The Northern short-tailed shrew is a small, energetic mammal that lives in central and eastern North America. The Mexican beaded lizard is a much larger reptile found in Mexico and Guatemala. These species are separated by a lot of a land and several million years of evolution, yet they share astonishing similarities. Not only are they both venomous, but the toxic proteins in their saliva have evolved in very similar ways from a common ancestor, converging on parallel lethal structures independently of one other.
This discovery, from Yael Aminetzach at Harvard University, shows that adaptations are sometimes very predictable. Despite the many changes that could have shaped the course of venom proteins in lizards and shrews, they seem to have gone down a consistent and similar route.
The northern short-tailed shrew is one of the few venomous mammals, but its poisonous bite is painful to humans and can kill smaller animals. The key to its venom is a protein called BLTX, whose job is to cut another protein in two. This chemical reaction frees a molecule called bradykinin, which widens blood vessels and lowers blood pressure. It's a necessary job, but BLTX is so active that if floods the body with bradykinin - an overdose that leads to paralysis and death.
BLTX is a dark, hyperactive descendant of an ancestral protein called kallikrein-1, which does the same thing but in a much more restrained way. Aminetzach found that BLTX is a longer version of kallikrein and the extra amino acids it has gained have changed the structure of the protein's 'active site'.
The active site is the protein's business end - it allows BLTX to latch onto the right targets and catalyse the relevant chemical reactions. It's also the part of the protein that has changed the most from the harmless kallikrein model; amino acids around BLTX's active site have changed about twice as much as the rest of the protein. As a result, the site is larger, more flexible and better at drawing in its target, and the protein as a whole has become hyperactive.
And amazingly, the Mexican beaded lizard has gone through similar changes. Its venom relies on a protein called GTX, which is also descended from kallikrein. Like BLTX, it too is a longer version of its ancestor, and while its extra amino acids have been shoved into different places, the results are the same. The changes have altered the protein's active site so that it's larger, more flexible and better at drawing in its target.
These changes are very specific to these toxic proteins. By studying 24 relatives of kallikrein, Aminetzach found that none of the non-toxic members of the family have any of the changes that BLTX and GTX share.
This study demonstrates that evolution doesn't work with infinite possibilities. Often, there are only a few roads leading to the same destination. Through different amino acid changes, both BLTX and GTX have evolved similar structures and have turned into weapons. This predictability of venom evolution may be useful to us - for example, Aminetzach suggests that it could allow scientists to more easily identify toxins from others species, even distantly related ones.
Reference: Current Biology 10.1016/j.cub.2009.09.022
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