Amino acid and cDNA sequences of a methionine‐rich 2S protein from sunflower seed (Helianthus annuus L.)

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Abstract
The amino acid sequence of a methionine‐rich 2S seed protein from sunflower (Helianthus annuus L.) and the sequence of a cDNA clone which codes for the entire primary translation product have been determined. The mature protein consists of a single polypeptide chain of 103 amino acids (molecular mass 12 133 Da) which contains 16 residues of methionine and 8 residues of cysteine. The cDNA sequence established that the protein is synthesized as a precursor of 141 residues with a typical hydrophobic signal sequence of 25 residues followed by a further 13‐residue hydrophobic pro‐sequence which is presumably removed by post‐translational cleavage. The sequence of the mature protein and that deduced from the cDNA were identical with no evidence of processing at the C‐terminus. Comparison of the sunflower methionine‐rich protein sequence with sequences of other seed 2S proteins from dicotyledons and monocotyledons showed limited but distinct sequence similarities; in particular the arrangement of the cysteine residues was conserved. The sunflower protein shows 34% identity with the methionine‐rich Brazil nut 2S protein and the prepro regions of the precursors of these two proteins show about 50% identity. This similarity indicates that these methionine‐rich 2S proteins have diverged as a subclass of the 2S superfamily of proteins which contain only 2–3% methionine. While the related 2S proteins from other dicotyledons are processed to a small and large subunit, the sunflower protein is not cleaved in this way.