How does complexity evolve? ATPase is a really complicated molecular machine, and how one aspect of its complexity evolved has now been figured out.
Few molecular machines are as complicated as the ATPase, which forms ATP (the “battery” at powers many function in the cell) from ADP, phosphate, and a protein gradient. ATPase consists of an 8-parted, vaguely windmill-like structure that sticks into the cytoplasm, with a wheel-like 6-parted unit anchored in the cell membrane. Protons move through membrane via the wheel and that ends what I know about how the ATP is made, but that’s not the point.
Finnigan et al. (2012) explored the history of a change in the 6-parted wheel-like part. In most eukaryotes (living things with complex cells), this wheel consists of 1 unit we’ll call A and 5 units we’ll call B. They’re arranged as ABBBBB, with the last B attached to the other side of A. In fungi, the wheel consists of 1 unit of A, 4 units of C, and 1 unit of D.
The researchers convincingly demonstrate that units C and D evolved from B via a gene duplication event that occurred about 800 million years ago. Subsequently, several mutation occurred in each gene. One mutation changed 1 amino acid in what became C and made it able to attach only to A at one side and B at the other. A change in one amino acid in what became A allowed it to attach to A on side and B on the other, but didn’t allow it to attach to the other side of A. To complete the ring, therefore, it was now necessary to have ACDDDD (with D attached to the other side of A, making a ring). Thus the complex ABBBBB ring became the even more complex ACDDDD ring.
The most impressive part of this to me was how the authors casually state, “we introduced historical mutations into [one part] by directed mutagenesis” and then tested whether mutations produced functional ATPase’s in live yeast. It’s amazing what humans can do with genetics now.
Finnigan, G. C., V. Hanson-Smith, T. H. Stevens, and J. W. Thornton. 2012. Evolution of increased complexity in a molecular machine. Nature 481: 360-364. doi: 10.1038/nature10724.