Nucleotide Sequences of cDNAs Encoding Two Members of the Brazil Nut Methionine-Rich 2S Albumin Gene Family

Abstract

Hawaii 96822 (S.S.M.S.) The 2S albumins ofBrazil nut (Bertholletia excelsa H.B.K.) are a group of abundant seed proteins containing high concentrations of the sulfur amino acids. Because these proteins contain about 18% methionine, the albumins have been used in gene transfer experiments to improve the methionine content of seed proteins from other crop plants (1, 3). The Brazil nut albumins consist of two subunits, a small subunit of 3 kD and a large subunit of 9 kD, which are joined by disulfide linkages. Both subunits are derived from a precursor polypep-tide in a series of proteolytic processing events (5, 8). Like many other seed storage proteins, the Brazil nut albumins are encoded by a multigene family. Amino acid sequences of five isoforms of the large subunit (2, 4) and one isoform of the small subunit (4) have been determined. In addition, the nucleotide sequences of cDNAs encoding two other isoforms have been reported (2, 5). Considerable homology is apparent among proteins encoded by members of the Brazil nut 2S albumin family. The Brazil nut protein also exhibits strong structural homology with the 2S albumins of other seeds, including those from rapeseed, Arabidopsis, sunflower, castor bean, barley, and lupin (2, 4, 6, 7). Indeed, the positions of seven to eight cysteine residues have been conserved in albu-mins from these species. These homologies have been important in defining regions of 2S seed proteins that might tolerate modifications without adversely affecting processing, transport , or stability of the proteins in seeds. In this paper, the nucleotide sequences of cDNAs encoding two new isoforms of the Brazil nut 2S albumin are reported (Fig. 1, Table I). These sequences are compared with the sequence of cDNA clone pHS-3, which encodes the entire protein precursor of a Brazil nut albumin (2). One cDNA, pHS-9, shows 99% ho-mology to pHS-3 at the nucleotide level and 96% homology at the protein level. The other cDNA, pHS-6, has 95% ho-'Present address: USDA-ARS,