Abstract
FliI is a protein needed for flagellar assembly in Salmonella typhimurium. It shows sequence similarity to the catalytic beta subunit of the F0F1-ATPase and is even more closely related to putative ATPases in Type III bacterial secretory pathways. A His-tagged version of FliI, which was fully functional in complementation tests, was purified to homogeneity. It had an ATPase activity of 0.16 s-1 at 25 degrees C and pH 7, and a Km for ATP of 0.3 mM; Mg2+ was required. The activity was not affected by inhibitors of the F-, V- or P-type ATPases, or inhibitors of the Type I or Type II bacterial secretory pathways. Mutations K188I and Y363S decreased the ATPase activity about 100-fold, increased the Km about 10-fold, blocked flagellar assembly, and were dominant. Other FliI mutations that disrupted flagellar protein export were found near the N terminus; they permitted essentially wild-type ATPase activity, were not dominant, and showed a dosage-dependent phenotype. We propose that FliI has a C-terminal ATPase domain and an N-terminal domain that interacts with other components in the flagellum-specific export apparatus.
Highlights
We propose that FliI has a C-terminal ATPase domain and an N-terminal domain that interacts with other components in the flagellum-specific export apparatus
The flagellum of Salmonella typhimurium and many other bacteria consists of a long helical filament, a short hook, and a basal body with a central rod and several rings
ATPase activity has been demonstrated for glutathione S-transferase fusions of the S. typhimurium Type III secretion component InvC [17] and FliI [18]
Summary
(Received for publication, August 6, 1996, and in revised form, September 27, 1996). Fan Fan and Robert M. Two of the structural components are exported by the general secretory pathway, which involves processing of an N-terminal signal peptide [2, 3]; these two proteins, FlgI and FlgH, are the subunits of the outer-membrane and periplasmic rings of the basal body. A third type of secretory pathway, Type III, has recently been identified in a variety of animal and plant pathogenic bacteria [1, 9, 15] This pathway is signal peptide-independent and involves many protein components. The flagellar protein export system and the Type III secretion systems all contain a putative ATPase component, which in the case of the flagellar system is FliI. ATPase activity has been demonstrated for glutathione S-transferase fusions of the S. typhimurium Type III secretion component InvC [17] and FliI [18]. We have examined the effects of selected mutations on ATPase activity and on flagellation and motility
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