A Soybean G2 Glycinin Allergen (1)
The 11S storage proteins of soybean are called glycinins. When isolated from seeds, these 350-kD globulins are hexamers. The region of IgE binding to a 15-kD peptide that corresponded to amino acid residues is 192–306. The glycinin G1 acidic subunit, when compared to the G2 acidic chain, had stronger IgE binding, even though the sequence homology was quite similar.
Peptides corresponding to the entire amino acid sequence of G2 proglycinin were prepared as 15-mer chains that were offset by 8 amino acids. 6 large regions that bound IgE were identified at amino acid sequence positions 1–23, 57–111, 169–215, 249–271, 329–383 and 449–471. Region 1 was completely soybean specific, with no detectable binding by peanut serum IgE. Region 2 amino acids 62–72 specifically bound serum IgE from peanut-sensitive individuals. Regions 3–6, identified with serum from soybean-sensitive individuals, shared soybean and peanut IgE-binding peptides.
Peptides representing 15 amino acids offset by 2 amino acids were synthesized for each of the 6 IgE-binding regions. Four of the 11 epitopes were located within the 22-kD C-terminal portion of the protein that corresponded to the basic chain of glycinin G2. Epitopes 2, 4, 8 and 11 reacted with IgE from serum from the majority of patients, these were considered to be immunodominant.
Although amino acid sequence homology could be identified between epitopes of glycinin G2 and epitopes of Ara h 3, the primary structure of epitopes 1, 2 and 3 from glycinin G2 do not share IgE binding sequences with the corresponding amino acid sequence in the peanut allergen. The structure of G2 with that of ß-phaseolin shows the trimer formed from ß-phaseolin subunits. Four of the 5 IgE-binding regions were located on one surface and edge of the trimer. Only a single region was found on the opposite surface, and it was located near the periphery of the trimer.
