FLIN IS A MAJOR STRUCTURAL PROTEIN OF THE C-RING IN THE SALMONELLA-TYPHIMURIUM FLAGELLAR BASAL BODY

The Salmonella typhimurium FliN protein has been proposed to form a mu tually interacting complex with FIG and FliM, the switch complex, that is required for flagellar morphogenesis and function. We have used af finity chromatography for purification of extended flagellar basal bod ies sufficient for quantitative analysis of their protein composition. The belied, extended structure is predominantly comprised of the swit ch complex proteins; with FliN present in the most copies (111 +/- 13) . This explains why single, missense fliN, fliG or fliM mutations, fou nd in many non-motile strains, can alter the belied morphology. Cell l ysates from these strains contained the wild-type complement of FliG, FliM and FliN; but the basal bodies lacked the outer, cytoplasmic(C)-r ing of the bell and were separated by sedimentation from FliM and FliN . The amount of FliG present in basal bodies from wild-type and one su ch mutant, FliN100LP, was comparable. These data show that: (1) the mu tations define a FliG and FliMFliN multiple contact interface importan t for motility. (2) FliG is responsible for the increased size of the membrane-embedded MS-ring complex of belied relative to acid-treated b asal bodies. (3) FliN, together with FliM, account for most of the C-r ing. As a major component of the C-ring, FliN is distinct from the oth er proteins implicated in axial flagellar protein export. Inner, cytop lasmic rod basal substructure, seen by negative-stain and quick-freeze replica electron microscopy, may gate such export. Lack of connectivi ty between the cytoplasmic rod and ring substructures places contacts between FliG and FliMFliN at the periphery of the basal body, proximal to the flagellar intramembrane ring particles. This topology is consi stent with models where torque results from interaction of circumferen tial arrays of the switch complex proteins with the ring particles. (C ) 1996 Academic Press Limited