS • Zein • Castor bean • Rice S-rich albumin prolamin 2S ALBUMIN STORAGE PROTEINS The 2s albumins were initially defined as a group on the basis of their sedimentation coefficients (S20.w). They are widely distributed in dicot seeds and have been most widely studied in the Cruciferae, notably oilseed rape (in which they are called napins) and Arabidopsis. They are synthesized as single precursor proteins that are proteolytically cleaved with the loss of a linker peptide and short peptides from both the N and C termini. GLOBULINS Present in embryo and outer aleurone layer of the endosperm. Readily soluble in dilute salt solution. Sedimentation coefficient is about 7. Usually removed by milling (wheat), polishing (rice), pearling (barley) or decortication (sorghum), before human consumption. Storage globulins of 11S located in the starchy endosperm are also present in atleast some cereal grains. HYPOTHETIC PATHWAY FOR THE EVOLUTION OF GERMINS AND GLOBULIN STORAGE PROTEINS PROLAMINS Major endosperm storage proteins of all cereal grains (except rice and oat). Generally rich in proline and amide nitrogen derived from glutamine. Soluble in alcohol/water mixtures (eg. 60-70% (v/v) ethanol). Much more variable in structure than the 7S and 11S globulins. Present in wheat, barley, rye, maize, sorghum, millets. Presence of amino acid sequences consisting of repeated blocks based on one or more short peptide motifs (eg. Methionine, glycine, histidine etc) eg. Zein. GLUTELINS Glutelins soluble in dilute acids or bases. They are generally prolamin like proteins in certain grass seeds eg. Wheat. Glutenin is the most common glutelin found in wheat. It imparts baking quality to wheat. it is known that glutelin, a major storage protein in rice, originates from the same ancestral gene as leguminous 11S globulin irrespective of difference in solubility. GLUTELIN STRUCTURE DECOMPOSITION OF SSPS AT SEED GERMINATION STAGE SSPs provide material for new protein synthesis, and nitrogen source and sulfur source during germination. SSP decomposition has been predominantly studied in seeds of cultivated plants such as cereal grains and legumes. SSPs are broken down into soluble peptides by endopeptidase, and subsequently these peptides are decomposed into smaller peptides or amino acids by exopeptidase. Peptidase is secreted from the cotyledons in dicots and from the aleurone layer and scutellum in monocots after water absorption by the seed. This peptidase secretion is triggered by the plant hormone gibberellin (GA). Amino acids produced from decomposition of SSPs are used as a source for new protein synthesis during germination and subsequent seedling growth. ENGINEERING PLANT SEED STORAGE PROTEINS Present strategy in the modification of seed storage protein is mainly focused on increasing concentration of sulphur containing amino acids (S-rich amino acids) in legume seeds and lysine content in cereals. Since animal system (including human) cannot synthesize certain essential amino acids, dietary supply can overcome this problem. Cereal grains containing lysine rich protein consumed as high profile energy source in the diet of humans and lives stock. Cereal seeds however, generally tend to be deficient in lysine, threonine and tryptophan. In contrast, legumes seed is the richest source for proteins (upto 40%) but shows, deficient in methionine and cysteine content as well as tryptophan (sulphur containing amino acid) for example, pea seed protein containing around 0.8% methionine and 1.0% cysteine. This level is insufficient for growth and development. Lives stock animals however, require 3.5% by weight of dietery protein. Besides, people who consumes only vegetarian food suffers health problem due to amino acid imbalance. Therefore, breeding and genetic engineering methods can drive increase in concentration of essential amino acid to overcome nutritional imbalance. TRANSGENIC TECHNOLOGY FOR TAILORING THE QUALITY OF SEED STORAGE PROTEIN Transgenic technology for tailoring the quality of seed storage protein Transgenic Improvement Gene Promoter plant 2S rich zein CAM V35S Alfa alfa S content (maize) 2S protein (brazil Vicia CAM V35S methionine nut) narborensis 2S protein Sulphur CAM V35S Legumes (sunflower) content Beta phaseolin Rice glu rice lysine (common bean) promoter Ferritin Rice glu B1 Rice Fe content (soyabean) promoter NOVEL APPLICATION OF SSPS Seeds, especially crop seeds, have attracted attention as a production platform for valuable proteins or peptides. Transgenic seed has many advantages as a bioreactor, because high-level expression of recombinant protein can be easily achieved. Engineered peptides can be produced and sequestered exclusively in the seed without causing deleterious effects in other vegetative tissues, when specifically expressed in seed. Furthermore, recombinant proteins are stable in transgenic seed for several years at room temperature. It is possible that transgenic seeds could easily provide bioactive function by oral feeding without extraction of bioactive components. APPLICATIONS CONTD.. SSPs are useful for high accumulation of recombinant protein as carrier protein. SSP-fused recombinant protein can be easily deposited in seed cells. SSP-fused recombinant protein (peptide) expressed under the control of the SSP promoter provides an ideal combination as an accumulation system for recombinant protein. Soybean glycinin, rice glutelin, and their promoters are actually used for accumulation of bioactive peptides or epitopic peptides. Transgenic seed accumulating human health promoting proteins may be practically used as bioactive foods in the future. REFERENCES http://www.biologydiscussion.com/plants/plants- with-improved-quality-of-nutrition/72129 http://www.plantcell.org/content/plantcell/7/7/945 .full.pdf H.B. Krishnan, E.H. CoeJr (2001), Seed Storage Proteins, Encyclopedia of Genetics. P.R. Shewry (2004), Protein Chemistry Of Dicotyledonous Grains, Encyclopedia of Grain Science. .
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