title: Archaeal flagellin combines a bacterial type IV pilin domain with an Ig-like domain.
abstract: The bacterial flagellar apparatus, which involves ∼40 different proteins, has been a model system for understanding motility and chemotaxis. The bacterial flagellar filament, largely composed of a single protein, flagellin, has been a model for understanding protein assembly. This system has no homology to the eukaryotic flagellum, in which the filament alone, composed of a microtubule-based axoneme, contains more than 400 different proteins. The archaeal flagellar system is simpler still, in some cases having ∼13 different proteins with a single flagellar filament protein. The archaeal flagellar system has no homology to the bacterial one and must have arisen by convergent evolution. However, it has been understood that the N-terminal domain of the archaeal flagellin is a homolog of the N-terminal domain of bacterial type IV pilin, showing once again how proteins can be repurposed in evolution for different functions. Using cryo-EM, we have been able to generate a nearly complete atomic model for a flagellar-like filament of the archaeon Ignicoccus hospitalis from a reconstruction at ∼4-Å resolution. We can now show that the archaeal flagellar filament contains a β-sandwich, previously seen in the FlaF protein that forms the anchor for the archaeal flagellar filament. In contrast to the bacterial flagellar filament, where the outer globular domains make no contact with each other and are not necessary for either assembly or motility, the archaeal flagellin outer domains make extensive contacts with each other that largely determine the interesting mechanical properties of these filaments, allowing these filaments to flex.
authors: Institute of Complex Systems, Forschungszentrum Jülich, 52425 Juelich, Germany;
authors: Tatjana
authors: T
authors: Braun
authors: Nanoport Europe, FEI Company, 5651 GG Eindhoven, The Netherlands;
authors: Matthijn R
authors: MR
authors: Vos
authors: Department of Biological Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel;
authors: Nir
authors: N
authors: Kalisman
authors: Biomolecular Analysis Facility, University of Virginia, Charlottesville, VA 22903;
authors: Nicholas E
authors: NE
authors: Sherman
authors: Department of Microbiology, Archaea Center, University of Regensburg, D-93053 Regensburg, Germany;
authors: Reinhard
authors: R
authors: Rachel
authors: Department of Microbiology, Archaea Center, University of Regensburg, D-93053 Regensburg, Germany;
authors: Reinhard
authors: R
authors: Wirth
authors: Institute of Complex Systems, Forschungszentrum Jülich, 52425 Juelich, Germany; Physics Department, Heinrich Heine University Düsseldorf, 40225 Duesseldorf, Germany; gu.schroeder@fz-juelich.de egelman@virginia.edu.
authors: Gunnar F
authors: GF
authors: Schröder
authors: Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, VA 22903 gu.schroeder@fz-juelich.de egelman@virginia.edu.
authors: Edward H
authors: EH
authors: Egelman
mesh_terms: D001105:Archaea / Q000737:chemistry / Q000235:genetics; D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics; D001419:Bacteria / Q000737:chemistry / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics; D002633:Chemotaxis; D018360:Crystallography, X-Ray; D019143:Evolution, Molecular; D036781:Fimbriae Proteins / Q000737:chemistry* / Q000235:genetics; D010861:Fimbriae, Bacterial / Q000737:chemistry / Q000235:genetics; D005408:Flagellin / Q000737:chemistry* / Q000235:genetics; D019617:Halobacterium salinarum / Q000737:chemistry / Q000235:genetics; D000070557:Immunoglobulin Domains / Q000235:genetics; D000072417:Protein Domains / Q000235:genetics
chemical_list: D019843:Archaeal Proteins; D001426:Bacterial Proteins; D005408:Flagellin; D036781:Fimbriae Proteins
keywords: archaea; cryo-EM; flagellar filaments; helical polymers