Structures of plant storage proteins and their functions

Abstract

The complete amino acid sequence of plant storage protein molecules has been determined over the past decade by sequence analysis of full‐length cDNA and genomic clones for most cereal and legume seeds. At the same time, knowledge on the biosynthesis of the storage proteins has also been accumulated at the molecular level. According to the data on the gene structures of the plant storage proteins, the homology of their amino acid sequences, and the mechanisms of their accumulation into the protein bodies in the biosynthesis, the storage proteins which are classified as glutelins by a traditional sequential extraction method should be classified as either globulins or prolamins on the basis of the molecular structures. For instance, both orizenin, the rice main storage protein, and glutenin, the most important protein component of wheat proteins, have been classified as glutelins, but they should be classified as globulins and prolamins from the molecular base standpoint, respectively. The amino acid sequences of 7S and 11S globulins of legumes such as soybeans, peas, kidney beans, etc., showed considerable sequence homology and predicted secondary structural identity among 7S globulins or among 11S globulins. In addition, there was a high degree of re‐latedness on the secondary structures, even between the 7S and 11S globulins. The 11S‐type globulins exist widely, not only in legumes, but also in sesame, rape seed, rice, oat, pumpkin, etc. Furthermore, the amino acid sequences of these globulins showed quite a high homology each other. These facts indicate that the 11S globulins, which are distributed very widely among the different species, have all evolved from a common origin. The storage proteins of corn, wheat, rye, and barley are the typical prolamins, of which amino acid sequences are quite different from those of the globulin. The characteristic structural feature of the prolamin molecules is the presence of a repetitive peptide structure in their polypeptide chains. The functional properties of storage proteins and their mechanisms are described at the molecular level both from the gelation of soybean 7S and 11S globulins, and from the viscoelastic properties of the high molecular weight (HMW) subunits of wheat glutenin. In the gel formation of 7S globulins, no ‐SH/S‐S interchange reaction participates and therefore the gels are soft and transparent, whereas in the gel formation of 11S globulins, the interchange reaction participates, which makes the resultant gels firm and turbid. In addition, the subunit compositions of the 11S globulin molecules markedly effect the hardness, turbidity, and rates of gelation of the 11S gels. The viscoelastic properties of wheat HMW subunit are ascribed to the β‐spiral structures of the repetitive central domain, which are assembled into long linear polymers through covalent crosslinks via cysteine residues between the ‐NH₂ and ‐COOH termini. A discussion on the improvement of the plant storage proteins as food by genetic engineering techniques is presented.