The Structure of F-Pili

Abstract

Exchange of DNA between bacteria involve 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 ∼ 12.8 Å; while the other has a one-start helical symmetry with an axial rise of ∼ 3.5 Å per subunit and a pitch of ∼ 12.2 Å. Both structures have a central lumen of ∼ 30 Å 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.

Force-dependent polymorphism in type IV pili reveals hidden epitopes

“…Our findings open an unforeseen chapter for the study of the dynamics of the Tfp structure as well as of its immunogenic properties. We predict that, besides explaining previously observed structural diversity in the Tfp (35), force-induced structural changes of N, gonorrhoeae Tfp will prove a useful model in understanding the structural diversity of other helical polymers (36,37). Finally, we postulate that the exact structural origin of the present transition may, in the future, facilitate the design of useful biomimetic nanostructures.…”

Evidence for VirB4-Mediated Dislocation of Membrane-Integrated VirB2 Pilin during Biogenesis of the Agrobacterium VirB/VirD4 Type IV Secretion System

“…The central hydrophilic loop (domain III), marked by Cys94, as well as the N-C cyclization junction site of domain I, marked by Cys54, Cys58, and Cys119, also appear to be buried in the interior of the T pilus. If the T pilus has a central lumen analogous to that detected for the F pilus (69,72), these hydrophilic regions might line the pilus lumen. Interestingly, whereas the central domain III of TraA F is also buried in the F pilus, its C terminus is surface displayed and can even accommodate epitope tags that are accessible to antibody (60).…”

Biological Diversity of Prokaryotic Type IV Secretion Systems

“…Recent microscopy studies of the F pili have supplied both high-resolution structural information and insights into the dynamics and biological importance of pilus extension and retraction (66,280). The structure of the F pilus was examined using CryoEM and single-particle methods.…”

“…These two packing arrangements seem to coexist within the pilus structure. The central lumen of the pilus is estimated to be ϳ30 Å, large enough to accommodate ssDNA and an unfolded protein(s), although the question of whether the conjugative pilus serves as a conduit for substrate transfer remains a subject of debate (280).…”