The molecular basis for allergen cross-reactivity: crystal structure and IgE-epitope mapping of birch pollen profilin.

The profilins are a group of ubiquitous actin monomer binding proteins that are responsible for regulating the normal distribution of filamentous actin networks in eukaryotic cells. Profilins also bind polyphosphoinositides, which can disrupt the profilin-action complex, and proline-rich ligands which localize profilin to sites requiring extensive actin filament accumulation. Profilins represent cross-reactive allergens for almost 20 % of all pollen allergic patients. We report the X-ray crystal structure of birch pollen profilin (BPP) at 2.4 resolution. The major IgE-reactive epitopes have been mapped and were found to cluster on the N- and C-terminal alpha helices and a segment of the protein containing two strands of the beta sheet. The overall fold of this protein is similar to that of the mammalian and amoeba profilins, however, there is a significant change in the orientation of the N-terminal alpha helix in BPP. This change in orientation alters the topography of a hydrophobic patch on the surface of the molecule, which is thought to be involved in the binding of proline-rich ligands. Profilin has been identified as an important cross-reactive allergen for patients suffering from multivalent type I allergy. The prevalent epitopic areas are located in regions with conserved sequence and secondary structure and overlap the binding sites for natural profilin ligands, indicating that the native ligand-free profilin acts as the original cross-sensitizing agent. Structural homology indicates that the basic features of the G actin-profilin interaction are conserved in all eukaryotic organisms, but suggests that mechanistic differences in the binding of proline-rich ligands may exist. The structure of BPP provides a molecular basis for understanding allergen cross-reactivity.