Figure 1: Structure of the flagellum. (a) An overview of the structure of the flagellum in Gram-negative bacteria. Abbreviations are as follows: L ring (L), P ring (P), outer membrane (OM), inner membrane (IM), and C ring (C). (b) Components of the flagellar basal body. Export apparatus proteins are shown in light blue. Abbreviations of the export apparatus proteins are as follows: FliR (R), FliP (P), FliO (O), and FliQ (Q). Details of the organization of the export apparatus proteins are not known, although results from genetic studies suggest associations between FlhA and the MS ring [10], FlhB and FliR [11], and FliO and FliP [12]. FliI is an ATPase which forms a heterotrimer with FliH. These proteins function together with other chaperones (not shown) to shuttle protein substrates to the export pore. Upon docking with a platform formed by the large cytoplasmic domains of FlhA and FlhB, a larger complex is formed. In most bacteria the C ring is composed of three different types of protein subunits (FliG, FliM, and FliN). The C ring in Salmonella contains an estimated 26 copies of FliG, 34 copies of FliM, and ~136 copies of FliN. FliG is closest to the membrane and interacts with the MS ring, while FliN is the most distal to the membrane and FliM is situated between FliG and FliN. The H. pylori C ring contains FliG, FliM, and FliN plus an additional protein subunit (FliY) that shares homology with FliN [13]. Additional information on the bacterial flagellar protein export apparatus can be found in reviews by Minamino et al. [1] and Macnab [14].