Crystallization and preliminary crystallographic analysis of the major horse allergen Equ c 1

The secreted protein Equ c 1 is the major component responsible for the induction of specific IgE antibodies in patients sensitized to horse allergens. Equ c 1 belongs to the lipocalin superfamily of hydrophobic ligand-binding proteins, which also includes other known allergens. Equilibrium sedimentation and gel-filtration studies demonstrate that both the glycosylated form of Equ c 1 purified from horse salivary glands and the non-glycosylated recombinant form expressed in bacteria exist predominantly as dimers in solution. As observed for other dimeric lipocalins, acidic pH and low protein concentration favour dimer dissociation. The recombinant form of Equ c 1 has been crystallized using ammonium sulfate as a precipitant. The crystals belong to the tetragonal space group P41212 with cell parameters a = b = 84.0, c = 56.1 A, and contain a single molecule in the asymmetric unit. A complete data set from native crystals was collected at the synchrotron source in Hamburg to 2.9 A resolution using a frozen crystal, and structure determination is in progress.