May Kihara

Substrate specificity of type III flagellar protein export in Salmonella is controlled by subdomain interactions in FlhB

FlhB, an integral membrane protein, gates the type III flagellar export pathway of Salmonella. It permits export of rod/hook‐type proteins before hook completion, whereupon it switches specificity to recognize filament‐type proteins. The cytoplasmic C‐terminal domain of FlhB (FlhBC) is cleaved between Asn‐269 and Pro‐270, defining two subdomains: FlhBCN and FlhBCC. Here, we show that subdomain interactions and cleavage within FlhB are central to substrate‐specificity switching. We found that deletions between residues 216 and 240 of FlhBCN permitted FlhB cleavage but abolished function, whereas a deletion spanning Asn‐269 and Pro‐270 abolished both. The mutation N269A prevented cleavage at the FlhBCN–FlhBCC boundary. Cells producing FlhB(N269A) exported the same amounts of hook‐capping protein as cells producing wild‐type FlhB. However, they exported no flagellin, even when the fliC gene was being expressed from a foreign promoter to circumvent regulation of expression by FlgM, which is itself a filament‐type substrate. Electron microscopy revealed that these cells assembled polyhook structures lacking filaments. Thus, FlhB(N269A) is locked in a conformation specific for rod/hook‐type substrates. With FlhB(P270A), cleavage was reduced but not abolished, and cells producing this protein were weakly motile, exported reduced amounts of flagellin and assembled polyhook filaments.

FlhB regulates ordered export of flagellar components via autocleavage mechanism.

The bacterial flagellum is a predominantly cell-external super-macromolecular construction whose structural components are exported by a flagellum-specific export apparatus. One of the export apparatus proteins, FlhB, regulates the substrate specificity of the entire apparatus; i.e. it has a role in the ordered export of the two main groups of flagellar structural proteins such that the cell-proximal components (rod-/hook-type proteins) are exported before the cell-distal components (filament-type proteins). The controlled switch between these two export states is believed to be mediated by conformational changes in the structure of the C-terminal cytoplasmic domain of FlhB (FlhBC), which is consistently and specifically cleaved into two subdomains (FlhBCN and FlhBCC) that remain tightly associated with each other. The cleavage event has been shown to be physiologically significant for the switch. In this study, the mechanism of FlhB cleavage has been more directly analyzed. We demonstrate that cleavage occurs in a heterologous host, Saccharomyces cerevisiae, deficient in vacuolar proteinases A and B. In addition, we find that cleavage of a slow-cleaving variant, FlhBC(P270A), is stimulated in vitro at alkaline pH. We also show by analytical gel-filtration chromatography and analytical ultracentrifugation experiments that both FlhBC and FlhBC(P270A) are monomeric in solution, and therefore self-proteolysis is unlikely. Finally, we provide evidence via peptide analysis and FlhB cleavage variants that the tertiary structure of FlhB plays a significant role in cleavage. Based on these results, we propose that FlhB cleavage is an autocatalytic process.

Interactions between C ring proteins and export apparatus components: a possible mechanism for facilitating type III protein export.

The flagellar switch proteins of Salmonella, FliG, FliM and FliN, participate in the switching of motor rotation, torque generation and flagellar assembly/export. FliN has been implicated in the flagellar export process. To address this possibility, we constructed 10‐amino‐acid scanning deletions and larger truncations over the C‐terminal domain of FliN. Except for the last deletion variant, all other variants were unable to complement a fliN null strain or to restore the export of flagellar proteins. Most of the deletions showed strong negative dominance effects on wild‐type cells. FliN was found to associate with FliH, a flagellar export component that regulates the ATPase activity of FliI. The binding of FliM to FliN does not interfere with this FliN–FliH interaction. Furthermore, a five‐protein complex consisting of FliG, His‐tagged FliM, FliN, FliH and FliI was purified by nickel‐affinity chromatography. FliJ, a putative general chaperone, is bound to FliM even in the absence of FliH. The importance of the C ring as a possible docking site for export substrates, chaperones and FliI through FliH for their efficient delivery to membrane components of the export apparatus is discussed.

Measurement of membrane potential inBacillus subtilis: A comparison of lipophilic cations, rubidium ion, and a cyanine dye as probes

SummaryTwo of the commonly used probes for measuring membrane potential—lipophilic cations and the cyanine dye diS-C3(5)—indicated nominally opposite results when tetraphenylarsonium ion was added as a drug to suspensions of metabolizingBacillus subtilis cells. [3H]-Triphenylmethylphosphonium uptake was enhanced by the addition, indicating hyperpolarization, yet fluorescence of diS-C3(5) was also enhanced, indicating depolarization. Evidence is presented that both effects are artifactual, and can occur without any change in membrane potential, as estimated by86Rb+ uptake in the presence of valinomycin. The fluorescence studies suggest that tetraphenylarsonium ion displaces the cyanine dye from the cell envelope, or other binding site, into the aqueous phase.The uptake characteristics of the radiolabeled lipophilic cations were quite unusual: At low concentrations (e.g., less than 10 μm for triphenylmethylphosphonium) there was potential-dependent uptake of the label to a stable level, but subsequent addition of nonradioactive lipophilic cation caused further uptake of label to a new stable level. Labeled triphenylmethylphosphonium ion taken up to the first stable level could be displaced by 10mm magnesium ion, whereas86Rb+ uptake was unperturbed. Association of the lipophilic cations with the surface of de-energized cells was concentration-dependent, but there was no evidence for cooperative binding. This phenomenon of stimulated uptake inB. subtilis (which was not seen inEscherichia coli cells or vesicles) is consistent with a two-compartment model with access to the second compartment only being possible above a critical cation concentration. We tentatively propose such a model, in which these compartments are the cell surface and the cytoplasm, respectively.Triphenylmethylphosphonium up to 0.5mm exhibited linear binding to de-energized cells; binding of tetraphenylphosphonium and tetraphenylarsonium was nonlinear but was not saturated at the highest concentration tested (1mm). The usual assumption, that association of the cation with cell surfaces is saturated and so can be estimated on de-energized cells, therefore leads to undercorrected estimates of cytoplasmic uptake inB. subtilis, and hence to overestimates of membrane potential. We describe a more realistic procedure, in which the estimate of extent of binding is based on a mean aqueous concentration related both to the external concentration and to the much higher internal concentration that exists in energized cells. Using this procedure we estimate the membrane potential inB. subtilis to be 120 mV, inside-negative. The procedure is of general applicability, and should yield more accurate estimates of membrane potential in any system where there is significant potential-dependent binding.

The ATPase FliI Can Interact with the Type III Flagellar Protein Export Apparatus in the Absence of Its Regulator, FliH

Salmonella FliI is the ATPase that drives flagellar protein export. It normally exists as a complex together with the regulatory protein FliH. A fliH null mutant was slightly motile, with overproduction of FliI resulting in substantial improvement of its motility. Mutations in the cytoplasmic domains of FlhA and FlhB, which are integral membrane components of the type III flagellar export apparatus, also resulted in substantially improved motility, even at normal FliI levels. Thus, FliH, though undoubtedly important, is not essential.

Intergenic Suppression between the Flagellar MS Ring Protein FliF of Salmonella and FlhA, a Membrane Component of Its Export Apparatus

The MS ring of the flagellar basal body of Salmonella is an integral membrane structure consisting of about 26 subunits of a 61-kDa protein, FliF. Out of many nonflagellate fliF mutants tested, three gave rise to intergenic suppressors in flagellar region II. The pseudorevertants swarmed, though poorly; this partial recovery of motile function was shown to be due to partial recovery of export function and flagellar assembly. The three parental mutants were all found to carry the same mutation, a six-base deletion corresponding to loss of Ala-174 and Ser-175 in the predicted periplasmic domain of the FliF protein. The 19 intergenic suppressors identified all lay in flhA, and they consisted of 10 independent examples at the nucleotide level or 9 at the amino acid level. Since two of the nine corresponded to different substitutions at the same amino acid position, only eight positions in the FlhA protein have given rise to suppressors. Thus, FliF-FlhA intergenic suppression is a fairly rare event. FlhA is a component of the flagellar protein export apparatus, with an integral membrane domain encompassing the N-terminal half of the sequence and a cytoplasmic C-terminal domain. All of the suppressing mutations lay within the integral membrane domain. These mutations, when placed in a wild-type fliF background, had no mutant phenotype. In the fliF mutant background, mutant FlhA was dominant, yielding a pseudorevertant phenotype. Wild-type FlhA did not exert significant negative dominance in the pseudorevertant background, indicating that it does not compete effectively with mutant FlhA for interaction with mutant FliF. Mutant FliF was partially dominant over wild-type FliF in both the wild-type and second-site FlhA backgrounds. Membrane fractionation experiments indicated that the fliF mutation, though preventing export, was mild enough to permit assembly of the MS ring itself, and also assembly of the cytoplasmic C ring onto the MS ring. The data from this study provide genetic support for a model in which at least the FlhA component of the export apparatus physically interacts with the MS ring within which it is housed.

Deletion Analysis of the Flagellar Switch Protein FliG of Salmonella

The flagellar motor/switch complex, consisting of the three proteins FliG, FliM, and FliN, plays a central role in bacterial motility and chemotaxis. We have analyzed FliG, using 10-amino-acid deletions throughout the protein and testing the deletion clones for their motility and dominance properties and for interaction of the deletion proteins with the MS ring protein FliF. Only the N-terminal 46 amino acids of FliG (segments 1 to 4) were important for binding to FliF; consistent with this, an N-terminal fragment consisting of residues 1 to 108 bound FliF strongly, whereas a C-terminal fragment consisting of residues 109 to 331 did not bind FliF at all. Deletions in the region from residues 37 to 96 (segments 4 to 9), 297 to 306 (segment 30), and 317 to 326 (segment 32) permitted swarming, though not at wild-type levels; all other deletions caused paralyzed or, more commonly, nonflagellate phenotype. Except for those near the N terminus, deletions had a dominant negative effect on wild-type cells.

Structure of the cytoplasmic domain of FlhA and implication for flagellar type III protein export.

FlhA is the largest integral membrane component of the flagellar type III protein export apparatus of Salmonella and is composed of an N‐terminal transmembrane domain (FlhATM) and a C‐terminal cytoplasmic domain (FlhAC). FlhAC is thought to form a platform of the export gate for the soluble components to bind to for efficient delivery of export substrates to the gate. Here, we report a structure of FlhAC at 2.8 Å resolution. FlhAC consists of four subdomains (ACD1, ACD2, ACD3 and ACD4) and a linker connecting FlhAC to FlhATM. The sites of temperature‐sensitive (ts) mutations that impair protein export are distributed to all four domains, with half of them at subdomain interfaces. Analyses of the ts mutations and four suppressor mutations to the G368C ts mutation suggested that FlhAC changes its conformation for its function. Molecular dynamics simulation demonstrated an open‐close motion with a 5–10 ns oscillation in the distance between ACD2 and ACD4. These results along with further mutation analyses suggest that a dynamic domain motion of FlhAC is essential for protein export.

Analysis of the Cytoplasmic Domains of Salmonella FlhA and Interactions with Components of the Flagellar Export Machinery

Most flagellar proteins are exported via a type III export apparatus which, in part, consists of the membrane proteins FlhA, FlhB, FliO, FliP, FliQ, and FliR and is housed within the membrane-supramembrane ring formed by FliF subunits. Salmonella FlhA is a 692-residue integral membrane protein with eight predicted transmembrane spans. Its function is not understood, but it is necessary for flagellar export. We have created mutants in which potentially important sequences were deleted. FlhA lacking the amino-terminal sequence prior to the first transmembrane span failed to complement and was dominant negative, suggesting that the sequence is required for function. Similar effects were seen in a variant lacking a highly conserved domain (FHIPEP) within a putative cytoplasmic loop. Scanning deletion analysis of the cytoplasmic domain (FlhAc) demonstrated that substantially all of FlhAc is required for efficient function. Affinity blotting showed that FlhA interacts with several other export apparatus membrane proteins. The implications of these findings are discussed, and a model of FlhA within the export apparatus is presented.

Role of the C-Terminal Cytoplasmic Domain of FlhA in Bacterial Flagellar Type III Protein Export

For construction of the bacterial flagellum, many of the flagellar proteins are exported into the central channel of the flagellar structure by the flagellar type III protein export apparatus. FlhA and FlhB, which are integral membrane proteins of the export apparatus, form a docking platform for the soluble components of the export apparatus, FliH, FliI, and FliJ. The C-terminal cytoplasmic domain of FlhA (FlhA(C)) is required for protein export, but it is not clear how it works. Here, we analyzed a temperature-sensitive Salmonella enterica mutant, the flhA(G368C) mutant, which has a mutation in the sequence encoding FlhA(C). The G368C mutation did not eliminate the interactions with FliH, FliI, FliJ, and the C-terminal cytoplasmic domain of FlhB, suggesting that the mutation blocks the export process after the FliH-FliI-FliJ-export substrate complex binds to the FlhA-FlhB platform. Limited proteolysis showed that FlhA(C) consists of at least three subdomains, a flexible linker, FlhA(CN), and FlhA(CC), and that FlhA(CN) becomes sensitive to proteolysis by the G368C mutation. Intragenic suppressor mutations were identified in these subdomains and restored flagellar protein export to a considerable degree. However, none of these suppressor mutations suppressed the protease sensitivity. We suggest that FlhA(C) not only forms part of the docking platform for the FliH-FliI-FliJ-export substrate complex but also is directly involved in the translocation of the export substrate into the central channel of the growing flagellar structure.