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 molecula r 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 comb ination of mass spectrometry and Edman degradation procedures. The protein consists of a single polypeptide chain of 157 amino acids, presenting at th e N-terminus a pyroglutamic acid residue. The two cysteine residues, occurr ing in the primary structure at positions 63 and 155, are involved in an in tramolecular disulphide bridge. Sequence comparison with other lipocalins r evealed a good similarity with bovine odorant-binding protein, the only mem ber 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 porci ne protein, suggesting structural similarities in this region of the molecu le. The synthesis of a fluorescent photoaffinity labelling agent and of two tin-containing thymol analogues is also described. These compounds togethe r with other ligands were able to bind the protein as revealed by competiti ve binding experiments.