Joe Thornton and colleagues at the University of Oregon have worked out the process by which evolution added a component to a cellular machine, according to the Nature News Blog. Cells rely on “machines” made of multiple different protein components to carry out many vital functions in the cell, and molecular and evolutionary biologists have puzzled about how they evolved. Part of this machine is a ring of six proteins that threads through the membrane.
The team first scoured databases and pulled out 139 genetic sequences that encode the ring’s component proteins in a range of eukaryotic organisms. They then used computational methods to work backwards and find the most likely sequences of these proteins hundreds of millions of years ago, at key branching points on the evolutionary tree: just before and just after the ring increased in complexity.
The team synthesized DNA that encoded these “ancestral” proteins and put it into yeast, which had had parts of its own proton pump deleted. The technique allowed Thornton’s team to test in yeast whether various combinations of ancestral proteins produced a working, proton-pumping machine.
The work reveals the pathway by which the two-component ancestral protein became a three-component one. The result challenges the assumption in biology that increased biological complexity evolves because it offers some kind of selective advantage. In this case, the more complex version doesn’t seem to work better or have any other obvious advantage compared with the simpler one; it is more likely that the two proteins were just corrupted by random mutation.
Thornton says that his and other groups will now probably use the same tools to dissect the evolution of more complex molecular machines.
The image above shows a RNA polymerase as it moves along the DNA molecule, copying its sequence of letters into ribonucleic acid.