Reducing irreducible complexity: divergence of quaternary structure and function in macromolecular assemblies.

The bacterial flagellar system is an intricate assembly (containing approximately 40 different proteins) that is involved in both protein secretion and bacterial motility. It has also become the icon of the neo-creationist movement in the United States, with the argument that it shows 'irreducible complexity' and could not have been the product of evolution. Recent studies provide new insights into the evolution of the flagellar system and lead to the suggestion that the divergence of quaternary structure in protein assemblies may be an underappreciated mechanism for rapid evolutionary divergence. Work on the enzyme FucU, involved in fucose metabolism, may suggest similar conclusions. Copyright 2009 Elsevier Ltd. All rights reserved.