A proton pump is a ring of proteins embedded in a cell wall, which moves molecules, atoms, or ions through the wall in a preferred direction. Research into the V-ATPase proton pump in eukaryotes has a ring of six linked protein molecules in the membrane wall and a ring of eight inside the cell. As its name implies, it moves hydrogen ions (protons), through the cell wall. Research into its history shows that greater complexity can evolve without a change in function. The “V” indicates a plant vacuolar pump. They are found not in the external cell wall but in the walls of vacuoles, or liquid-filled spaces, within the cell.
…the complexity of an essential molecular machine—the hexameric transmembrane ring of the eukaryotic V-ATPase proton pump—increased hundreds of millions of years ago. We show that the ring of Fungi, which is composed of three paralogous proteins, evolved from a more ancient two-paralogue complex because of a gene duplication that was followed by loss in each daughter copy of specific interfaces by which it interacts with other ring proteins. These losses were complementary, so both copies became obligate components with restricted spatial roles in the complex. Reintroducing a single historical mutation from each paralogue lineage into the resurrected ancestral proteins is sufficient to recapitulate their asymmetric degeneration and trigger the requirement for the more elaborate three-component ring. Our experiments show that increased complexity in an essential molecular machine evolved because of simple, high-probability evolutionary processes, without the apparent evolution of novel functions. They point to a plausible mechanism for the evolution of complexity in other multi-paralogue protein complexes.
“Evolution of increased complexity in a molecular machine,” Gregory C. Finnigan, Victor Hanson-Smith, Tom H. Stevens & Joseph W. Thornton. Nature (2012). doi:10.1038/nature10724.
Look here for a diagram of V-ATPase. This diagram shows the ancestral molecule, in green, and its more specialized descendents, which need their increased complexity to properly assemble and function. The ancestor is on the left.