biostudies-literatureEinenkel REuropean Research CouncilHuman Frontier Science Program (HFSP)Biotechnology and Biological Sciences Research Council1741-1757https://www.ebi.ac.uk/biostudies/studies/S-EPMC12221982biostudies-literatureUnknown10(7)The bacterial flagellum is essential for motility, adhesion and colonization in pathogens such as Salmonella enterica and Campylobacter jejuni. Its extracellular structure comprises the hook, hook-filament junction, filament and filament cap. Native structures of the hook-filament junction and the cap are lacking, and molecular mechanisms of cap-mediated filament assembly are largely uncharacterized. Here we use cryo-electron microscopy to resolve structures of the complete Salmonella extracellular flagellum including the pentameric FliD cap complex (3.7 Å) and the FlgKL hook-filament junction (2.9 Å), as well as the Campylobacter extracellular flagellum before filament assembly (6.5 Å). This, coupled with structure-guided mutagenesis and functional assays, reveals intermediates of filamenNature microbiologyThe structure of the complete extracellular bacterial flagellum reveals the mechanism of flagellin incorporation.PMC12221982BB/R009759/2864971RGY0080/2021BB/V019732/1Gonzalez-Rodriguez NHarrowell JQin KAl-Otaibi NSEinenkel RSchmidt JErhardt MBeeby MMann DBergeron JRCDrobnic TCohen EJShmakova EfalseThe structure of the complete extracellular bacterial flagellum reveals the mechanism of flagellin incorporation.The bacterial flagellum is essential for motility, adhesion and colonization in pathogens such as Salmonella enterica and Campylobacter jejuni. Its extracellular structure comprises the hook, hook-filament junction, filament and filament cap. Native structures of the hook-filament junction and the cap are lacking, and molecular mechanisms of cap-mediated filament assembly are largely uncharacterized. Here we use cryo-electron microscopy to resolve structures of the complete Salmonella extracellular flagellum including the pentameric FliD cap complex (3.7 Å) and the FlgKL hook-filament junction (2.9 Å), as well as the Campylobacter extracellular flagellum before filament assembly (6.5 Å). This, coupled with structure-guided mutagenesis and functional assays, reveals intermediates of filamen2025-01-01T00:00:00Z2025 Jul2026-06-01T15:32:13.875Z2026-04-08T13:46:46.609ZS-EPMC122219824059528710.1038/s41564-025-02037-0