STRUCTURE OF A MAJOR BIRCH
POLLEN ALLERGEN MUTANT WITH REDUCED IGE BINDING PROPERTIES.
Osman Mirza1, Henrik Ipsen2,
Jørgen N. Larsen2, Michael D. Spangfort2
and Michael Gajhede1
1Protein Structure Group, Department of
Chemistry, University of Copenhagen, Universitetsparken 5,
DK-2100, Copenhagen, Denmark
2ALK-ABELLO Group, Bøge Alle 10-12, DK-2970
Hørsholm, Denmark
Keywords: Bet v 1, allergy, Immune therapy, X-ray structure.
Around 90% of all birch pollen allergic patients show specific serum IgE reactivity towards the 17.5 kDa major allergen Bet v 1.
The crystal structure of a single site mutant of bet v 1 has been solved to 3.1Å resolution. The main feature of the structure is a seven stranded anti-parallel b-sheet, that wraps around a 25 residues long C-terminal amphipatic a-helix. The b-sheet and the C-terminal part of the long helix are seperated by two consecutive helices. Between the two helices and the b-sheet is a larger cavity and a forked tunnel through the structure.
The mutation introduced, Glu45Ser, seems to abolish the binding of this mutant to a murine monoclonal IgG antibody BV16, and has been shown to have a 50% reduction in the binding to human specific IgE when tested against a pool of serum IgE derived from birch pollen allergic patients. The residue 45 lies in a region that has previously been suggested as a potential IgE binding site (Gajhede et al. 1996), based on sequence similarity and tendency of cross reactivity between several taxonomically related trees (Fagales). The 50% decrease in IgE binding suggests that this region is an overlapping epitope, recognized by several IgE's.
The major use of mutant allergens with low affinity towards IgE's is in the immune therapeutic treatment of allergy patients. In immune therapy the dose of allergen given to the patient is very significant, the higher the dose the better the treatment. By using recombinant allergens one can reduce the anaphylactic side effects, and thereby give the patient a higher dose (Bousquet et al. 1994)
The introduction of a Ser in the place of a Glu has no effect on the overall backbone structure of the protein, indicating the importance of this single amino acid in the binding to IgE's. Secondly it seems that the replacement of the Glu residue has made the binding of an additional water molecule possible, changing the molecular surface in the region significantly.