Storage Protein Synthesis

Abstract

Seed storage proteins are synthesised in the endoplasmic reticulum and accumulate in protein bodies in the cotyledon and endosperm storage tissues during seed development. Their synthesis relies on remobilisation of nitrogen from vegetative tissues. They are adapted for storing rapidly and in an inert form a high concentration of remobilisable nitrogen. Upon germination, they provide the seedling with amino acids during the heterotrophic growth phase. They also represent a major protein source for humans and livestock. Although having limiting quantities of essential amino acids, notably lysine and methionine, a combination of cereal and legume seed proteins, for example constitutes a good dietary amino acid balance for monogastric animals. Seed proteins have been classified by their differing solubilities. With the benefit of sequence data, we now know that within a solubility class different sequence types may exist. Interestingly, despite their contrasted primary sequences, the conservation of short sequence motifs shows that different storage protein classes are derived from a common evolutionary ancestor. Storage proteins embrace a large range of valuable technological properties, which find varied uses in the food industry.

Key Concepts

  • Seed storage proteins have evolved to store nitrogen efficiently as a source for the germinating seedling.
  • Most monocotyledons store mainly prolamin storage proteins, whereas dicotyledons store mainly globulins.
  • Prolamins consist mainly of short tandem repeats of noncharged amino acids, which renders them insoluble in water.
  • Globulins are assembled into multimers that reduce their solubility.
  • Seeds contain antinutritional compounds that reduce digestibility of the storage proteins by humans or livestock unless eliminated by breeding or processing.
  • The secondary/tertiary structure of cereal glutelins confers the property of viscoelasticity, of fundamental importance to breadmaking.
  • Storage proteins are encoded by multigene families, and mutations having a major effect on their accumulation are mostly in trans, at loci regulating their synthesis or deposition in protein bodies.
  • The seed's structure and its development are highly adapted to permit a rapid accumulation of storage products during the limited period of seed maturation.
  • Storage protein deposition relies on remobilisation of nitrogen from vegetative tissue.
  • Storage protein remobilisation during germination involves activation of preexisting proteases and induction of de novo synthesis.

Keywords: endosperm; cotyledon; prolamin; glutelin; globulin; vacuole; protein body; remobilisation; embryo; heterotrophic

Figure 1. Thin section of immature (20 days after fertilisation) seed of barrel medick (Medicago truncatula). Immunofluorescence labelling of embryo cells with antivicilin antibodies which localises this storage protein in the protein storage vacuoles (green). The remaining cellular structures are visualised with propidium iodide fluorescence (red). Micrograph kindly provided by M. Abirachid‐Darmency and S. Ochatt (INRA Agroécologie, Dijon, France)
Figure 2. Storage protein structures. (a) Schematic view of a globulin hexamer (glycinin from soya bean), based on X‐ray crystallography studies. The six subunits are indicated in different colours (Adachi, M., et al. (2003). Crystal structure of soybean 11S globulin: Glycinin A3B4 homohexamer. Proceedings of the National Academy of Sciences 100 (12): 7395–7400.) (b) Predicted HMW glutenin structures. The polypeptide backbone consists of a central rod‐like beta‐spiral (blue) with alpha‐helical globular N‐ and C‐terminal regions containing cysteine residues. The rods may associate to form HCP (‘hexagonal closed polymers’, Kuktaite et al., ). They are also thought to form longer chains by cystine cross‐bridge formation involving subterminal residues.
Figure 3. Protein body formation in rice endosperm cells. Prolamin mRNA (blue) and glutelin mRNA (green) locate to different ER domains, the prolamin mRNA to the sites of formation of type‐I protein bodies (PB‐1) and glutelin mRNA to the cisternal ER (C‐ER). Prolamins coalesce in situ to form type‐I protein bodies, whereas glutelins are transported via the Golgi apparatus to the protein storage vacuoles, where type‐II protein bodies form. Figure reproduced with permission from: Crofts et al., Biochem. Cell Biol. 83: 728–737 (2005).
Figure 4. Interactions between the LAFL ‘master regulators’ of storage protein synthesis in Arabidopsis thaliana. LEC1 controls the synthesis of ABI3, FUS3 and LEC2. LEC1, ABI3, LEC2 and FUS3 all act positively on transcription of seed storage protein genes. ABI3 and FUS3 autoregulate their own synthesis and are activated by LEC2. Adapted from Fatihi et al. .
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Further Reading

Gutierrez L, Van Wuytswinkel O, Castelain M and Bellini C (2007) Combined networks regulating seed maturation. Trends in Plant Science 12 (7): 294–300.

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Santos‐Mendoza M, Dubreucq B, Baud S, et al. (2008) Deciphering gene regulatory networks that control seed development and maturation in Arabidopsis. Plant Journal 54 (4): 608–620.

Washida H, Sugino A, Messing J, Esen A and Okita TW (2004) Asymmetric localization of seed storage protein RNAs to distinct subdomains of the endoplasmic reticulum in developing maize endosperm cells. Plant and Cell Physiology 45 (12): 1830–1837.

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Thompson, Richard D(Jun 2018) Storage Protein Synthesis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023693]