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Mucus, the result of an astonishing evolutionary recycling

Evolution is economical. Making new out of old is his mantra. “Evolution behaves like a handyman who (…) would slowly modify his work, (…) cutting here, lengthening there, seizing every opportunity to gradually adapt to its new use”said in 1977 the biologist François Jacob, Nobel Prize in Medicine, in the journal Science. From this random tinkering – it proceeds from pure chance – were born fabulous innovations such as the eye, a well-known example.

A new example of this molecular construction game is told to us in the review Science Advances of August 26. This time, it is described at work in the saliva and mucus that lines the surface of our organs! These viscous fluids lubricate food, protect the mucous membranes from mechanical, chemical or microbial attack, but also allow slugs to slide on the ground…

For more than thirty years, a team from the University of Buffalo (United States) has been interested in the molecules of saliva called mucins, which preserve teeth from cavities. In this new work, she compared the genomes of 49 mammalian species, targeting mucins, which give mucus its viscosity and elasticity.

close to tears

The researchers first noticed that a small mucin found in mouse saliva, Muc10, is unrelated to a similarly sized mucin, MUC7, found in human saliva. In contrast, this mouse mucin shares part of its structure with a human tear protein, PROL1, which is not a mucin.

But there was a size difference. This human tear protein lacks a key region that all mucins have in common – with variations. It is a region formed from the repetition (more than 10 to 100 times) of a sequence which can have up to 169 amino acids, the building blocks of proteins. Crucially, it is rich in three amino acids (proline, threonine and serine, hence its name “PTS domain”) to which cling, like the hairs of a bottle brush, thousands of sugar chains . It is because they are bristling with sugars that mucins make mucus viscous.

“Mucins are made up of 80% sugars and 20% amino acids.explains Isabelle Van Seuningen, director of research at the CNRS, specialist in these molecules at the University of Lille. In our species, this family comprises about twenty different molecules, each having privileged territories. » The mucus that covers the cavities of our mouth, our nose, our lungs or our intestines, for example, thus have different compositions.

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