Seed Storage Proteins; Strategies for Developing Crops Promoting Human Health

Plant seeds contain high amounts of storage proteins. These are classified on basis of their solubility as water-soluble albumins, salt-soluble globulins, alcohol-soluble prolamins, and acidor alkaline-soluble glutelins (Osborne 1924, Utsumi, 1992). The compositions of seed storage proteins differ among plant species. For examples, monocot seeds contain mainly glutelins and prolamins (Ogawa et al., 1987; Li & Okita, 1993; Cagampang et al., 1966), whereas legume seeds contain mainly 7S/11S globulins (Utsumi, 1992). Rice is the staple food of approximately half of the population of the world. The major seed storage proteins of rice are glutelins and prolamin, similarly to the other monocots. The seed storage proteins present in rice, however, offer little significant benefit to human physiology. Therefore, improving the nutritional and physiological values of rice would be of benefit to the health of considerable numbers of people. A candidate protein that might be of interest in the context of improving the physiological values of rice is β-conglycinin. The seed storage protein of soybean, β-conglycinin, lowers plasma cholesterol and triglyceride levels in humans (Sirtori et al., 1995; Aoyama et al., 2001). Moreover, βconglycinin increases adiponectin levels and improves glucose tolerance (Tachibana et al., 2010). The α’ subunit of β-conglycinin has LDL-cholesterol-lowering activity (Sirtori and Lovati, 2001) and contains a phagocytosis-stimulating peptide (Tsuruki et al., 2003). Therefore, development of rice that can accumulate β-conglycinin should produce a staple food with several important physiological benefits to human health. β-Conglycinin has the trimeric structure common to 7S globulins of other plant species and is composed of three subunits, α, α’ and β. The α and α’ subunits contain an N-terminal extension in addition to a core region common to all the subunits (Maruyama et al., 1998, 2001 & 2004). The β subunit consists of only the core domain. The α and α’ subunits and the β subunit are synthesized on polysomes as preproand pre-forms, respectively. The signal peptides are co-translationally removed, the polypeptides are N-glycosylated with highmannose glycans and assemble into trimers in the ER (Yamauchi & Yamagishi, 1979; Utsumi, 1992). They are transported from the ER to the protein storage vacuoles through the Golgi apparatus (Mori et al., 2004). The pro regions of the α and α’ subunits are