Novel plant pathogenesis-related protein family involved in food allergy

BACKGROUND: Members belonging to 9 different families of plant pathogenesis-related (PR) proteins have been identified as pollen and food allergens. However, no PR-1 protein, a family widely distributed throughout the plant kingdom, has been involved so far in allergic reactions. On the other hand, melon ranges among the most relevant fruits causing food allergy in some countries, but the majority of its allergens remain still unidentified. OBJECTIVE: We sought to identify melon allergens related to plant PR proteins. METHODS: A serum pool or individual sera from 17 patients with allergy to melon confirmed by means of double-blind, placebo-controlled food challenge were used to detect IgE binding proteins of extracts from melon pulp and juice. Cuc m 3 was isolated from melon juice by reverse-phase HPLC and characterized by means of N-terminal amino acid sequencing of internal peptides, matrix-assisted laser desorption/ionization mass spectrometry analysis, direct and inhibition ELISA assays, and skin prick tests. RESULTS: Cuc m 3 was a minor component of the melon juice, with a molecular weight of 16,097 d and a blocked N-terminus. N-terminal amino acid sequences of 3 different peptides derived from endo-Lys C digestion (overall 41 residues) showed more than 60% of sequence identity with PR-1 proteins from grape and cucumber. Cuc m 3 bound IgE from 12 of 17 sera from patients allergic to melon and inhibited approximately 40% and 70% of the IgE binding to melon pulp and juice extracts, respectively. Positive skin prick test responses to purified Cuc m 3 were found in 2 of 14 allergic patients. CONCLUSION: A new melon allergen belonging to the PR-1 protein family has been isolated and characterized. It is the first evidence of the involvement of this plant protein family in food allergy.