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.

Discovery

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). Since they were named, the proteins have been found to be differentially expressed in other glands that secrete products directly into the external environment. These include lacrimal, parotid, submaxillary, sublingual, preputial and mammary glands. In some species, such as cats and pigs, Mups appear not to be expressed in urine at all and are mainly found in saliva. Sometimes the term urinary Mups (uMups) is used to distinguish those Mups expressed in urine from those in other tissues.

Mup genes Between 1979 and 1981, it was estimated that Mups are encoded by a gene family of between 15 and 35 genes and pseudogenes in the mouse and by an estimated 20 genes in the rat. In 2008 a more precise number of Mup genes in a range of species was determined by analyzing the DNA sequence of whole genomes.

Rodents

The mouse reference genome has at least 21 distinct Mup genes (with open reading frames) and a further 21 Mup pseudogenes (with reading frames disrupted by a nonsense mutation or an incomplete gene duplication). They are all clustered together, arrayed side by side across 1.92 megabases of DNA on chromosome 4. The 21 functional genes have been divided into two sub-classes based on position and sequence similarity: 6 peripheral Class A Mups and 15 central Class B Mups. The central Class B Mup gene cluster formed through a number of sequential duplications from one of the Class A Mups. As all the Class B genes are almost identical to each other, researchers have concluded that these duplications occurred very recently in mouse evolution. Indeed, the repetitive structure of these central Mup genes means they are likely to be unstable and may vary in number among wild mice. The Class A Mups are more different from each other and are therefore likely to be more stable, older genes, but what, if any, functional differences the classes have are unknown. The similarity between the genes makes the region difficult to study using current DNA sequencing technology. Consequently, the Mup gene cluster is one of the few parts of the mouse whole genome sequence with gaps remaining, and further genes may remain undiscovered. Rat urine also contains homologous urinary proteins; although they were originally given a different name, α2u-globulins, they have since become known as rat Mups. Rats have 9 distinct Mup genes and a further 13 pseudogenes clustered together across 1.1 megabases of DNA on chromosome 5. Like in mice, the cluster formed by multiple duplications. However, this occurred independently of the duplications in mice, meaning that both rodent species expanded their Mup gene families separately, but in parallel.

Nonrodents Most other mammals studied, including the pig, cow, cat, dog, bushbaby, macaque, chimpanzee and orangutan, have a single Mup gene. Some, however, have an expanded number: horses have three Mup genes, and gray mouse lemurs have at least two. Insects, fish, amphibia, birds and marsupials appear to have disrupted synteny at the chromosomal position of the Mup gene cluster, suggesting the gene family may be specific to placental mammals. Humans are the only placental mammals found not to have any active Mup genes; instead, they have a single Mup pseudogene containing a mutation that causes missplicing, rendering it dysfunctional.

Function

Transport proteins

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