Accumulation of Soybean Glycinin and Its Assembly with the Glutelins in Rice1

Abstract

Saline-soluble glycinins and insoluble glutelins are the major storage proteins in soybean (Glycine max) and rice (Oryza sativa), respectively. In spite of their differences in solubility properties, both proteins are members of the 11S globulin gene family based on their similarities in primary sequences and processing of the coded protein. Wild-type and methionine-modified glycinin coding sequences were expressed in transgenic rice plants under the control of the rice glutelin GluB-1 promoter. Glycinins were specifically synthesized in the endosperm tissue and co-localized with glutelins in type II protein bodies. They assembled into 7S and 11S species, similar to what was observed in developing soybean seeds. This pattern was quite different from that displayed by the rice glutelins in untransformed plants, in which processed subunits sedimenting at 2S were apparent. In glycinin-expressing transgenic plants, however, glutelins were observed sedimenting at 7S and 11S with lesser amounts in the 2S region. A portion of the glycinins was also found associated in the insoluble glutelin fraction. Renaturation experiments suggested that the hybrid glycinin-glutelin oligomers were formed through specific interactions. Overall, these results indicate that despite significant differences in the assembly of soybean glycinin and rice glutelin, both proteins can assemble with each other to form soluble hexameric oligomers or insoluble aggregates.Seed storage proteins were initially classified into albumins (water soluble), globulins (saline soluble), prolamins (alcohol soluble), and glutelins (residue) by Osborne (1924) according to their solubility properties. Based on more recent and extensive molecular and biochemical analysis of the storage protein genes and their coded products, the storage proteins fall into two major groups, the globulins and the prolamins (Shewry and Tatham, 1990). The rice (Oryza sativa) glutelins and soybean (Glycine max) glycinins are excellent examples of this reclassification of storage proteins. The rice glutelins, which comprise up to 70% to 80% of the total seed protein, are insoluble in a neutral saline solution but soluble in a diluted acid/alkaline solution. They exist as large macromolecular complexes formed by disulfide and hydrophobic interactions of acidic and basic polypeptides. The soybean glycinins, which account for 40% of the total proteins (Utsumi, 1992; Utsumi et al., 1997), are soluble in neutral saline solutions. These proteins accumulate as 11S oligomers comprised of six pairs (subunits) of acidic and basic polypeptides interlinked by a conserved disulfide bond. Although glutelin and glycinin have different properties (such as their solubility), they nevertheless are related and are both members of the 11S globulin family of storage proteins. These proteins share 32% to 37% identity in their primary sequences. Moreover, both proteins are synthesized as a larger precursor on the ER, are proteolytically processed into acidic and basic polypeptides, and are accumulated and...

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