Mucus-producing mucin proteins have emerged repeatedly and independently in mammals, possibly through co-option of existing proteins in mucus factories
August 26, 2022
The proteins that make mucus appear to have evolved in at least 15 independent cases in mammals, possibly by co-opting existing proteins into mucus producers.
From the gooey saliva of a dog to the slippery coating of a snail, slime is almost everywhere in the animal kingdom. “Almost every animal, even yeasts and bacteria, have mucus,” says Omer Gokcumen of the University at Buffalo in New York State. “It is a substance that is essential for life.”
Mammals produce mucus via toilet brush-shaped proteins called mucins, which give stickiness and slide into body fluids. Most animals have numerous mucins whose slimy products combine to provide the right thickness and smoothness in different parts of the body.
Gokcumen first investigated mucins after an unexpected discovery in mice. He noted that the primary mucin in human saliva, called MUC7, is absent in rodents. Conversely, the saliva of mice is thickened with a mucin called MUC10 that humans don’t have. Investigating it, he and his team found that the two mucins were evolutionarily unrelated — a break from the usual trend in which animals share proteins from a common ancestral gene.
Then the team found another surprise. MUC10, the mouse saliva protein, was remarkably similar to the protein that lubricates human tears, called PROL1. Unlike mouse mucin, PROL1 lacked repeats of specific amino acids, the sugar-coated building blocks of a protein.
“We had these two different mucins with two different evolutionary origins. We’re like, that’s really cool, and we want to know if this is really happening somewhere else — or is this just one of those weird, finicky, evolutionary once-in- a-lifetime stories?” says Gokcumen.
Through a genetic analysis of 49 different mammals, from pangolins to rhinoceroses, the team was able to locate 15 different mucins not present in other species, which Gokcumen calls “orphan mucins.” It would have been surprising to find one new mucin, he says, but finding more than a dozen was a shock.
“[These mucins] do not even occur in other species. They are just specific to cows, only specific to ferrets, only specific to humans,” says Gokcumen. “The reason why [mucins] its weird is that they don’t come from a single genetic ancestor, but they seem to evolve independently in different lineages in different ways,” he says.
The team suspects that the new mucins are co-opted from existing proteins. By duplicating sections of specific amino acids, the proteins grow longer and transform into a mucus-producing mucin.
Most species with unique mucins have only one, but others were striking: Ferrets have a total of five mucins that are unique only to them.
Gokcumen expects that there are still many unique mucins to be discovered. Next, he hopes to investigate how many times the slimy stuff has evolved into slugs and snails.
Magazine reference: Scientific progressDOI: 10.1126/sciaadv.abm8757
More on these topics: