Slime reveals mechanism of evolution

New mucus molecules were created 15 times in mammals – independently of one another, but according to the same evolutionary mechanism. This is the result of a research group led by Ömer Gökçümen from the State University of New York in Buffalo based on an investigation of the so-called mucins of 49 species of mammals. As the team reports in Science Advances, many of these mucus proteins evolved from proteins with an entirely different function, simply by duplicating certain parts of their genes multiple 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 previously unknown. Normally, groups of proteins with a similar function are formed by duplicating an original gene.

Mucins are the main component of the mucus layer on the mucous membranes. These proteins, which contain a large number of water-binding, branched sugar chains, fulfill important biological functions; the mucus forms a chemical and physical barrier that stops viruses and bacteria and many of the mucins are also signaling substances that control cell functions in the mucous membrane. Despite their common function, however, not all mucins are evolutionarily related. As the working group writes, there are several mucin genes in humans that are “orphans” – that is, they are not related to other mucins or even to other known genes.

A comparison with the mucin genes of the mouse provided a first indication of the solution to the riddle. It has been shown that humans form a small mucin in their saliva, which mice lack. Mice, on the other hand, produce a mucin with a very similar function, but which is not at all related to the human protein. To this end, 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 that covered it with a slimy coat of sugar.

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