Characterization of Enterococcus faecalis alkaline phosphatase and use in identifying Streptococcus agalactiae secreted proteins

Dual colorimetric and fluorogenic probes for visualizing tyrosine phosphatase activity

Two resorufin-based substrates were developed, providing sensitive fluorogenic readouts for PTP activityin vitroand in living cells.

A novel transposon construct expressing PhoA with potential for studying protein expression and translocation in Mycoplasma gallisepticum

BACKGROUND

Mycoplasma gallisepticum is a major poultry pathogen and causes severe economic loss to the poultry industry. In mycoplasmas lipoproteins are abundant on the membrane surface and play a critical role in interactions with the host, but tools for exploring their molecular biology are limited.

RESULTS

In this study we examined whether the alkaline phosphatase gene (phoA ) from Escherichia coli could be used as a reporter in mycoplasmas. The promoter region from the gene for elongation factor Tu (ltuf) and the signal and acylation sequences from the vlhA 1.1 gene, both from Mycoplasma gallisepticum , together with the coding region of phoA , were assembled in the transposon-containing plasmid pISM2062.2 (pTAP) to enable expression of alkaline phosphatase (AP) as a recombinant lipoprotein. The transposon was used to transform M. gallisepticum strain S6. As a control, a plasmid containing a similar construct, but lacking the signal and acylation sequences, was also produced (pTP) and also introduced into M. gallisepticum . Using a colorimetric substrate for detection of alkaline phosphatase activity, it was possible to detect transformed M. gallisepticum . The level of transcription of phoA in organisms transformed with pTP was lower than in those transformed with pTAP, and alkaline phosphatase was not detected by immunoblotting or enzymatic assays in pTP transformants, eventhough alkaline phosphatase expression could be readily detected by both assays in pTAP transformants. Alkaline phosphatase was shown to be located in the hydrophobic fraction of transformed mycoplasmas following Triton X-114 partitioning and in the membrane fraction after differential fractionation. Trypsin proteolysis confirmed its surface exposure. The inclusion of the VlhA lipoprotein signal sequence in pTAP enabled translocation of PhoA and acylation of the amino terminal cysteine moiety, as confirmed by the effect of treatment with globomycin and radiolabelling studies with [14C]palmitate. PhoA could be identified by mass-spectrometry after separation by two-dimensional electrophoresis.

CONCLUSION

This is the first study to express PhoA as a lipoprotein in mycoplasmas. The pTAP plasmid will facilitate investigations of lipoproteins and protein translocation across the cell membrane in mycoplasmas, and the ease of detection of these transformants makes this vector system suitable for the simultaneous screening and detection of cloned genes expressed as membrane proteins in mycoplasmas.

Maltodextrin utilization plays a key role in the ability of group A Streptococcus to colonize the oropharynx

Analysis of multiple group A Streptococcus (GAS) genomes shows that genes encoding proteins involved in carbohydrate utilization comprise some 15% of the core GAS genome. Yet there is a limited understanding of how carbohydrate utilization contributes to GAS pathogenesis. Previous genome-wide GAS studies led us to a focused investigation of MalE, a putative maltodextrin-binding protein. Analysis of 28 strains of 22 distinct M protein serotypes showed that MalE is highly conserved among diverse GAS strains. malE transcript levels were significantly increased during growth in human saliva compared to growth in a chemically defined glucose-containing medium or a nutrient-rich medium. MalE was accessible to antibody binding, indicating that it is expressed on the GAS cell surface. Moreover, growth in human saliva appeared to increase MalE surface expression compared to growth in a nutrient-rich medium. Analysis of a delta malE isogenic mutant strain revealed decreased growth in human saliva compared to wild-type GAS. Radiolabeled carbohydrate binding assays showed that MalE was required for the binding of maltose but not glucose. The delta malE isogenic mutant strain colonized a lower percentage of GAS-challenged mice compared to wild-type and genetically complemented strains. Furthermore, decreased numbers of CFU were recovered from mice infected with the delta malE strain compared to those infected with wild-type GAS. These data demonstrate that maltodextrin acquisition is likely to be a key factor in the ability of GAS to successfully infect the oropharynx. Further investigation into carbohydrate transport and metabolism pathways may yield novel insights into GAS pathogenesis.

Construction and characterization of transposon TnphoZ for the identification of genes encoding exported proteins in Streptococcus agalactiae

Bacterial virulence often depends on exported proteins. To identify genes encoding exported proteins in the neonatal pathogen, group B streptococcus, the transposon TnphoZ was constructed. Here, the coding sequence for the secretion-dependent enzyme alkaline phosphatase from Enterococcus faecalis was fused to the left terminal repeat of Tn917, generating TnphoZ. A collection of TnphoZ mutants was isolated and the DNA flanking the transposon insertion sites was sequenced. Sequence data correlated the expression of high AP activity with transposon insertion into genes encoding predicted exported proteins. It is anticipated that TnphoZ will be suitable for use in other Gram-positive hosts.

Novel protein vaccine candidates against Group B streptococcal infection identified using alkaline phosphatase fusions

Using an alkaline phosphatase-based genetic screening method, we identified a number of proteins that are potentially located on the outer surface of Group B streptococcus (Streptococcus agalactiae). In an enzyme-linked immunosorbent assay, antisera raised against two of the proteins, the streptococcal yutD homologue and a subunit of an ABC transporter, recognised clinically important serotypes of Group B streptococcus. In a neonatal rat model, purified IgG from the sera conferred significant levels of protection against a lethal challenge infection. The proteins identified show potential as protein subunit candidates for vaccines against Group B streptococcal disease in neonates.

A eukaryotic type serine/threonine kinase and phosphatase in Streptococcus agalactiae reversibly phosphorylate an inorganic pyrophosphatase and affect growth, cell segregation, and virulence

Protein phosphorylation is essential for the regulation of cell growth, division, and differentiation in both prokaryotes and eukaryotes. Signal transduction in prokaryotes was previously thought to occur primarily by histidine kinases, involved in two-component signaling pathways. Lately, bacterial homologues of eukaryotic-type serine/threonine kinases and phosphatases have been found to be necessary for cellular functions such as growth, differentiation, pathogenicity, and secondary metabolism. The Gram-positive bacteria Streptococcus agalactiae (group B streptococci, GBS) is an important human pathogen. We have identified and characterized a eukaryotic-type serine/threonine protein kinase (Stk1) and its cognate phosphatase (Stp1) in GBS. Biochemical assays revealed that Stk1 has kinase activity and localizes to the membrane and that Stp1 is a soluble protein with manganese-dependent phosphatase activity on Stk1. Mutations in these genes exhibited pleiotropic effects on growth, virulence, and cell segregation of GBS. Complementation of these mutations restored the wild type phenotype linking these genes to the regulation of various cellular processes in GBS. In vitro phosphorylation of cell extracts from wild type and mutant strains revealed that Stk1 is essential for phosphorylation of six GBS proteins. We have identified the predominant endogenous substrate of both Stk1 and Stp1 as a manganese-dependent inorganic pyrophosphatase (PpaC) by liquid chromatography/tandem mass spectrometry. These results suggest that these eukaryotic-type enzymes regulate pyrophosphatase activity and other cellular functions of S. agalactiae.

Penicillin-binding proteins in Streptococcus agalactiae: a novel mechanism for evasion of immune clearance

Group B streptococci (GBS) remain the most significant bacterial pathogen causing neonatal sepsis, pneumonia and meningitis in the USA despite CDC-recommended chemoprophylaxis strategies for preventing infection. To cause infection pathogens such as GBS must evade recognition and clearance by the host's immune system. Strategies for avoidance of opsonization and phagocytic killing include elaboration of antiopsonophagocytic capsules and surface proteins. During screening for mutants of GBS that were attenuated for virulence in a neonatal rat sepsis model, we identified a mutant with a transposon insertion in the ponA gene. ponA encodes an extra-cytoplasmic penicillin-binding protein PBP1a, a newly identified virulence trait for GBS that promotes resistance to phagocytic killing independent of capsular polysaccharide. Complementation analysis in vivo and in vitro confirmed that the altered phenotypes observed in the mutant were due to the transposon insertion in ponA. Deletion of PBP1a does not affect C3 deposition on GBS suggesting that mechanism by which PBP1a protects GBS from phagocytic killing is distinct from the antiopsonic activity of capsular polysaccharide. This is the first report describing expression of an antiphagocytic surface protein by GBS and represents a novel mechanism for evasion of immune recognition and clearance that may explain the decreased virulence observed in Gram-positive bacterial species for penicillin-binding protein mutants.

Altering of the metal specificity of Escherichia coli alkaline phosphatase

Analysis of sequence alignments of alkaline phosphatases revealed a correlation between metal specificity and certain amino acid side chains in the active site that are metal-binding ligands. The Zn(2+)-requiring Escherichia coli alkaline phosphatase has an Asp at position 153 and a Lys at position 328. Co(2+)-requiring alkaline phosphatases from Thermotoga maritima and Bacillus subtilis have a His and a Trp at these positions, respectively. The mutations D153H, K328W, and D153H/K328W were induced in E. coli alkaline phosphatase to determine whether these residues dictate the metal dependence of the enzyme. The wild-type and D153H enzymes showed very little activity in the presence of Co(2+), but the K328W and especially the D153H/K328W enzymes effectively use Co(2+) for catalysis. Isothermal titration calorimetry experiments showed that in all cases except for the D153H/K328W enzyme, a possible conformation change occurs upon binding Co(2+). These data together indicate that the active site of the D153H/K328W enzyme has been altered significantly enough to allow the enzyme to utilize Co(2+) for catalysis. These studies suggest that the active site residues His and Trp at the E. coli enzyme positions 153 and 328, respectively, at least partially dictate the metal specificity of alkaline phosphatase.

mlpB, a gene encoding a new lipoprotein in Myxococcus xanthus

AIMS

To search for and study the genes involved in the regulation of phosphate in the soil developmental bacterium Myxococcus xanthus.

METHODS AND RESULTS

The mlpB gene encoding a 149 residue polypeptide was identified while screening for genes with products related to phosphate metabolism. The amino terminal 19 residues of MlpB encode a typical prokaryotic signal sequence with a putative lipoprotein cleavage site.

CONCLUSIONS

In this study, a new myxobacterial putative lipoprotein is reported. The data suggest that MlpB may be involved in the secretion of phosphate-related proteins.

SIGNIFICANCE AND IMPACT OF THE STUDY

Soil bacteria have complex regulatory systems for using inorganic phosphate. This nutrient is limiting in the environment, and has a critical importance for growth and in the initiation of differentiation for developmental bacteria. A number of proteins are involved in all these processes, including membrane lipoproteins, which are being increasingly studied in M. xanthus.

Alkaline phosphatase from the hyperthermophilic bacterium T. maritima requires cobalt for activity

The hyperthermophilic bacterium Thermotoga maritima encodes a gene sharing sequence similarities with several known genes for alkaline phosphatase (AP). The putative gene was isolated and the corresponding protein expressed in Escherichia coli, with and without a predicted signal sequence. The recombinant protein showed phosphatase activity toward the substrate p-nitrophenyl-phosphate with a k(cat) of 16 s(-1) and a K(m) of 175 microM at a pH optimum of 8.0 when assayed at 25 degrees C. T. maritima phosphatase activity increased at high temperatures, reaching a maximum k(cat) of 100 s(-1), with a K(m) of 93 microM at 65 degrees C. Activity was stable at 65 degrees C for >24 h and at 90 degrees C for 5 h. Phosphatase activity was dependent on divalent metal ions, specifically Co(II) and Mg(II). Circular dichroism spectra showed that the enzyme gains secondary structure on addition of these metals. Zinc, the most common divalent metal ion required for activity in known APs, was shown to inhibit the T. maritima phosphatase enzyme at concentrations above 0.3 moles Zn: 1 mole monomer. All activity was abolished in the presence of 0.1 mM EDTA. The T. maritima AP primary sequence is 28% identical when compared with E. coli AP. Based on a structural model, the active sites are superimposable except for two residues near the E. coli AP Mg binding site, D153 and K328 (E. coli numbering) corresponding to histidine and tryptophan in T. maritima AP, respectively. Sucrose-density gradient sedimentation experiments showed that the protein exists in several quaternary forms predominated by an octamer.

Cloning of cold-active alkaline phosphatase gene of a psychrophile, Shewanella sp., and expression of the recombinant enzyme

A psychrophilic alkaline phosphatase (EC 3.1.3.1) from Shewanella sp. is a cold-active enzyme that has high catalytic activity at low temperature [Ishida et al. (1998) Biosci. Biotechnol. Biochem., 62, 2246-2250]. Here, we identified the nucleotide sequence of a gene encoding the enzyme after cloning with the polymerase chain reaction (PCR) and inverted PCR techniques. The deduced amino acid sequence of the enzyme contained conserved amino acids found among mesophilic alkaline phosphatases and showed some structural characteristics including a high content of hydrophobic amino acid residues and the lack of single alpha-helix compared with the alkaline phosphatase of Escherichia coli, which were possibly efficient for catalytic reaction at low temperatures. The recombinant enzyme expressed in E. coli was purified to homogeneity with the molecular mass of 41 kDa. The recombinant enzyme had a specific activity of 1,500 units/mg and had high catalytic activity at low temperatures.

Alkaline phosphatase reporter transposon for identification of genes encoding secreted proteins in gram-positive microorganisms

We describe the construction of TnFuZ, a genetic tool for the discovery and mutagenesis of proteins exported from gram-positive bacteria. This tool combines a transposable element (Tn4001) of broad host range in gram-positive bacteria and an alkaline phosphatase gene (phoZ) derived from a gram-positive bacterium that has been modified by removal of the region encoding its export signal. Mutagenesis of Streptococcus pyogenes with TnFuZ ("FuZ" stands for fusions to phoZ) identified genes encoding secreted proteins whose expression was enhanced during growth in an aerobic environment. Thus, TnFuZ should be valuable for analysis of protein secretion, gene regulation, and virulence in gram-positive bacteria.

The Chryseobacterium meningosepticum PafA enzyme: prototype of a new enzyme family of prokaryotic phosphate-irrepressible alkaline phosphatases?

Chryseobacterium meningosepticum is an aerobic Gram-negative rod widely distributed in natural environments. Unlike many bacteria, it produces a phosphate-irrepressible periplasmic alkaline phosphatase (AP). This work describes cloning of the gene encoding that enzyme from C. meningosepticum CCUG 4310 (NCTC 10585), and preliminary characterization of its product. The gene, named pafA, encodes a protein (PafA) of 546 amino acids with a calculated molecular mass of the mature peptide of 58682 Da. PafA exhibits high sequence identity with the PhoV AP of Synechococcus PCC 7942 (49.9% identity) and with the Cda Ca(2+)-dependent ATPase of Myroides odoratus (51.9% identity), while being more distantly related to the PhoD AP of Zymomonas mobilis (22.1% identity) and to the PhoA AP of Escherichia coli (14.0% identity). PafA was partially purified; it exhibits optimal activity at pH 8.5 and is active towards a broad spectrum of substrates including both phosphomonoesters and ATP, with preferential activity for the latter compound. The present findings allow definition of a new family of APs including 60 kDa, periplasmic enzymes whose expression is not influenced by freely available P(i) in the medium. Moreover, PafA can be considered an evolutionary intermediate between Ca(2+)-ATPase of M. odoratus and the APs PhoV of Synechococcus PCC 7942 and PhoD of Z. mobilis.

Alkaline phosphatase activity in whitefly salivary glands and saliva

Alkaline phosphatase activity was histochemically localized in adult whiteflies (Bemisia tabaci B biotype, syn. B. argentifolii) with a chromogenic substrate (5-bromo-4-chloro-3-indolylphosphate) and a fluorogenic substrate (ELF-97). The greatest amount of staining was in the basal regions of adult salivary glands with additional activity traced into the connecting salivary ducts. Other tissues that had alkaline phosphatase activity were the accessory salivary glands, the midgut, the portion of the ovariole surrounding the terminal oocyte, and the colleterial gland. Whitefly nymphs had activity in salivary ducts, whereas activity was not detected in two aphid species (Rhodobium porosum and Aphis gossypii). Whitefly diet (15% sucrose) was collected from whitefly feeding chambers and found to have alkaline phosphatase activity, indicating the enzyme was secreted in saliva. Further studies with salivary alkaline phosphatase collected from diet indicated that the enzyme had a pH optimum of 10.4 and was inhibited by 1 mM cysteine and to a lesser extent 1 mM histidine. Dithiothreitol, inorganic phosphate, and ethylenediaminetetraacetic acid (EDTA) also inhibited activity, whereas levamisole only partially inhibited salivary alkaline phosphatase. The enzyme was heat tolerant and retained approximately 50% activity after a 1-h treatment at 65 degrees C. The amount of alkaline phosphatase activity secreted by whiteflies increased under conditions that stimulate increased feeding. These observations indicate alkaline phosphatase may play a role during whitefly feeding.

Identification and characterization of a second extracellular collagen-like protein made by group A Streptococcus: control of production at the level of translation

A recent study found that group A Streptococcus (GAS) expresses a cell surface protein with similarity to human collagen (S. Lukomski, K. Nakashima, I. Abdi, V. J. Cipriano, R. M. Ireland, S. R. Reid, G. G. Adams, and J. M. Musser, Infect. Immun. 68:6542-6553, 2000). This streptococcal collagen-like protein (Scl) contains a long region of Gly-X-X motifs and was produced by serotype M1 GAS strains. In the present study, a second member of the scl gene family was identified and designated scl2. The Scl2 protein also has a collagen-like region, which in M1 strains is composed of 38 contiguous Gly-X-X triplet motifs. The scl2 gene was present in all 50 genetically diverse GAS strains studied. The Scl2 protein is highly polymorphic, and the number of Gly-X-X motifs in the 50 strains studied ranged from 31 in one serotype M1 strain to 79 in serotype M28 and M77 isolates. The scl1 and scl2 genes were simultaneously transcribed in the exponential phase, and the Scl proteins were also produced. Scl1 and Scl2 were identified in a cell-associated form and free in culture supernatants. Production of Scl1 is regulated by Mga, a positive transcriptional regulator that controls expression of several GAS virulence factors. In contrast, production of Scl2 is controlled at the level of translation by variation in the number of short-sequence pentanucleotide repeats (CAAAA) located immediately downstream of the GTG (Val) start codon. Control of protein production by this molecular mechanism has not been identified previously in GAS. Together, the data indicate that GAS simultaneously produces two extracellular human collagen-like proteins in a regulated fashion.

The RofA binding site in Streptococcus pyogenes is utilized in multiple transcriptional pathways

Understanding the regulation of adhesins defines a pathogenic bacterium's interaction with the local environment within the host. In certain strains of Streptococcus pyogenes, transcription of prtF, the gene which encodes the fibronectin-binding adhesin protein F, is activated by RofA under anaerobic conditions. RofA binds specifically to DNA in its target promoters and autoregulates its own expression. In this study, we have used DNase I protection assays to further investigate the interaction of RofA with its target promoters. In the region between rofA and the gene which encodes protein F (prtF), RofA binds to two distinct sites: a smaller site (17 bp) adjacent to the rofA promoter, and a larger site (40 bp) adjacent to the prtF promoter. Analysis of fusions to a novel reporter gene whose product consists of the fusion of the N-terminal secretion domain of protein F with the C-terminal enzymatic domain of the enterococcal alkaline phosphatase (PhoZ) revealed that the small RofA binding site had no direct role in control of prtF transcription but contributed to regulation of rofA. Comparison in several strains representing different patterns of prtF expression indicated that the larger site was required for activation of rofA and of prtF in all strains by both RofA-dependent and -independent pathways. Thus, it would appear that a common recognition sequence provides separate entries to a final common pathway in S. pyogenes virulence gene expression. The identification of multiple RofA-like proteins and promoters with RofA binding sites implies the existence of a widespread interacting regulatory network.