So in mammals, proteins called mucins evolved and with them the often beneficial characteristics of saliva in our mouths. The research, which uses a gel electrophoresis technique to separate mucins from other proteins in the saliva of different mammals, reveals a multiple and often surprising evolution of proteins called mucins. While these proteins are best known as components of mucus, promoting its gooey consistency, they actually perform several functions.
A frequent and continuous phenomenon of mucinization
The scientists thus identify several events or stages of evolution or “mucinization” from a protein which, at the start, was not a mucin. At some point, the evolutionary process adds a new section to this non-mucin base, i.e. a short chain of building blocks or amino acids in the form of sugar molecules. Over time, this new region duplicates, adding more copies to further lengthen the protein, which eventually becomes mucin.
Thus, a protein which is not a mucin becomes a mucin by gaining repeats. This is an important way in which evolution makes mucus.
“It’s an evolutionary trick”,
summarizes the researcher Gokcumen, professor of biological sciences at UB.
Viscosity above all: it is these duplicate regions or “repeats” that are essential to the function of mucin. Especially for viscosity: the sugars covering these sections protrude outward like spiky hairs that give mucins that viscous property vital to the many functions of these proteins.
What biological functions? The “PTS” repeats named so because of their high content of amino acids proline, threonine and serine help mucins in their important biological functions which range from lubrication and protection of tissue and mucosal surfaces to “slipperiness”. of our food. Beneficial microbes have evolved to live on mucus-covered surfaces, and mucus can at the same time act as a protective barrier to defend us against disease by shielding us from unwanted pathogens.
Towards new drugs? While the first mucin that was purified and biochemically characterized came from a salivary gland, many research teams have been working for decades on the mucins present in saliva, in particular because of their protective capacity: thus, mucins , among other benefits, protects our teeth from cavities and helps balance the oral microbiota.
A small salivary mucin in humans called MUC7: the scientists note that this mucin was not present in mice. Mice have a similar sized salivary mucin called MUC10. So did these 2 proteins, in humans and in mice, follow the same evolutionary process? While this does not appear to be the case and MUC10 does not appear to be related to MUC7, a protein found in human tears called PROL1 does, on the other hand, share part of the MUC10 structure. PROL1 looked a lot like MUC10, minus some repeats that make MUC10 a mucin. The researchers here make the exciting hypothesis that “this tear gene” ends up being reused. It gains repeats that confer mucin function, and it becomes abundantly expressed in the saliva of mice. Other research, which provides other examples of the same phenomenon, confirms this evolutionary process which converts non-mucin proteins into mucins via the addition of PTS repeats.
“If these mucins keep evolving from non-mucins over and over again in different species at different times,
this suggests that there is some sort of adaptive pressure that makes or confirms them as beneficial.
If this evolutionary mechanism is derailed, it can also be an explanation for the development of certain diseases such as cancers or mucosal diseases”.
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