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

Abstract

The bacterial flagellum is a remarkable molecular motor, present at the surface of many bacteria, whose primary function is to allow motility through the rotation of a long filament protruding from the bacterial cell. A cap complex, consisting of an oligomeric assembly 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. This structure reveals that FliD is pentameric, with the N-terminal region of the protomer forming an unexpected 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 and propose a molecular model for the filament-cap interaction.