Molecular cloning and characterization of hazel pollen protein (70 kD) as a luminal binding protein (BiP): a novel cross-reactive plant allergen

Background: Tree pollen contains many allergens showing cross-reactivity to proteins from pollen, seeds, and fruits of different plant species. Amongst Fagales, responsible for several allergenic responses, hazel provides the best material to study pollen as well as food allergens in one species. The aim of this study was to identify and characterize the physiological function of an allergen from hazel pollen and to determine possible cross-reactivity to proteins from hazelnut. Methods: Monoclonal antibodies (mAbs) against hazel pollen crude extract were produced. On the basis of IgE binding, demonstrated by sera from patients allergic to hazel pollen, one mAb indicating the best correlation has been selected, and the putative allergen was purified by preparative gel electrophoresis. Isoforms were investigated by two-dimensional PAGE, and for molecular identification a hazel pollen cDNA library was constructed. In situ localization of the allergen during pollen development was performed by immunofluorescence labelling. Results: Immunological staining of crude hazel pollen extract with specific IgE and mAb revealed a 70-kD protein. Immunoblot studies with mAb showed cross-reactive proteins of 70-72 kD in different plant tissues and species. After protein purification, the IgE-binding reactivity of the allergen has been reconfirmed, and two isoforms were detected. Molecular cloning identified the allergen as a luminal binding protein (BiP) of the Hsp70 family with 88-92% sequence identity in various plants. Further immunocytological studies indicated involvement of BiP during pollen development. Conclusions: Chaperons like BiP play an important role in protein synthesis and in the protection of cellular structures during stress-related processes. Because of their highly conserved protein sequences, we propose that such allergens could be responsible for at least a part of the allergenic cross-reactivity between proteins from different pollens and plant foods.