Most plant seeds contain 11S globulins as major storage proteins for their nutrition. Soybean glycinin belongs to the 11S globulin family and consists of five kinds of subunits. We determined the crystal structure of a homohexamer of the glycinin A3B4 subunit at 2.1-Å resolution. The crystal structure shows that the hexamer has 32-point group symmetry formed by face-to-face stacking of two trimers. The interface buries the highly conserved interchain disulfide. Based on the structure, we propose that an ingenious face-to-face mechanism controls the hexamer formation of the 11S globulin by movement of a mobile disordered region to the side of the trimer after posttranslational processing. Electrostatic analysis of the faces suggests that the interchain disulfide-containing face has high positive potential at acidic pH, which induces dissociation of the hexamer into trimers that may be susceptible to proteinases after seed imbibition. This dissociation might result in the degradation and mobilization of 11S globulins as storage proteins in embryos during germination and seedling growth. P lants accumulate protein reserves in developing seeds to act as a sink of nitrogen, sulfur, and carbon. Most dicotyledonous plant seeds contain 7S and͞or 11S globulins and albumins as the major storage proteins in the embryo or cotyledons, whereas some cereals such as wheat, barley, and corn contain prolamins in the endosperm (1, 2). The 11S globulins are distributed more widely than the 7S globulins among plant seeds. Although the quaternary structures of the 7S and 11S globulins are different from each other (the former being a trimeric protein and the latter a hexameric protein), they are believed to be derived from a common ancestor because of the partial homologies in their amino acid sequences and limited proteolysis patterns (3). This assumption has been confirmed by x-ray crystallography of 7S globulins from kidney bean (4), jack bean (5), and soybean (6) and of the trimeric soybean 11S globulin precursor (7).In general, 11S globulins are composed of several kinds of subunits. For example, five major subunits have been identified from soybean: A1aB1b, A2B1a, A1bB2, A3B4, and A5A4B3. In developing seeds, the constituent subunits of 11S globulin are synthesized as a single polypeptide precursor, preproprotein, the signal sequence of which is removed cotranslationally. The resultant proproteins assemble into trimers of Ϸ8S in the endoplasmic reticulum. The proprotein trimers are transported from the endoplasmic reticulum to protein storage vacuoles (PSVs), where they then are cleaved to form acidic and basic polypeptides that are linked by a disulfide bond (8). Finally, the mature proteins assemble into hexamers. The protein reserves are stored in the dormant seed until its germination.Recently we determined the crystal structure of the soybean proglycinin A1aB1b homotrimer at 2.8-Å resolution (7). The protomer consists of N-and C-terminal modules that each include a jelly-roll -barrel and a helix domain. However, the packa...
The crystal structures of recombinant and native b homotrimers of soybean b-conglycinin were determined by X-ray crystallography at 2.7 and 2.8 A Ê resolutions, respectively. The crystals of the recombinant and native b homotrimers belong to space group P2 1 with cell parameters a 80. Keywords: b-conglycinin; crystal structure; N-linked glycan; soybean; vicilin.Soybean proteins are one of the most attractive plant food proteins for man, as they exhibit a hypocholesterolemic effect [1,2] and have good nutritional and physicochemical properties (such as gel-forming and emulsifying abilities required for food systems) [3,4]. Soybean proteins are composed of two major components, b-conglycinin (vicilin class) and glycinin (legumin class), which account for about 30% and 40% of the total seed proteins, respectively. These two components are responsible for the nutritional, physicochemical and physiological properties of soybean proteins.b-Conglycinin has a trimeric structure analogous to that of other 7S globulins [4,5] and is composed of three kinds of subunit a, a H and b. Many molecular species with different subunit compositions, including homotrimers with almost random combinations, are present in soybean seeds [6,7]. The amino-acid sequences of these subunits deduced from their nucleotide sequences indicate that the a and a H subunits contain extension regions in addition to core regions common to all subunits, and that the core regions exhibit high absolute homologies (90.4, 76.2 and 75.5% between a and a H , a and b, and a H and b, respectively [8]). However, it is known that the contributions of individual subunits to physicochemical and physiological properties and allergenicity differ as follows: (a) the orders of thermal stabilities, which are one of the structural features related to food processing and conferred by the core regions, are b . a H . a [8]; (b) b-conglycinin mainly contributes to the up-regulation of low-density lipoprotein receptors, which is one of the mechanisms of the hypocholesterolemic effect of soybean proteins [2,9], and only the a H subunit is responsible for the up-regulation activity of b-conglycinin [10]; (c) b-conglycinin is the third main allergen responsible for soybean allergy, and only the a subunit is identified as having a reactivity with soybean-sensitive-patients H sera [11].In order to investigate why individual subunits of b-conglycinin have different properties as described above, it is necessary to elucidate their crystal structures by means of X-ray crystallography. Although a large amount of homogeneous homotrimers is required for X-ray crystallography, they are very difficult to obtain from natural soybean seeds because of molecular heterogeneity [6,7] and the high homologies of their amino-acid sequences [8]. Recently, a mutant soybean cultivar has been developed which contains b-conglycinin composed of only the b subunit [12]. Therefore, b homotrimers can be purified easily using the mutant soybean. Alternatively, the Escherichia coli expression systems we cons...
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