Glycerol-3-phosphate transport in Haemophilus influenzae: cloning, sequencing, and transcription analysis of the glpT gene

Vaccine Candidates against Nontypeable Haemophilus influenzae: a Review

Nonencapsulated, nontypeable Hemophilus influenzae (NTHi) remains an important cause of acute otitis and respiratory diseases in children and adults. NTHi bacteria are one of the major causes of respiratory tract infections, including acute otitis media, cystic fibrosis, and community-acquired pneumonia among children, especially in developing countries. The bacteria can also cause chronic diseases such as chronic bronchitis and chronic obstructive pulmonary disease in the lower respiratory tract of adults. Such bacteria express several outer membrane proteins, some of which have been studied as candidates for vaccine development. Due to the lack of effective vaccines as well as the spread and prevalence of NTHi worldwide, there is an urgent need to design and develop effective vaccine candidates against these strains.

Salt-bridge dynamics control substrate-induced conformational change in the membrane transporter GlpT

Active transport of substrates across cytoplasmic membranes is of great physiological, medical and pharmaceutical importance. The glycerol-3-phosphate (G3P) transporter (GlpT) of the E. coli inner membrane is a secondary active antiporter from the ubiquitous major facilitator superfamily that couples the import of G3P to the efflux of inorganic phosphate (P(i)) down its concentration gradient. Integrating information from a novel combination of structural, molecular dynamics simulations and biochemical studies, we identify the residues involved directly in binding of substrate to the inward-facing conformation of GlpT, thus defining the structural basis for the substrate-specificity of this transporter. The substrate binding mechanism involves protonation of a histidine residue at the binding site. Furthermore, our data suggest that the formation and breaking of inter- and intradomain salt bridges control the conformational change of the transporter that accompanies substrate translocation across the membrane. The mechanism we propose may be a paradigm for organophosphate:phosphate antiporters.

A genomically/chemically complete module for synthesis of lipid membrane in a minimal cell

A minimal cell is a hypothetical cell defined by the essential functions required for life. We have developed a module for the synthesis of membrane precursors for a mathematical minimal cell model. This module describes, with chemical and genomic detail the production of the constituents required to build a cell membrane and identifies the corresponding essential genes. Membranes allow selective nutrient passage, harmful substance exclusion, and energy generation. Bacterial membrane components range from lipids to fatty acids with embedded proteins and are structurally similar to eukaryotic cell membranes. Membranes are dynamic structures and experimental analyses show great variations in bacterial membrane composition. The flexibility of the model is such that different membrane compositions could be obtained in response to simulated changes in culture conditions. The model's predictions are in close agreement with the observed biological trends. The model's predictions correspond well with the experimental values of total lipid content in cells grown in chemostat culture, but less well with data from batch growth. Cell shape and size results agree especially well for data for growth rate relative to maximum growth rate larger than 0.5; and DNA, RNA, and protein predictions are consistent with experimental observations. A better understanding of the simplest bacterial membrane should lead to insights on the more complex behavior of membranes of higher species as well as identification of potential targets for antimicrobials.

Glycerol-3-phosphate transporter of Escherichia coli: structure, function and regulation

Glycerol-3-phosphate (G3P) plays a major role in glycolysis and phospholipid biosynthesis in the cell. Escherichia coli uses a secondary membrane transporter protein, GlpT, to uptake G3P into the cytoplasm. The crystal structure of the protein was recently determined to 3.3 A resolution. The protein consists of an N- and a C-terminal domain, each formed by a compact bundle of six transmembrane alpha-helices. The substrate-translocation pore is found at the domain interface and faces the cytoplasm. At the closed end of the pore is the substrate binding site, which is formed by two arginine residues. In combination with biochemical data, the crystal structure suggests a single binding site, alternating access mechanism for substrate translocation, namely, the substrate bound at the N- and C-terminal domain interface is transported across the membrane via a rocker-switch type of movement of the domains. Furthermore, GlpT may serve as a structural and mechanistic paradigm for other secondary active membrane transporters.

Differences in genetic and transcriptional organization of the glpTQ operons between Haemophilus influenzae type b and nontypeable strains

The glpTQ operon of Haemophilus influenzae type b (Hib) and nontypeable H. influenzae (NTHi) strains is highly conserved, except for a 1.4-kb glpTQ intergenic region that was found in most Hib strains. The presence of this intergenic region results in divergent glpTQ transcriptional profiles for Hib and NTHi where Hib strains appear to have evolved an alternative promoter for glpQ expression. Based on the intergenic region's low G+C content, we speculate that this DNA fragment was acquired by lateral transfer.

Functional characterization of cysteine residues in GlpT, the glycerol 3-phosphate transporter of Escherichia coli

In Escherichia coli, the GlpT transporter, a member of the major facilitator superfamily, moves external glycerol 3-phosphate (G3P) into the cytoplasm in exchange for cytoplasmic phosphate. Study of intact cells showed that both GlpT and HisGlpT, a variant with an N-terminal six-histidine tag, are inhibited (50% inhibitory concentration approximately 35 microM) by the hydrophilic thiol-specific agent p-mercurichlorobenzosulfonate (PCMBS) in a substrate-protectable fashion; by contrast, two other thiol-directed probes, N-maleimidylpropionylbiocytin (MPB) and [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET), have no effect. Use of variants in which the HisGlpT native cysteines are replaced individually by serine or glycine implicates Cys-176, on transmembrane helix 5 (TM5), as the major target for PCMBS. The inhibitor sensitivity of purified and reconstituted HisGlpT or its cysteine substitution derivatives was found to be consistent with the findings with intact cells, except that a partial response to PCMBS was found for the C176G mutant, suggesting the presence of a mixed population of both right-side-out (RSO) (resistant) and inside-out (ISO) (sensitive) orientations after reconstitution. To clarify this issue, we studied a derivative (P290C) in which the RSO molecules can be blocked independently due to an MPB-responsive cysteine in an extracellular loop. In this derivative, comparisons of variants with (P290C) and without (P290C/C176G) Cys-176 indicated that this residue shows substrate-protectable inhibition by PCMBS in the ISO orientation in proteoliposomes. Since PCMBS gains access to Cys-176 from both periplasmic and cytoplasmic surfaces of the protein (in intact cells and in a reconstituted ISO orientation, respectively) and since access is unavailable when the substrate is present, we propose that Cys-176 is located on the transport pathway and that TM5 has a role in lining this pathway.

Cloning and characterization of a putative human glycerol 3-phosphate permease gene (SLC37A1 or G3PP) on 21q22.3: mutation analysis in two candidate phenotypes, DFNB10 and a glycerol kinase deficiency

Using multiple exons trapped from human chromosome 21 (HC21)-specific cosmids with homology to a putative Arabidopsis thaliana glycerol 3-phosphate permease, we have determined the full-length cDNA sequence of a novel HC21 gene encoding a putative sugar-phosphate transporter (HGMW-approved symbol SLC37A1, aka G3PP). The predicted protein has 12 putative transmembrane domains and is also highly homologous to bacterial glpT proteins. The transcript was precisely mapped to 21q22.3 between D21S49 and D21S113. Comparison of the SLC37A1 cDNA to genomic sequence revealed that the gene encompasses 82 kb, and it is split into 19 coding exons and 7 untranslated exons, which are alternatively spliced in a complex and tissue-specific manner. Glycerol 3-phosphate (G3P) is produced by glycerol kinase (GK) and is found in several biochemical pathways in different cellular compartments, such as the glycerol phosphate shuttle and glycerophospholipid synthesis. Thus SLC37A1 mutations may cause a phenotype similar to GK deficiency. Mutational analyses of SLC37A1 in seven patients with no mutations in the GK gene and low GK activity revealed only nonpathogenetic sequence variants, excluding SLC37A1 as the gene for the phenotype in these patients. SLC37A1 maps in the refined critical region of the autosomal recessive deafness locus, DFNB10, on 21q22.3. Mutation analyses also excluded SLC37A1 as the gene for DFNB10.

Fragmentation heterogeneity of 23S ribosomal RNA in Haemophilus species

The fragmentation of 23S rRNA of 23 Haemophilus influenzae strains and eight strains belonging to other Haemophilus species was investigated. Instead of intact molecules, the 23S rRNA molecules were found to be cleaved into two to five smaller conserved fragments in most strains examined, especially in H. influenzae type b (5/6) and nontypeable strains (5/5). One or two conserved potential cleavage sites were identified by PCR analysis of the strains showing a fragmented 23S rRNA pattern. The relevant nucleotide sequences were determined and compared to H. influenzae Rd, which contains intact 23S rRNA molecules. An identical 112bp long intervening sequence (IVS) at position 542 and a conserved 121-123bp IVS sequence at position 1171 were found in two H. influenzae type b strains and one nontypeable strain. Among the strains with fragmented 23S rRNA, nearly half showed a heterogeneous cleavage pattern due to the dispersion of IVSs among different 23S rRNA operons. The localization of the conserved H. influenzae IVSs coincided well with the extensively studied IVSs among other bacteria, but differed in nucleotide sequence from any other reported IVSs. Therefore, the IVSs of Haemophilus 23S rRNA may originate from a common source that is independent of other bacteria.