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

The secreted protein Equ c I is the major component responsible for the ind uction 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 sedimenta tion and gel-filtration studies demonstrate that both the glycosylated form of Equ c 1 purified from horse salivary glands and the non-glycosylated re combinant form expressed in bacteria exist predominantly as dimers in solut ion. As observed for other dimeric lipocalins, acidic pH and low protein co ncentration favour dimer dissociation. The recombinant form of Equ c 1 has been crystallized using ammonium sulfate as a precipitant. The crystals bel ong to the tetragonal space group P4(1)2(1)2 with cell, parameters a = b = 84.0, c = 56.1 Angstrom, and contain a single molecule in the asymmetric un it. A complete data set from native crystals was collected at the synchrotr on source in Hamburg to 2.9 Angstrom resolution using a frozen crystal, and structure determination is in progress.