THE STRUCTURE OF THE MONOMERIC PORCINE ODORANT BINDING-PROTEIN SHEDS LIGHT ON THE DOMAIN SWAPPING MECHANISM

The X-ray structure of the porcine odorant binding protein (OBPp) was determined at 2.25 Angstrom resolution. This lipocalin is a monomer an d is devoid of naturally occurring bound ligand, contrary to what was observed in the case of bovine OBP [Tegoni, M., et al. (1996) Nat. Str uct. Biol. 3, 863-867; Bianchet, M. A., et al. (1996) Nat. Struct. Bio l. 3, 934-939]. In this latter protein, a dimer without any disulfide bridges, domain swapping was found to occur between the beta- and alph a-domains. A single Gly (121) insertion was found in OBPp when it was compared to OBPb, which may prevent domain swapping from taking place. The presence of a disulfide bridge between the OBPp beta- and alpha-d omains (cysteines 63 and 155) may lock the resulting fold in a nonswap ped monomeric conformation. Comparisons with other OBPs indicate that the two cysteines involved in the OBPp disulfide bridge are conserved in the sequence, suggesting that OBPp may be considered a prototypic O BP fold, and not OBPb.