Cloning and sequencing of a gene encoding a 2S albumin seed storage protein precursor from English walnut (Juglans regia), a major food allergen.

BACKGROUND: Walnuts rank third in per capita consumption of tree nuts in the United States and can be associated with systemic IgE-mediated reactions in some individuals.
OBJECTIVE: The objectives of the study were to clone a gene encoding one of the major food allergens in the walnut kernel and to characterize the recombinant allergen.
METHODS: A cDNA expression library in the lambda vector Uni-ZAP, which was prepared from walnut somatic embryos, was screened by using a patient's sera that reacted with multiple protein bands on immunoblotting.
RESULTS: A cDNA clone containing an insert of 663 bp was identified and named Jug r 1. DNA sequence analysis of this clone revealed that it encoded a protein 142 amino acids in length. Comparison of the encoded protein sequences with protein databases revealed that this clone exhibits a 46.1% identity with the Brazil nut (Bertholletia excelsa) methionine-rich 2S albumin seed storage protein precursor, Ber e 1. Jug r 1 appears to be an important walnut food allergen; 12 of 16 sera from patients allergic to walnuts demonstrated IgE binding to the 2S albumin seed storage protein precursor fusion protein. An IgE-binding inhibition study suggests that the walnut 2S protein precursor undergoes posttranslational modification into a large and small subunit that is similar to castor seed, cottonseed, mustard seed, and Brazil nut 2S seed storage protein allergens. Interestingly, the gene encoding this allergenic protein in Brazil nuts has recently gained notoriety because of its experimental use as a transgene to enhance the nutritional quality of legumes.
CONCLUSION: This is now the sixth definitive 2S albumin seed storage protein demonstrated to bind IgE, suggesting that this class of proteins is inherently allergenic.