Evidence for a precursor molecule of Brazil nut 2 S seed proteins from biosynthesis and cDNA analysis

Summary Seed storage proteins were extracted from Brazil nut (Bertholletia excelsa H.B.K.) seed embryos at various maturation stages. Salt-soluble and water-soluble proteins (globulins and albumins) were separated by gel chromatography and exhaustive dialysis against water. Both fractions were analysed by one- and two-dimensional polyacrylamide gel electrophoresis. Amino acid analysis revealed that both fractions are unusually high in methionine. The albumins consist of a family of low molecular weight polypeptides that are heterogeneous with respect to pI and are identical to the high methionine 2 S proteins described by Youle and Huang (1981). The biosynthesis of this class of proteins in maturing embryos was followed by in vivo labelling combined with immunological studies. Western blotting with monospecific antibodies against purified 2 S albumins and sequencing of a nearly complete cDNA clone revealed that they are synthesized via a precursor polypeptide.