Three odorant-binding proteins from rabbit nasal mucosa

Following the purification of an odorant-binding protein (OBP) from rabbit nasal mucosa, we have identified, purified and partially characterized two additional OBPs from the nasal tissue of the same animal species. OBP-II is a monomer of 21 kDa and isoelectric point 4.2; OBP-III is a dimer with subunits of 23 kDa and isoelectric point 4.8. Like OBP-I, both these new members bind the odorant 2-isobutyl-3-methoxypyrazine. The partial amino acid sequences of the three OBPs, determined by Edman degradation, confirm that they are members of the OBP family, but reveal poor similarity between them. However, higher similarity is found between each OBP and other members of the lipocalin family. In particular, OBP-I is most similar to bovine OBP (55% identity in the N-terminal region), OBP-II is > 50% identical, limited to its first 18 amino acids, to mouse OBP-I and porcupine OBP-II, while OBP-III shares 26 out of the first 40 amino acids with major urinary protein (MUP) 4, a member of the mouse salivary proteins. The possible role of these proteins in olfactory transduction is also discussed.