The structure of F-pili

Exchange of DNA between bacteria involves conjugative pili. While the prevailing view has been that F-pili are completely retracted before single-stranded DNA is passed from one cell to another, it has recently been reported that the F-pilus, in addition to establishing the contact between mating cells, serves as a channel for passing DNA between spatially separated cells during conjugation. The structure and function of F-pili are poorly understood. They are built from a single subunit having only 70 residues, and the small size of the subunit has made these filaments difficult to study. Here, we have applied electron cryo-microscopy and single-particle methods to solve the long-existing ambiguity in the packing geometry of F-pilin subunits. We show that the F-pilus has an entirely different symmetry from any of the known bacterial pili as well as any of the filamentous bacteriophages, which have been suggested to be structural homologs. Two subunit packing schemes were identified: one has stacked rings of four subunits axially spaced by approximately 12.8 A, while the other has a one-start helical symmetry with an axial rise of approximately 3.5 A per subunit and a pitch of approximately 12.2 A. Both structures have a central lumen of approximately 30 A diameter that is more than large enough to allow for the passage of single-stranded DNA. Remarkably, both schemes appear to coexist within the same filaments, in contrast to filamentous phages that have been described as belonging to one of two possible symmetry classes. For the segments composed of rings, the twist between adjacent rings is quite variable, while the segments having a one-start helix are in multiple states of both twist and extension. This coexistence of two very different symmetries is similar to what has recently been reported for an archaeal Methanococcus maripaludis pili filament and an archaeal Sulfolobus shibatae flagellar filament.