Cupin Superfamily

The 7S and 11S globulins are classified into the cupin superfamily (31,32).

Cupin comes from "cupa," the Latin term for a small barrel. The characteristic cupin domain comprises two conserved motifs, G(x)5HxH(x)3,4E(x)6G and G(x)5PxG(x)3N (31,32). Based on structural features, cupin proteins are classified as either a monocupin having one cupin motif per one monomer, and a bicupin having two cupin motifs per one monomer. The 7S and 11S globulins are bicupins. Cupin proteins are also known to have diverse functions. For example, in addition to seed storage proteins, an oxalate oxidase and a sucrose binding protein belong to this group (33,34).

In the three dimensional structure of the cupin protein phaseolin, which was determined at the early stage of research of the cupin superfamily, the cupin motifs are located at the C and D strands and at the G and H strands of the ^-barrel (Figure 2.7). The length between motifs varies largely among species of proteins. Later, the three dimensional structures of many cupin proteins were shown to have a jellyroll type ^-barrel structure. Further, it was elucidated that in addition to a tertiary structure, a quaternary structure is common among cupin proteins. Three dimensional structures of many cupin proteins indicate that an arabinose binding domain of AraC transcription factor from E. coli, a dimeric dTDP-4-dehydrorhamnose 3,5-epimerase from Salmonella enterica, a hexameric germin from barley and an oxalate decarboxylase from Bacillus subtilis are very similar to the domain, protomer, and trimer of proglycinin and P-conglycinin, and the hexamer of glycinin, respectively (27,29,33,34) (Figure 2.7).

Figure 2.7 Ribbon diagrams of the three dimensional structures of cupin superfamily. (a) arabi-nose-binding domain of AraC transcription factor from E. coli (PDB code: 2ARA); (b) dimeric dTDP-4-dehydrorhamnose 3,5-epimerase from Salmonella enterica (1DZR); (c) hexameric germin from barley (1FI2); (d) oxalate decarboxylase from Bacillus subtilis (IJ58). The cupin motifs in (b) are shown in dark gray. Manganese ions are indicated by balls in (c) and (d). (From: Dunwell, J.M., A. Culham, C.E. Carter, C.R. Sosa-Aguirre, PW. Goodenough. Trends Biochem. Sci. 26:740-746, 2001; and Anand, R., PC. Dorrestein, C. Kinsland, T.P Begley, S.E. Ealick. Biochemistry 41:76597669, 2002.)

Figure 2.7 Ribbon diagrams of the three dimensional structures of cupin superfamily. (a) arabi-nose-binding domain of AraC transcription factor from E. coli (PDB code: 2ARA); (b) dimeric dTDP-4-dehydrorhamnose 3,5-epimerase from Salmonella enterica (1DZR); (c) hexameric germin from barley (1FI2); (d) oxalate decarboxylase from Bacillus subtilis (IJ58). The cupin motifs in (b) are shown in dark gray. Manganese ions are indicated by balls in (c) and (d). (From: Dunwell, J.M., A. Culham, C.E. Carter, C.R. Sosa-Aguirre, PW. Goodenough. Trends Biochem. Sci. 26:740-746, 2001; and Anand, R., PC. Dorrestein, C. Kinsland, T.P Begley, S.E. Ealick. Biochemistry 41:76597669, 2002.)

The cupin fold is evolutionally conserved, although amino acid sequences are not conserved except the cupin motifs. This indicates that the cupin motif is essential for the formation and maintenance of the structures of glycinin and P-conglycinin.

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