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Volume 272, Number 45, Issue of November 7, 1997 pp. 28630-28637
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Immunological and Biological Properties of Bet v 4, a Novel Birch Pollen Allergen with Two EF-hand Calcium-binding Domains

(Received for publication, June 20, 1997, and in revised form, August 26, 1997)

Edwin Engel , Klaus Richter , Gerhard Obermeyer ^§ , Peter Briza , Andreas J. Kungl ^¶ , Birgit Simon , Manfred Auer ^¶ , Christof Ebner ^** , Hans-Jörg Rheinberger , Michael Breitenbach and Fátima Ferreira

From the  Institut für Genetik und Allgemeine Biologie and the ^§ Institut für Pflanzenphysiologie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, the ^¶ Novartis Forschungsinstitut, Brunnerstrasse 59, A-1235 Wien, and the ^** Institut für Allgemeine und Experimentelle Pathologie, Währinger Gürtel 18-20, A-1090 Wien, Austria

We have isolated a cDNA clone coding for a birch pollen allergen, Bet v 4. The deduced amino acid sequence of Bet v 4 contained two typical EF-hand calcium-binding domains. Sequence similarities of Bet v 4 to calmodulin are primarily confined to the calcium-binding domains. However, significant sequence similarities extending outside the Ca²⁺-binding sites were found with a recently described group of pollen-specific allergens of Brassica and Bermuda grass. Both EF-hand domains of Bet v 4 are able to bind Ca²⁺, as demonstrated by ⁴⁵Ca²⁺ blot overlay of wild type and calcium-binding deficient mutants of Bet v 4. Among pollen-allergic patients, protein-bound Ca²⁺ was not an absolute requirement for IgE recognition of Bet v 4. However, disruption of the carboxyl-terminal Ca²⁺-binding domain indicated that most IgE antibodies from allergic patients are directed against this site. IgE inhibition experiments suggested that Bet v 4 represents a highly cross-reactive pollen allergen. Pre-absorption of allergic sera with Bet v 4 drastically reduced IgE binding to proteins of similar molecular weight in pollen extracts from distantly related plant species (e.g. timothy grass, mugwort, lily) but not in extracts from plant-derived foodstuff. To test for a possible biological role in pollen germination and tube growth, we introduced recombinant Bet v 4 protein into growing lily pollen tubes by iontophoresis. As a result, cytoplasmic streaming stopped in the vicinity of the electrode tip, and a slight depolarization of the membrane voltage was measured. These effects were not observed with Ca²⁺-binding deficient mutants of Bet v 4. Thus, Bet v 4 and homologous proteins represent a new class of pollen-specific Ca²⁺-binding allergens that may have a physiological role as inhibitors of cytoplasmic streaming in outgrowing pollen tubes.

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