Bacterial Flagella

Abstract

Abstract Bacteria propel themselves through liquid or over semisolid media using rotation of a propeller‐like organelle, the flagellum. The membrane‐embedded basal body is connected via a flexible coupling structure, the hook, to the rigid, external filament. The flagellum is a sophisticated, molecular nanomachine, and to build a flagellum, several thousand copies of more than two dozen proteins assemble in an ordered process. Flagellar gene expression is temporally coupled to its assembly state, and complex self‐assembly mechanisms control the accurate size and subunit composition of flagellar substructures. A specialised protein export machine, termed type‐III secretion system, transports flagellar substrates across the inner membrane. Secreted substrates diffuse through a narrow channel within the flagellum and self‐assemble at the distal end. Intermittent secretion of a molecular ruler protein controls the hook length, while diffusion of flagellin proteins intrinsically limits the length of the flagellar filament. Key Concepts Bacteria swim through their environment by rotating an extracellular appendage, the flagellum. Rotation of the flagellum is energised by influx of ions through stator proteins that are attached to the cell body, which is called the ion motive force. The bacterial flagellum consists of three major parts: (1) the basal body as the engine, (2) a flexible joint structure, the hook, that connects the engine to (3) the long external filament. Assembly of the flagellum involves dozens of proteins and the coordination of gene expression to the assembly state of the flagellum. A specialised protein export system, a type‐III secretion apparatus, is responsible for the export of flagellar secretion substrates, including the majority of the external structures of the flagellum. Protein export via the flagellar‐specific type‐III secretion system is dependent on the proton motive force. The length of the hook structure is regulated to an optimal length of 55 nm in Salmonella . A molecular ruler controls the final length and catalyses a switch in secretion specificity from early to late substrate secretion mode. After proton motive force‐dependent injection, flagellin molecules, the building blocks of the filament, diffuse through a narrow channel and self‐assemble at the distal tip of the structure.