The cryo-EM structure of the bacterial flagellum cap complex suggests a molecular mechanism for filament elongation

Natalie S Al-Otaibi,Frank Dimaio,Daniel P Farrell,Svetomir B Tzokov,Julien R C Bergeron,Aidan J Taylor,David J Kelly

doi:10.1038/s41467-020-16981-4

Abstract

The bacterial flagellum is a remarkable molecular motor, whose primary function in bacteria is to facilitate motility through the rotation of a filament protruding from the bacterial cell. A cap complex, consisting of an oligomer of the protein FliD, is localized at the tip of the flagellum, and is essential for filament assembly, as well as adherence to surfaces in some bacteria. However, the structure of the intact cap complex, and the molecular basis for its interaction with the filament, remains elusive. Here we report the cryo-EM structure of the Campylobacter jejuni cap complex, which reveals that FliD is pentameric, with the N-terminal region of the protomer forming an extensive set of contacts across several subunits, that contribute to FliD oligomerization. We also demonstrate that the native C. jejuni flagellum filament is 11-stranded, contrary to a previously published cryo-EM structure, and propose a molecular model for the filament-cap interaction.

Highlights

The bacterial flagellum is a remarkable molecular motor, whose primary function in bacteria is to facilitate motility through the rotation of a filament protruding from the bacterial cell
The flagellum is composed of >25 different proteins, and consists of three main regions: the basal body acts as an anchor in the bacterial membrane, and includes the molecular machinery for rotation and protein secretion; the hook forms a junction which protrudes from the outer membrane; and the filament, consisting of multiple repeats of a single protein, forms the propeller[3]
Low-resolution cryo-EM studies of the cap complex in Salmonella enterica have suggested that it consists of five copies of FliD, forming a “stool”-shaped complex with a core “head” domain and five flexible “leg” domains, that interact with the growing end of the filament[5,6]

Summary

Introduction

The bacterial flagellum is a remarkable molecular motor, whose primary function in bacteria is to facilitate motility through the rotation of a filament protruding from the bacterial cell. Crystal structures of the FliD head domain have been reported for several species, and revealed a range of crystallographic symmetries, from tetramers in Serratia marscecens (FliDsm), pentamers in S. enterica (FliDse) to hexamers in E. coli (FliDec) and Pseudomonas aeruginosa (FliDpa)[7,8,9,10]. This observation led to the hypothesis that the cap complex can have different, species-specific oligomeric states. FliDcj is the major antigen in C. jejuni and a target for vaccine design[17,18,19]

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Results

Conclusion