MUPs

Major urinary proteins (MUPs) are a family of small secreted proteins belonging to the lipocalin superfamily. In the house mouse (Mus musculus) and several related species, MUPs are mainly synthesized in the liver and excreted in urine, where they form a dominant component of urinary scent marks.

MUPs function as pheromone carriers: they bind and stabilize volatile small molecules that act as pheromones and release them slowly from urine, extending the duration of chemical signals. The combined set of MUP isoforms in an individual creates a distinctive chemical fingerprint that can convey information about identity, kinship, reproductive status, and social dominance.

Genetically, MUPs are encoded by a cluster of genes and pseudogenes on a chromosome. In mice, the Mup gene family shows extensive copy number variation and allelic diversity between individuals, producing a large repertoire of MUP proteins even within a single population.

Behavioral studies link MUP variation to social and mating behaviors. Mice can distinguish individuals by urine scent, and MUP profiles influence mate choice, territorial aggression, and pup recognition. Expression levels of MUPs can correlate with social status or breeding condition, though these relationships are complex and context-dependent.

Beyond Mus musculus, MUPs or related lipocalins are found in other rodents, with the precise complement and functions varying among species. The study of MUPs has contributed to understanding chemical communication, olfaction, and the evolution of protein families involved in pheromone signaling.