Abstract
Recent advances in cryo-electron microscopy (cryo-EM) have opened up the possibility that a large class of biological structures, helical polymers, may now be readily reconstructed at near-atomic resolution. This will have a huge impact, since most of these structures are unlikely to be crystallized. This review focuses on new cryo-EM studies involving three classes of bacterial pili (chaperone-usher, mating, and Type IV) as well as on archaeal flagellar filaments. While it has long been known that one domain within archaeal flagellar filaments is homologous to a domain within bacterial Type IV pilins, the new studies shed light on how homologous and even highly conserved subunits can pack together in different ways with only small changes in sequence.