Sialic acid transport in Haemophilus influenzae is essential for lipopolysaccharide sialylation and serum resistance and is dependent on a novel tripartite ATP-independent periplasmic transporter

Nontypeable Haemophilus influenzae: understanding virulence and commensal behavior

Haemophilus influenzae is genetically diverse and exists as a near-ubiquitous human commensal or as a pathogen. Invasive type b disease has been almost eliminated in developed countries; however, unencapsulated strains - nontypeable H. influenzae (NTHi) - remain important as causes of respiratory infections. Respiratory tract disease occurs when NTHi adhere to or invade respiratory epithelial cells, initiating one or more of several proinflammatory pathways. Biofilm formation explains many of the observations seen in chronic otitis media and chronic bronchitis. However, NTHi biofilms seem to lack a biofilm-specific polysaccharide in the extracellular matrix, a source of controversy regarding their relevance. Successful commensalism requires dampening of the inflammatory response and evasion of host defenses, accomplished in part through phase variation.

Crystal structures of an Extracytoplasmic Solute Receptor from a TRAP transporter in its open and closed forms reveal a helix-swapped dimer requiring a cation for alpha-keto acid binding

BACKGROUND

The import of solutes into the bacterial cytoplasm involves several types of membrane transporters, which may be driven by ATP hydrolysis (ABC transporters) or by an ion or H+ electrochemical membrane potential, as in the tripartite ATP-independent periplasmic system (TRAP). In both the ABC and TRAP systems, a specific periplasmic protein from the ESR family (Extracytoplasmic Solute Receptors) is often involved for the recruitment of the solute and its presentation to the membrane complex. In Rhodobacter sphaeroides, TakP (previously named SmoM) is an ESR from a TRAP transporter and binds alpha-keto acids in vitro.

RESULTS

We describe the high-resolution crystal structures of TakP in its unliganded form and as a complex with sodium-pyruvate. The results show a limited "Venus flytrap" conformational change induced by substrate binding. In the liganded structure, a cation (most probably a sodium ion) is present and plays a key role in the association of the pyruvate to the protein. The structure of the binding pocket gives a rationale for the relative affinities of various ligands that were tested from a fluorescence assay. The protein appears to be dimeric in solution and in the crystals, with a helix-swapping structure largely participating in the dimer formation. A 30 A-long water channel buried at the dimer interface connects the two ligand binding cavities of the dimer.

CONCLUSION

The concerted recruitment by TakP of the substrate group with a cation could represent a first step in the coupled transport of both partners, providing the driving force for solute import. Furthermore, the unexpected dimeric structure of TakP suggests a molecular mechanism of solute uptake by the dimeric ESR via a channel that connects the binding sites of the two monomers.

Succinate dehydrogenase functioning by a reverse redox loop mechanism and fumarate reductase in sulphate-reducing bacteria

Sulphate- or sulphur-reducing bacteria with known or draft genome sequences (Desulfovibrio vulgaris, Desulfovibrio desulfuricans G20, Desulfobacterium autotrophicum [draft], Desulfotalea psychrophila and Geobacter sulfurreducens) all contain sdhCAB or frdCAB gene clusters encoding succinate : quinone oxidoreductases. frdD or sdhD genes are missing. The presence and function of succinate dehydrogenase versus fumarate reductase was studied. Desulfovibrio desulfuricans (strain Essex 6) grew by fumarate respiration or by fumarate disproportionation, and contained fumarate reductase activity. Desulfovibrio vulgaris lacked fumarate respiration and contained succinate dehydrogenase activity. Succinate oxidation by the menaquinone analogue 2,3-dimethyl-1,4-naphthoquinone depended on a proton potential, and the activity was lost after degradation of the proton potential. The membrane anchor SdhC contains four conserved His residues which are known as the ligands for two haem B residues. The properties are very similar to succinate dehydrogenase of the Gram-positive (menaquinone-containing) Bacillus subtilis, which uses a reverse redox loop mechanism in succinate : menaquinone reduction. It is concluded that succinate dehydrogenases from menaquinone-containing bacteria generally require a proton potential to drive the endergonic succinate oxidation. Sequence comparison shows that the SdhC subunit of this type lacks a Glu residue in transmembrane helix IV, which is part of the uncoupling E-pathway in most non-electrogenic FrdABC enzymes.

Role of lgtC in resistance of nontypeable Haemophilus influenzae strain R2866 to human serum

We are investigating a nontypeable Haemophilus influenzae (NTHI) strain, R2866, isolated from a child with meningitis. R2866 is unusually resistant to killing by normal human serum. The serum 50% inhibitory concentration (IC50) for this strain is 18%, approaching that of encapsulated H. influenzae. R3392 is a derivative of R2866 that was found to have increased sensitivity to human serum (IC50, 1.5%). Analysis of tetrameric repeat regions within lipooligosaccharide (LOS) biosynthetic genes in both strains indicated that the glycosyltransferase gene lgtC was out of frame ("off") in most colonies of R3392 but in frame with its start codon ("on") in most colonies of the parent. We sought antigenic and biochemical evidence for modification of the LOS structure. In a whole-cell enzyme-linked immunosorbent assay, strain R3392 displayed reduced binding of the Galalpha1,4Gal-specific monoclonal antibody 4C4. Mass spectrometry analysis of LOS from strain R2866 indicated that the primary oligosaccharide glycoform contained four heptose and four hexose residues, while that of R3392 contained four heptose and three hexose residues. We conclude that the R2866 lgtC gene encodes a galactosyltransferase involved in synthesis of the 4C4 epitope, as in other strains, and that expression of lgtC is associated with the high-level serum resistance that has been observed for this strain. This is the first description of the genetic basis of high-level serum resistance in NTHI, as well as the first description of LOS composition in an NTHI strain for which the complete genome sequence has been determined.

Conservation of Structure and Mechanism in Primary and Secondary Transporters Exemplified by SiaP, a Sialic Acid Binding Virulence Factor from Haemophilus influenzae

Extracytoplasmic solute receptors (ESRs) are important components of solute uptake systems in bacteria, having been studied extensively as parts of ATP binding cassette transporters. Herein we report the first crystal structure of an ESR protein from a functionally characterized electrochemical ion gradient dependent secondary transporter. This protein, SiaP, forms part of a tripartite ATP-independent periplasmic transporter specific for sialic acid in Haemophilus influenzae. Surprisingly, the structure reveals an overall topology similar to ATP binding cassette ESR proteins, which is not apparent from the sequence, demonstrating that primary and secondary transporters can share a common structural component. The structure of SiaP in the presence of the sialic acid analogue 2,3-didehydro-2-deoxy-N-acetylneuraminic acid reveals the ligand bound in a deep cavity with its carboxylate group forming a salt bridge with a highly conserved Arg residue. Sialic acid binding, which obeys simple bimolecular association kinetics as determined by stopped-flow fluorescence spectroscopy, is accompanied by domain closure about a hinge region and the kinking of an alpha-helix hinge component. The structure provides insight into the evolution, mechanism, and substrate specificity of ESR-dependent secondary transporters that are widespread in prokaryotes.

Heterogeneity in tandem octanucleotides within Haemophilus influenzae lipopolysaccharide biosynthetic gene losA affects serum resistance

Haemophilus influenzae is subject to phase variation mediated by changes in the length of simple sequence repeat regions within several genes, most of which encode either surface proteins or enzymes involved in the synthesis of lipopolysaccharides (LPS). The translational repeat regions that have been described thus far all consist of tandemly repeated tetranucleotides. We describe an octanucleotide repeat region within a putative LPS biosynthetic gene, losA. Approximately 20 percent of nontypeable H. influenzae strains contain copies of losA and losB in a genetic locus flanked by infA and ksgA. Of 30 strains containing losA at this site, 24 contained 2 tandem copies of the octanucleotide CGAGCATA, allowing full-length translation of losA (on), and 6 strains contained 3, 4, 6, or 10 tandem copies (losA off). For a serum-sensitive strain, R3063, with losA off (10 repeat units), selection for serum-resistant variants yielded a heterogeneous population in which colonies with increased serum resistance had losA on (2, 8, or 11 repeat units), and colonies with unchanged sensitivity to serum had 10 repeats. Inactivation of losA in strains R3063 and R2846 (strain 12) by insertion of the cat gene decreased the serum resistance of these strains compared to losA-on variants and altered the electrophoretic mobility of LPS. We conclude that expression of losA, a gene that contributes to LPS structure and affects serum resistance, is determined by octanucleotide repeat variation.

Novel ligands for the extracellular solute receptors of two bacterial TRAP transporters

Tripartite ATP-independent periplasmic (TRAP) transporters are relatively common prokaryotic secondary transporters which comprise an extracytoplasmic solute receptor (ESR) protein and two dissimilar membrane proteins or domains, yet the substrates and physiological functions of only a few of these systems are so far known. In this study, a biophysical approach was used to identify the ligands for the purified Rhodobacter capsulatus RRC01191 and Escherichia coli YiaO proteins, which are members of two phylogenetically distinct families of TRAP-ESRs found in diverse bacteria. In contrast to previous indirect evidence pointing to RRC01191 orthologues being involved in polyol uptake, it was shown that RRC01191 binds pyruvate, 2-oxobutyrate and a broad range of aliphatic monocarboxylic 2-oxoacid anions with varying affinities (K(d) values 0.08-3 muM), consistent with a predicted role in monocarboxylate transport related to branched-chain amino-acid biosynthesis. The E. coli YiaMNO TRAP transporter has previously been proposed to be an l-xylulose uptake system [Plantinga et al. (2004) Mol Membr Biol 21, 51-57], but purified YiaO did not bind l- or d-xylulose as judged by fluorescence spectroscopy, circular dichroism or mass spectrometry. Instead, these techniques showed that a breakdown product of l-ascorbate, 2,3-diketo-l-gulonate (2,3-DKG), binds by a simple one-step mechanism with sub-micromolar affinity. The data provide the first evidence for the existence of ESR-dependent transporters for 2-oxoacids and 2,3-DKG, homologues of which appear to be widespread amongst prokaryotes. The results also underline the utility of direct ESR ligand-binding studies for TRAP transporter characterization.