New mucus molecules were created 15 times in mammals - independently of each other, but according to the same evolutionary mechanism. This is the conclusion reached by a working group led by Ömer Gökçümen from the State University of New York in Buffalo on the basis of a study of the so-called mucins of 49 mammal species. As the team reports in "Science Advances", many of these mucus proteins were created from proteins with a completely different function, simply by doubling certain parts of their genes several times. The repeating sequences are rich in binding sites for branched sugar molecules, through which proteins bind water and become "slimy". According to Gökçümen, this mechanism of how proteins evolve new functions was not previously known. Normally, groups of proteins of similar function are formed by doubling an origin gene.
Mucine are the most important part of the mucus layer on the mucous membranes. These proteins, which are binding with a lot of water, fulfill important biological functions; The tough mucus forms a chemical and physical barrier that is viruses and bacteria; Many of the mucine are also signal substances that control cell functions in the mucous membrane. Despite their common function, however, not all Mucine belong together evolutionarily. As the working group writes, there are several mucin genes in humans who are "orphans"-so there are no relatives to other mucins or even other known genes.
A first clue to the solution of the puzzle was provided by a comparison with the mucin genes of the mouse. As it turned out, humans form a small mucin in saliva, which mice lack. Mice, on the other hand, form a mucin with a very similar function, but which is not at all related to the human protein. For this, Gökçümen and his team found a protein in human tears that closely resembles mouse mucin, but is not mucin itself. In the mouse, however, the original tear protein accumulated the crucial sequence repeats, which covered it with a slimy fur made of sugar.
Further analyzes on Mucin genes of 49 mammals showed: It is by no means an isolated case. In fact, 15 other Mucine appeared in the very limited study, which showed the signs of multiplication of the sugar -rich protein sections. According to the team, this indicates that the evolution of new Mucine is evolutionarily favored - why is still unclear. However, it would be possible, for example, that new components play a role in the defense against diseases in the mucus. In addition, the frequency of the mechanism suggests that doubling of protective protecting with certain properties could also lead to other protein classes to develop biomolecules new functions.
