Major urinary proteins
Major urinary proteins (Mups), also known as α2u-globulins, are a subfamily of proteins found in abundance in the urine and other secretions of many animals. Mups provide a small range of identifying information about the donor animal, when detected by the vomeronasal organ of the receiving animal. They belong to a larger family of proteins known as lipocalins. Mups are encoded by a cluster of genes, located adjacent to each other on a single stretch of DNA, that varies greatly in number between species: from at least 21 functional genes in mice to none in humans. Mup proteins form a characteristic glove shape, encompassing a ligand-binding pocket that accommodates specific small, organic chemicals.
Urinary proteins were first reported in rodents in 1932, during studies by Thomas Addis into the cause of proteinuria. They are potent human allergens, and are largely responsible for a number of animal allergies, including to cats, horses and rodents. Their endogenous function within an animal is unknown, but may involve regulating energy expenditure. However, as secreted proteins they play multiple roles in chemical communication between animals, functioning as pheromone transporters and stabilizers in rodents and pigs. Mups can also act as protein pheromones themselves. They have been demonstrated to promote aggression in male mice, and one specific Mup protein found in male mouse urine is sexually attractive to female mice. Mups can also function as signals between different species: mice display an instinctive fear response on the detection of Mups derived from predators such as cats and rats.
Humans in good health excrete urine that is largely free of protein. Therefore, since 1827 physicians and scientists have been interested in proteinuria, the excess of protein in human urine, as an indicator of kidney disease. To better understand the etiology of proteinuria, some scientists attempted to study the phenomenon in laboratory animals. Between 1932 and 1933 a number of scientists, including Thomas Addis, independently reported the surprising finding that some healthy rodents have protein in their urine. However, it was not until the 1960s that the major urinary proteins of mice and rats were first described in detail. It was found that the proteins are primarily made in the liver of males and secreted through the kidneys into the urine in large quantities (milligrams per day).
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