Crystal structure of a truncated form of porcine odorant-binding protein

The odorant-binding proteins (OBPs) are a family of structurally related molecules that are found in high concentrations in the nasal mucus of vertebrates and bind with moderate affinity a large family of hydrophobic odorants. On the basis of their quaternary structure, the OBPs have been classified as monomers, homodimers, and heterodimers. Porcine OBP was believed for a long time to be a monomer under physiological conditions but there are recent data that support the existence of a monomer-dimer equilibrium. We have determined the crystal structure of a monoclinic form of porcine OBP and found that the truncated molecules, which lack the first 8 amino acids, pack in the cell as dimers that appear to have physiological relevance. The presence in the maps of electron density for an endogenous ligand has also let us identify the side chain of the amino acids that are at the ligand-binding site. In addition, an alternative way of access to the central cavity that binds the ligands is suggested by the particular packing of the molecules in this unit cell.