FIG. 10.

Model for FliN subunit arrangement and location. (A) Doughnut-like arrangement seen in the FliN crystal structure 1yab (5). Positions that gave a high yield of cross-linked dimer are indicated. At each dimer-dimer interface, the structure shows interpretable density for residue 136 of only one subunit, extending only to residue 135 in the other subunit. Residue 136 occurs in an α helix; to obtain a rough estimate of its position, this helix from the first subunit was overlaid on the (shorter) helix of the second. (B) Proposed location of the FliN tetramers in the flagellar basal body. The location and orientation of one FliN tetramer are indicated; the flagellum would contain approximately 34 FliN tetramers in all. The side view of the basal body is from reference 40 (with permission of the publisher), and the end view is from reference 46 (with permission of the publisher). Given this location for FliN and evidence that FliG must lie near the top (15, 20, 25, 30, 48), it follows that FliM is located in the middle of the C-ring wall, as indicated. (C, left) Arrangement of N-terminal segments that can account for the cross-linking results and for the contribution of the segment to stability of the tetramer. The segment containing residues 15 to 25 is hypothesized to form an α helix. Two such helices are in proximity to each other and to the hydrophobic patch of the other FliN dimer, bringing residues 19 and 68 into proximity. Although the arrangement pictured is stable enough to allow high-yield cross-linking between positions 19 and 68, the results also indicate that the segments are dynamic (see Discussion). (Right) Detailed view of the hypothesized interface between the N-terminal segments and the body of the dimer. The view is along the segments, one pointing toward the viewer and the other pointing away. Positions that cross-link most efficiently to position 68 are shaded, and positions that allow some cross-linking between the two N-terminal segments are circled.