The archaeabacterial flagellar filament: a bacterial propeller with a pilus-like structure

Common prokaryotic motility modes are swimming by means of rotating internal or external flagellar filaments or gliding by means of retracting pili. The archaeabacterial flagellar filament differs significantly from the eubacterial flagellum: (1) Its diameter is 10-14 nm, compared to 18-24 nm for eubacterial flagellar filaments. (2) It has 3.3 subunits/turn of a 1.9 nm pitch left-handed helix compared to 5.5 subunits/turn of a 2.6 nm pitch right-handed helix for plain eubacterial flagellar filaments. (3) The archaeabacterial filament is glycosylated, which is uncommon in eubacterial flagella and is believed to be one of the key elements for stabilizing proteins under extreme conditions. (4) The amino acid composition of archaeabacterial flagellin, although highly conserved within the group, seems unrelated to the highly conserved eubacterial flagellins. On the other hand, the archaeabacterial flagellar filament shares some fundamental properties with type IV pili: (1) The hydrophobic N termini are largely homologous with the oligomerization domain of pilin. (2) The flagellin monomers follow a different mode of transport and assembly. They are synthesized as pre-flagellin and have a cleavable signal peptide, like pre-pilin and unlike eubacterial flagellin. (3) The archaeabacterial flagellin, like pilin, is glycosylated. (4) The filament lacks a central channel, consistent with polymerization occurring at the cell-proximal end. (5) The diameter of type IV pili, 6-9 nm, is closer to that of the archaeabacterial filament, 10-14 nm. A large body of data on the biochemistry and molecular biology of archaeabacterial flagella has accumulated in recent years. However, their structure and symmetry is only beginning to unfold. Here, we review the structure of the archaeabacterial flagellar filament in reference to the structures of type IV pili and eubacterial flagellar filaments, with which it shares structural and functional similarities, correspondingly.