Type III secretion systems (TTSSs) mediate translocation of virulence factors into host cells. We report the 17-angstrom resolution structures of a central component of Salmonella typhimurium TTSS, the needle complex, and its assembly precursor, the bacterial envelope-anchored base. Both the base and the fully assembled needle complex adopted multiple oligomeric states in vivo, and needle assembly was accompanied by recruitment of the protein PrgJ as a structural component of the base. Moreover, conformational changes during needle assembly created scaffolds for anchoring both PrgJ and the needle substructure and may provide the basis for substrate-specificity switching during type III secretion.

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Three-dimensional reconstructions from electron cryomicrographs of the rotor of the flagellar motor reveal that the symmetry of individual M rings varies from 24-fold to 26-fold while that of the C rings, containing the two motor/switch proteins FliM and FliN, varies from 32-fold to 36-fold, with no apparent correlation between the symmetries of the two rings. Results from other studies provided evidence that, in addition to the transmembrane protein FliF, at least some part of the third motor/switch protein, FliG, contributes to a thickening on the face of the M ring, but there was no evidence as to whether or not any portion of FliG also contributes to the C ring. Of the four morphological features in the cross section of the C ring, the feature closest to the M ring is not present with the rotational symmetry of the rest of the C ring, but instead it has the symmetry of the M ring. We suggest that this inner feature arises from a domain of FliG. We present a hypothetical docking in which the C-terminal motor domain of FliG lies in the C ring, where it can interact intimately with FliM.The bacterial flagellum of Salmonella enterica serovar Typhimurium has a reversible rotary motor powered by the proton gradient across the cell's plasma membrane. The flagellar filament, with its corkscrew shape, converts the motor's torque to thrust. Counterclockwise (CCW) rotation of the filament by the motor pushes the cell through the liquid medium, whereas a brief intervening burst of clockwise (CW) rotation causes the cell to tumble. Following the CW burst, the motor resumes CCW rotation, and the cell swims off in a new direction. Approximately 40 proteins are involved in the regulation, assembly, and operation of the flagellum. At least 24 of them are components of the completed flagellum. Of the 24, only 5, i.e., MotA, MotB, FliG, FliM, and FliN, appear to be involved in torque generation (for reviews, see references 1, 2, and 25). MotA and MotB (8, 37) form a proton channel through the plasma membrane (3, 38); they are assumed to form the stator (7, 18). The remaining three are cytoplasmic proteins that form the switch complex (49) and are assumed to be part of the rotor. FliG appears to be most directly involved in torque generation (14,22), and the C-terminal domain of FliG contains key charged residues that interact with charged residues in MotA (53). The N-terminal portion of FliG interacts with FliF, the transmembrane component of the rotor (20, 31), which functions as a mechanical mount for the rotor and couples the motor to the rod or drive shaft. FliM is involved in switching between CCW and CW rotation (34) and binds phospho-CheY (48), tipping the motor's bias toward CW rotation. FliN appears to play a smaller role in rotation and switching (14, 22) but has an important role in flagellar export (46).The gross structural features of the flagellum are a filamentous axial structure and a set of coaxial rings (Fig. 1). MotA and MotB, the putative stator, form a ring of about 10 studs in the plasma membrane (17, 18)...