The Seed Biotinylated Protein of Soybean (Glycine max): A Boiling-Resistant New Allergen (Gly m 7) with the Capacity To Induce IgE-Mediated Allergic Responses

Soybean is a common allergenic food; thus, a comprehensive characterization of all the proteins that cause allergy is crucial to the development of effective diagnostic and immunotherapeutic strategies. A cDNA library was constructed from seven stages of developing soybean seeds to investigate candidate allergens. We searched the library for cDNAs encoding a seed-specific biotinylated protein (SBP) based on its allergenicity in boiled lentils. A full-length cDNA clone was retrieved and expressed as a 75.6-kDa His-tagged recombinant protein (rSBP) in Escherichia coli. Western immunoblotting of boiled bacterial extracts demonstrated specific IgE binding to rSBP, which was further purified by metal affinity and anion exchange chromatographies. Of the 23 allergic sera screened by ELISA, 12 contained IgEs specific to the purified rSBP. Circular dichroism spectroscopy revealed a predominantly unordered structure consistent with SBP's heat stability. The natural homologues (nSBP) were the main proteins isolated from soybean and peanut embryos after streptavidin affinity purification, yet they remained low-abundance proteins in the seed as confirmed by LC-MS/MS. Using capture ELISAs, the soybean and peanut nSBPs were bound by IgEs in 78 and 87% of the allergic sera tested. The soybean nSBP was purified to homogeneity and treatments with different denaturing agents before immunoblotting highlighted the diversity of its IgE epitopes. In vitro activation of basophils was assessed by flow cytometry in a cohort of peanut-allergic children sensitized to soybean. Stronger and more frequent (38%) activations were induced by nSBP-soy compared to the major soybean allergen, Gly m 5. SBPs may represent a novel class of biologically active legume allergens with the structural resilience to withstand many food-manufacturing processes.