Background: Walnuts and other tree nuts are important food-allergen sources that have the potential to be associated with life-threatening, IgE-mediated systemic reactions in some individuals.
Objective: The purpose of this study was to characterize a complementary (c)DNA clone encoding one of the walnut food allergens.
Methods: A cDNA expression library prepared from walnut somatic embryo was screened for IgE reactivity with patient serum. A reactive clone of 2060 bp, which encoded a protein of 593 amino acids in length, was subcloned by excision into the pGEX expression vector. IgE-binding inhibition experiments were performed.
Results: A recombinant fusion protein was induced and shown to bind serum IgE from 9 of 15 patients tested, thus identifying a major allergen. This clone, named Jug r 2, exhibited significant homology with genes encoding the vicilin group of seed proteins. An IgE-binding inhibition experiment suggested that the encoded protein undergoes posttranslational modification into at least one major polypeptide (47 kd) and possibly several others, which is similar to the vicilin-like proteins characterized in cocoa bean (Theobroma cacao) and cottonseed (Gossypium hirsutum). N-terminal sequencing of the 47-kd band, Jug r 2, identified it as a mature protein obtained from the precursor. A second IgE-binding inhibition experiment showed that there is minimal or no cross-reactivity between Jug r 2 and pea vicilin, peanut proteins, or cacao proteins.
Conclusion: Jug r 2 is the third vicilin food allergen identified in addition to vicilins from soy and peanut. The availability of recombinant food allergens should help advance studies on the immunopathogenesis and possible treatment of IgE-mediated food hypersensitivity.