Ligand binding and partial characterization of sex-specifically expressed lipocalins of submandibular and lacrimal glands of Syrian hamsters

Lipocalins are abundantly expressed secretory proteins that perform diverse roles such as ligand-transport, immunomodulation, cell-signaling, chemical communication etc. In Syrian hamsters (Mesocricetus auratus), male-specific submandibular gland proteins (MSP) and female-specific lacrimal gland proteins (FLP) are sex-specifically secreted in saliva and tears respectively. MSP and FLP are lipocalins having 85% protein sequence identity between themselves and they have 58-61% identity with odorant-binding lipocalins (OBP) of rat and mouse. We purified natural MSP and FLP from hamster tissues and recombinant MSP and FLP after cloning and overexpression in E. coli. We found that MSP and FLP have very similar far ultraviolet-circular dichroic (UV-CD) spectra, typical of lipocalins. Natural MSP was found to be expressed as glycosylated and non-glycosylated forms and had no phosphorylation while FLP had no glycosylation or phosphorylation. In vitro ligand binding studies showed that MSP and FLP bind with high affinity to 2-isobutyl-3-methoxypyrazine (IBMP; a potent bell-pepper odorant) and also other test odorants. Therefore, we conclude that the sex-specific MSP and FLP lipocalins are odorant-binding proteins and they may have a role in odor-mediated communication in male and female hamsters.