major urinary proteins (MUPs) and similar proteins
Mouse urine contains major urinary proteins (MUPs) which bind low molecular weight hydrophobic organic compounds such as urinary volatile pheromones such as the male-specific 2-sec-butyl-4,5-dihydrothiazole (SB2HT) which hastens puberty in female mice. The association between MUPs and these volatiles slows the release of the volatiles into the air from urine marks. MUPs may also act as pheromones themselves. MUPs, expressed in the nasal and vomeronasal mucosa, may be important for delivering urinary volatiles to receptors in the vomeronasal organ. This group includes MUPs encoded by central genes in the MUP cluster, as well as those encoded by peripheral genes such as Darcin/Mup20 which binds most of the male pheromone SB2HT in urine and was the first MUP shown to have male pheromonal activity in its own right. This group includes rat MUPs (also called alpha-2U globulins) and other lipocalins such as major horse allergen Equ c 1 and boar salivary lipocalin, a pheromone-binding protein specifically expressed in the submaxillary glands of the boar. It belongs to the lipocalin/cytosolic fatty-acid binding protein family which have a large beta-barrel ligand-binding cavity. Lipocalins are mainly low molecular weight extracellular proteins that bind principally small hydrophobic ligands, and form covalent or non-covalent complexes with soluble macromolecules, as well as membrane bound-receptors. They participate in processes such as ligand transport, modulation of cell growth and metabolism, regulation of immune response, smell reception, tissue development and animal behavior. Cytosolic fatty-acid binding proteins, also bind hydrophobic ligands in a non-covalent, reversible manner, and have been implicated in intracellular uptake, transport and storage of hydrophobic ligands, regulation of lipid metabolism and sequestration of excess toxic fatty acids, as well as in signaling, gene expression, inflammation, cell growth and proliferation, and cancer development.