Bacterial Nanomachines: The Flagellum and Type III Injectisome

Abstract

The bacterial flagellum and the virulence-associated injectisome are complex, structurally related nanomachines that bacteria use for locomotion or the translocation of virulence factors into eukaryotic host cells.The assembly of both structures and the transfer of extracellular proteins is mediated by a unique, multicomponent transport apparatus, the type III secretion system.Here, we discuss the significant progress that has been made in recent years in the visualization and functional characterization of many components of the type III secretion system, the structure of the bacterial flagellum, and the injectisome complex.B acteria swim in their environment by rotat- ing a rigid, helical filament, the flagellum (Berg and Anderson 1973).The flagellum is a sophisticated molecular nanomachine made of about 25 different proteins (Berg and Anderson 1973;Namba and Vonderviszt 1997;Macnab 2003;Kojima and Blair 2004;Chevance and Hughes 2008) (Fig. 1A).Closely related to the flagellum is the type III injectisome, another complex nanomachine that allows Gram-negative bacteria to deliver effector proteins into eukaryotic host cells (Gala ´n and Collmer 1999;Cornelis and Van Gijsegem 2000;Cornelis 2006;Gala ´n and Wolf-Watz 2006) (Fig. 1B).Most extracellular components of the flagellum and the injectisome are exported via a specific type III secretion system (T3SS).Importantly, the export apparatus of both the flagellum and the injectisome are closely related on structural and functional levels (Fig. 2 and Table 1).