Immunological and biological properties of Bet v 4, a novel birch polen allergen with two EF-hand calcium-binding domains

E. Engel, K. Richter, P. Briza, B. Simon, H.-J. Rheinberger, M. Breitenbach, F. Ferreira, G. Obermeyer, A.J. Kungl, M. Auer, C. Ebner

Research output: Contribution to journalArticlepeer-review

Abstract

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 Ca-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 Ca, as demonstrated by Ca blot overlay of wild type and calcium-binding deficient mutants of Bet v 4. Among pollen-allergic patients, protein-bound Ca was not an absolute requirement for IgE recognition of Bet v 4. However, disruption of the carboxyl-terminal Ca-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 Ca -binding deficient mutants of Bet v 4. Thus, Bet v 4 and homologous proteins represent a new class of polle-specific Ca-binding allergens that may have a physiological role as inhibitors of cytoplasmic streaming in outgrowing pollen tubes.
Original languageEnglish
Pages (from-to)28630-28637
Number of pages8
JournalJournal of Biological Chemistry
Volume272
Issue number45
DOIs
Publication statusPublished - 7 Nov 1997

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