Amino acid sequence, post-translational modifications, binding and labelling of porcine odorant-binding protein

An odorant-binding protein, migrating in SDS-PAGE with an apparent molecular weight of 22 kDa and an isoelectric point of 4.2, has been purified from pig nasal mucosa. Its complete amino acid sequence was determined by a combination of mass spectrometry and Edman degradation procedures. The protein consists of a single polypeptide chain of 157 amino acids, presenting at the N-terminus a pyroglutamic acid residue. The two cysteine residues, occurring in the primary structure at positions 63 and 155, are involved in an intramolecular disulphide bridge. Sequence comparison with other lipocalins revealed a good similarity with bovine odorant-binding protein, the only member of this class which does not contain disulphide bonds and of which the three-dimensional structure recently has been resolved. Nine out of the 16 residues lining the binding pocket in bovine OBP are conserved in the porcine protein, suggesting structural similarities in this region of the molecule. The synthesis of a fluorescent photoaffinity labelling agent and of two tin-containing thymol analogues is also described. These compounds together with other ligands were able to bind the protein as revealed by competitive binding experiments.