Manella TEGONI-1, Roberto RAMONI-2, Enrico BIGNETTI-2 , Silvia SPINELLI-1 and Christian CAMBILLAU-1
1 Architecture et Fonction des Macro-molecules Biologiques, UPR 9039.
CNRS 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.
2 Istituto di Biochimica Veterinaria, Facolta di Medicina Veterinaria.
Universita di Parma. Via del Taglio 8, 43100 Parma. Italy
Odorants are known to be generally small hydrophobic molecules which travel through the air and cross the aqueous compartment of the nasal mucus, towards specific olfactory receptors on sensory neurones in mammals, OBPs may play an important role in this transport process 1. Bovine OBP is a homodimer (2 x 19 kDa) belonging to the lipocalin family 2. It shows 7% and 27% sequence identity with two other members of this protein transport family serum retinol binding protein (SRBP) and major urinary protein (MUP), respectively. We have solved the X ray structure of bovine OBP at 2.0. A resolution using molecular replacement methods taking the structure of rat MUP as the starting model. The extended ß-barrel of 0BP is similar to that of MUP and SRBP. However, OBP displays a domain swapped fold 3 which is unique among the lipocalins the a-helix of each monomer stacks against the ß-barrel of the other monomer, yielding a very large dimer interface. Each monomer turns out to have an internal cavity which accomodates a naturally occurring molecule. Previous data in the literature indicate that one odour molecule is bound per dimer 4. The added ligands used for in-vitro studies may bind at a site formed by the swapped domains at the dimer interface. Whether or not this may mean that OBP has a dual function. and that this function results from the existence of binding sites of two kinds, is still a matter of speculation