Organization of the Flagellar Switch Complex of Bacillus subtilis.

While the protein complex responsible for controlling the direction (clockwise [CW] or counterclockwise [CCW]) of flagellar rotation has been fairly well studied in and , less is known about the switch complex in or other Gram-positive species. Two component proteins (FliG and FliM) are shared between and , but in place of the protein FliN found in , the complex contains the larger protein FliY. Notably, in the signaling protein CheY-phosphate induces a switch from CW to CCW rotation, opposite to its action in Here, we have examined the architecture and function of the switch complex in using targeted cross-linking, bacterial two-hybrid protein interaction experiments, and characterization of mutant phenotypes. In major respects, the switch complex appears to be organized similarly to that in The complex is organized around a ring built from the large middle domain of FliM; this ring supports an array of FliG subunits organized in a similar way to that of , with the FliG C-terminal domain functioning in the generation of torque via conserved charged residues. Key differences from involve the middle domain of FliY, which forms an additional, more outboard array, and the C-terminal domains of FliM and FliY, which are organized into both FliY homodimers and FliM heterodimers. Together, the results suggest that the CW and CCW conformational states are similar in the Gram-negative and Gram-positive switches but that CheY-phosphate drives oppositely directed movements in the two cases. Flagellar motility plays key roles in the survival of many bacteria and in the harmful action of many pathogens. Bacterial flagella rotate; the direction of flagellar rotation is controlled by a multisubunit protein complex termed the switch complex. This complex has been extensively studied in Gram-negative model species, but little is known about the complex in or other Gram-positive species. Notably, the switch complex in Gram-positive species responds to its effector CheY-phosphate (CheY-P) by switching to CCW rotation, whereas in or CheY-P acts in the opposite way, promoting CW rotation. In the work here, the architecture of the switch complex has been probed using cross-linking, protein interaction measurements, and mutational approaches. The results cast light on the organization of the complex and provide a framework for understanding the mechanism of flagellar direction control in and other Gram-positive species.