A phosphoprotein from the archaeon Sulfolobus solfataricus with protein-serine/threonine kinase activity

Novel Tyrosine Kinase-Mediated Phosphorylation With Dual Specificity Plays a Key Role in the Modulation of Streptococcus pyogenes Physiology and Virulence

Streptococcus pyogenes (Group A Streptococcus, GAS) genomes do not contain a gene encoding a typical bacterial-type tyrosine kinase (BY-kinase) but contain an orphan gene-encoding protein Tyr-phosphatase (SP-PTP). Hence, the importance of Tyr-phosphorylation is underappreciated and not recognized for its role in GAS pathophysiology and pathogenesis. The fact that SP-PTP dephosphorylates Abl-tyrosine kinase-phosphorylated myelin basic protein (MBP), and SP-STK (S. pyogenes Ser/Thr kinase) also autophosphorylates its Tyr¹⁰¹-residue prompted us to identify a putative tyrosine kinase and Tyr-phosphorylation in GAS. Upon a genome-wide search of kinases possessing a classical Walker motif, we identified a non-canonical tyrosine kinase M5005_Spy_1476, a ∼17 kDa protein (153 aa) (SP-TyK). The purified recombinant SP-TyK autophosphorylated in the presence of ATP. In vitro and in vivo phosphoproteomic analyses revealed two key phosphorylated tyrosine residues located within the catalytic domain of SP-TyK. An isogenic mutant lacking SP-TyK derived from the M1T1 strain showed a retarded growth pattern. It displayed defective cell division and long chains with multiple parallel septa, often resulting in aggregates. Transcriptomic analysis of the mutant revealed 287 differentially expressed genes responsible for GAS pathophysiology and pathogenesis. SP-TyK also phosphorylated GAS CovR, WalR, SP-STP, and SDH/GAPDH proteins with dual specificity targeting their Tyr/Ser/Thr residues as revealed by biochemical and mass-spectrometric-based phosphoproteomic analyses. SP-TyK-phosphorylated CovR bound to PcovR efficiently. The mutant displayed sustained release of IL-6 compared to TNF-α during co-culturing with A549 lung cell lines, attenuation in mice sepsis model, and significantly reduced ability to adhere to and invade A549 lung cells and form biofilms on abiotic surfaces. SP-TyK, thus, plays a critical role in fine-tuning the regulation of key cellular functions essential for GAS pathophysiology and pathogenesis through post-translational modifications and hence, may serve as a promising target for future therapeutic developments.

The biology of thermoacidophilic archaea from the order Sulfolobales

Thermoacidophilic archaea belonging to the order Sulfolobales thrive in extreme biotopes, such as sulfuric hot springs and ore deposits. These microorganisms have been model systems for understanding life in extreme environments, as well as for probing the evolution of both molecular genetic processes and central metabolic pathways. Thermoacidophiles, such as the Sulfolobales, use typical microbial responses to persist in hot acid (e.g. motility, stress response, biofilm formation), albeit with some unusual twists. They also exhibit unique physiological features, including iron and sulfur chemolithoautotrophy, that differentiate them from much of the microbial world. Although first discovered >50 years ago, it was not until recently that genome sequence data and facile genetic tools have been developed for species in the Sulfolobales. These advances have not only opened up ways to further probe novel features of these microbes but also paved the way for their potential biotechnological applications. Discussed here are the nuances of the thermoacidophilic lifestyle of the Sulfolobales, including their evolutionary placement, cell biology, survival strategies, genetic tools, metabolic processes and physiological attributes together with how these characteristics make thermoacidophiles ideal platforms for specialized industrial processes.

Autophosphorylation and Cross-Phosphorylation of Protein Kinases from the Crenarchaeon Sulfolobus islandicus

Protein phosphorylation, one of the most important post-translational modifications, regulates almost every cellular process. Although signal transduction by protein phosphorylation is extensively studied in Eukaryotes and Bacteria, the knowledge of this process in archaea is greatly lagging behind, especially for Ser/Thr/Tyr phosphorylation by eukaryotic-like protein kinases (ePKs). So far, only a few studies on archaeal ePKs have been reported, most of which focused on the phosphorylation activities in vitro, but their physiological functions and interacting network are still largely unknown. In this study, we systematically investigated the autophosphorylation and cross-phosphorylation activities of ePKs from Sulfolobus islandicus REY15A using proteins expressed in Escherichia coli or S. islandicus. In vitro kinase assay showed that 7 out of the 11 putative ePKs have autophosphorylation activity. A protein Ser/Thr phosphatase, SiRe_1009, was able to dephosphorylate various autophosphorylated ePKs, confirming that these proteins are Ser/Thr kinases. Two ePKs, SiRe_2030 and SiRe_2056, homologs of typical eukaryotic PKs involved in peptide synthesis in response to various cellular stresses, exhibit highly efficient phosphorylation activities on both themselves and other ePKs. Overexpression of the protein kinases in vivo revealed that elevated level of either SiRe_1531 or SiRe_2056 inhibited the cell growth of S. islandicus cells. Finally, a phosphorylation network of the protein kinases was proposed and their putative physiological roles were discussed.

Expanding the archaellum regulatory network - the eukaryotic protein kinases ArnC and ArnD influence motility of Sulfolobus acidocaldarius

Expression of the archaellum, the archaeal‐type IV pilus‐like rotating motility structure is upregulated under nutrient limitation. This is controlled by a network of regulators, called the archaellum regulatory network (arn). Several of the components of this network in Sulfolobus acidocaldarius can be phosphorylated, and the deletion of the phosphatase PP2A results in strongly increased motility during starvation, indicating a role for phosphorylation in the regulation of motility. Analysis of the motility of different protein kinase deletion strains revealed that deletion of saci_0965, saci_1181, and saci_1193 resulted in reduced motility, whereas the deletion of saci_1694 resulted in hypermotility. Here ArnC (Saci_1193) and ArnD (Saci_1694) are characterized. Purified ArnC and ArnD phosphorylate serine and threonine residues in the C‐terminus of the repressor ArnB. arnC is upregulated in starvation medium, whereas arnD is constitutively expressed. However, while differences in the expression and levels of flaB were observed in the ΔarnD strain during growth under rich conditions, under nutrient limiting conditions the ΔarnC and ΔarnD strains showed no large differences in the expression levels of the archaellum or of the studied regulators. This suggests that next to the regulation via the archaellum regulatory network additional regulatory mechanisms of expression and/or activity of the archaellum exist.

Protein phosphorylation and its role in archaeal signal transduction

Reversible protein phosphorylation is the main mechanism of signal transduction that enables cells to rapidly respond to environmental changes by controlling the functional properties of proteins in response to external stimuli. However, whereas signal transduction is well studied in Eukaryotes and Bacteria, the knowledge in Archaea is still rather scarce. Archaea are special with regard to protein phosphorylation, due to the fact that the two best studied phyla, the Euryarchaeota and Crenarchaeaota, seem to exhibit fundamental differences in regulatory systems. Euryarchaeota (e.g. halophiles, methanogens, thermophiles), like Bacteria and Eukaryotes, rely on bacterial-type two-component signal transduction systems (phosphorylation on His and Asp), as well as on the protein phosphorylation on Ser, Thr and Tyr by Hanks-type protein kinases. Instead, Crenarchaeota (e.g. acidophiles and (hyper)thermophiles) only depend on Hanks-type protein phosphorylation. In this review, the current knowledge of reversible protein phosphorylation in Archaea is presented. It combines results from identified phosphoproteins, biochemical characterization of protein kinases and protein phosphatases as well as target enzymes and first insights into archaeal signal transduction by biochemical, genetic and polyomic studies.

The authors review the current knowledge about protein phosphorylation in Archaea and its impact on signaling in this organism group.

Activation of SsoPK4, an Archaeal eIF2α Kinase Homolog, by Oxidized CoA

The eukaryotic protein kinase (ePK) paradigm provides integral components for signal transduction cascades throughout nature. However, while so-called typical ePKs permeate the Eucarya and Bacteria, atypical ePKs dominate the kinomes of the Archaea. Intriguingly, the catalytic domains of the handful of deduced typical ePKs from the archaeon Sulfolobus solfataricus P2 exhibit significant resemblance to the protein kinases that phosphorylate translation initiation factor 2α (eIF2α) in response to cellular stresses. We cloned and expressed one of these archaeal eIF2α protein kinases, SsoPK4. SsoPK4 exhibited protein-serine/threonine kinase activity toward several proteins, including the S. solfataricus homolog of eIF2α, aIF2α. The activity of SsoPK4 was inhibited in vitro by 3ʹ,5ʹ-cyclic AMP (K_i of ~23 µM) and was activated by oxidized Coenzyme A, an indicator of oxidative stress in the Archaea. Activation enhanced the apparent affinity for protein substrates, K_m, but had little effect on V_max. Autophosphorylation activated SsoPK4 and rendered it insensitive to oxidized Coenzyme A.

Carbohydrate metabolism in Archaea: current insights into unusual enzymes and pathways and their regulation

The metabolism of Archaea, the third domain of life, resembles in its complexity those of Bacteria and lower Eukarya. However, this metabolic complexity in Archaea is accompanied by the absence of many "classical" pathways, particularly in central carbohydrate metabolism. Instead, Archaea are characterized by the presence of unique, modified variants of classical pathways such as the Embden-Meyerhof-Parnas (EMP) pathway and the Entner-Doudoroff (ED) pathway. The pentose phosphate pathway is only partly present (if at all), and pentose degradation also significantly differs from that known for bacterial model organisms. These modifications are accompanied by the invention of "new," unusual enzymes which cause fundamental consequences for the underlying regulatory principles, and classical allosteric regulation sites well established in Bacteria and Eukarya are lost. The aim of this review is to present the current understanding of central carbohydrate metabolic pathways and their regulation in Archaea. In order to give an overview of their complexity, pathway modifications are discussed with respect to unusual archaeal biocatalysts, their structural and mechanistic characteristics, and their regulatory properties in comparison to their classic counterparts from Bacteria and Eukarya. Furthermore, an overview focusing on hexose metabolic, i.e., glycolytic as well as gluconeogenic, pathways identified in archaeal model organisms is given. Their energy gain is discussed, and new insights into different levels of regulation that have been observed so far, including the transcript and protein levels (e.g., gene regulation, known transcription regulators, and posttranslational modification via reversible protein phosphorylation), are presented.

Protein Ser/Thr/Tyr phosphorylation in the Archaea

The third domain of life, the Archaea (formerly Archaebacteria), is populated by a physiologically diverse set of microorganisms, many of which reside at the ecological extremes of our global environment. Although ostensibly prokaryotic in morphology, the Archaea share much closer evolutionary ties with the Eukarya than with the superficially more similar Bacteria. Initial genomic, proteomic, and biochemical analyses have revealed the presence of "eukaryotic" protein kinases and phosphatases and an intriguing set of serine-, threonine-, and tyrosine-phosphorylated proteins in the Archaea that may offer new insights into this important regulatory mechanism.

Archaeal signal transduction: impact of protein phosphatase deletions on cell size, motility, and energy metabolism in Sulfolobus acidocaldarius

In this study, the in vitro and in vivo functions of the only two identified protein phosphatases, Saci-PTP and Saci-PP2A, in the crenarchaeal model organism Sulfolobus acidocaldarius were investigated. Biochemical characterization revealed that Saci-PTP is a dual-specific phosphatase (against pSer/pThr and pTyr), whereas Saci-PP2A exhibited specific pSer/pThr activity and inhibition by okadaic acid. Deletion of saci_pp2a resulted in pronounced alterations in growth, cell shape and cell size, which could be partially complemented. Transcriptome analysis of the three strains (Δsaci_ptp, Δsaci_pp2a and the MW001 parental strain) revealed 155 genes that were differentially expressed in the deletion mutants, and showed significant changes in expression of genes encoding the archaella (archaeal motility structure), components of the respiratory chain and transcriptional regulators. Phosphoproteome studies revealed 801 unique phosphoproteins in total, with an increase in identified phosphopeptides in the deletion mutants. Proteins from most functional categories were affected by phosphorylation, including components of the motility system, the respiratory chain, and regulatory proteins. In the saci_pp2a deletion mutant the up-regulation at the transcript level, as well as the observed phosphorylation pattern, resembled starvation stress responses. Hypermotility was also observed in the saci_pp2a deletion mutant. The results highlight the importance of protein phosphorylation in regulating essential cellular processes in the crenarchaeon S. acidocaldarius.

Atypical protein kinases of the RIO family in archaea

Atypical protein kinases of the RIO (right open reading frame) kinase family are found in all three domains of life, emphasizing their essential function. In all archaeal genomes sequenced to date, typically two, but at least one, members of the RIO kinase family have been identified. Although the function of RIO kinases in Archaea remains to be resolved, bioinformatics analysis (e.g. comparison of the phylogenetic distribution and gene neighbourhood analysis, as well as interaction analysis) in combination with the available phosphoproteome study of Sulfolobus solfataricus provided some first hints to the possible function as well as revealed some putative target proteins for RIO kinases in Archaea. This study suggests a possible function of archaeal RIO kinases in RNA and/or DNA binding/processing translation initiation or ribosomal biogenesis resembling the assumed physiological role in yeast.

Change of carbon source causes dramatic effects in the phospho-proteome of the archaeon Sulfolobus solfataricus

Protein phosphorylation is known to occur in Archaea. However, knowledge of phosphorylation in the third domain of life is rather scarce. Homology-based searches of archaeal genome sequences reveals the absence of two-component systems in crenarchaeal genomes but the presence of eukaryotic-like protein kinases and protein phosphatases. Here, the influence of the offered carbon source (glucose versus tryptone) on the phospho-proteome of Sulfolobus solfataricus P2 was studied by precursor acquisition independent from ion count (PAcIFIC). In comparison to previous phospho-proteome studies, a high number of phosphorylation sites (1318) located on 690 phospho-peptides from 540 unique phospho-proteins were detected, thus increasing the number of currently known archaeal phospho-proteins from 80 to 621. Furthermore, a 25.8/20.6/53.6 Ser/Thr/Tyr percentage ratio with an unexpectedly high predominance of tyrosine phosphorylation was detected. Phospho-proteins in most functional classes (21 out of 26 arCOGs) were identified, suggesting an important regulatory role in S. solfataricus. Focusing on the central carbohydrate metabolism in response to the offered carbon source, significant changes were observed. The observed complex phosphorylation pattern hints at an important physiological function of protein phosphorylation in control of the central carbohydrate metabolism, which might particularly operate in channeling carbon flux into the respective metabolic pathways.

The activity of an ancient atypical protein kinase is stimulated by ADP-ribose in vitro

The piD261/Bud32 protein kinases are universal amongst the members of the Eucarya and Archaea. Despite the fact that phylogenetic analyses indicate that the piD261/Bud32 protein kinases descend directly from the primordial ancestor of the "eukaryotic" protein kinase superfamily, our knowledge of their physiological role is relatively fragmentary and largely limited to two eucaryal representatives: piD261/Bud32 from yeast and the p53-related protein kinase from humans. A deduced archaeal homolog, SsoPK5, is encoded by open reading frame sso0433 from the acidothermophile Sulfolobus solfataricus. Recombinantly-expressed SsoPK5 exhibited protein kinase activity, with a noticeable preference for phosphorylating proteins of acidic character and for Mn(2+) as cofactor. The protein kinase also can phosphorylate itself on serine and threonine residues. The activity of rSsoPK5 was increased several-fold upon preincubation with either millimolar concentrations of 5'-AMP or submicromolar concentrations of ADP-ribose. Other mono- and di-nucleotides were ineffective. While activation was enhanced by the presence of ATP, no autophosphorylation of the protein kinase could be detected prior to addition of exogenous substrate proteins. We therefore suggest that ADP-ribose acts by evoking a conformational transition in the enzyme. Activation by ADP-ribose represents a potential regulatory link between chromatin remodeling and the activity of SsoPK5.

A framework for classification of prokaryotic protein kinases

BACKGROUND

Overwhelming majority of the Serine/Threonine protein kinases identified by gleaning archaeal and eubacterial genomes could not be classified into any of the well known Hanks and Hunter subfamilies of protein kinases. This is owing to the development of Hanks and Hunter classification scheme based on eukaryotic protein kinases which are highly divergent from their prokaryotic homologues. A large dataset of prokaryotic Serine/Threonine protein kinases recognized from genomes of prokaryotes have been used to develop a classification framework for prokaryotic Ser/Thr protein kinases.

METHODOLOGY/PRINCIPAL FINDINGS

We have used traditional sequence alignment and phylogenetic approaches and clustered the prokaryotic kinases which represent 72 subfamilies with at least 4 members in each. Such a clustering enables classification of prokaryotic Ser/Thr kinases and it can be used as a framework to classify newly identified prokaryotic Ser/Thr kinases. After series of searches in a comprehensive sequence database we recognized that 38 subfamilies of prokaryotic protein kinases are associated to a specific taxonomic level. For example 4, 6 and 3 subfamilies have been identified that are currently specific to phylum proteobacteria, cyanobacteria and actinobacteria respectively. Similarly subfamilies which are specific to an order, sub-order, class, family and genus have also been identified. In addition to these, we also identify organism-diverse subfamilies. Members of these clusters are from organisms of different taxonomic levels, such as archaea, bacteria, eukaryotes and viruses.

CONCLUSION/SIGNIFICANCE

Interestingly, occurrence of several taxonomic level specific subfamilies of prokaryotic kinases contrasts with classification of eukaryotic protein kinases in which most of the popular subfamilies of eukaryotic protein kinases occur diversely in several eukaryotes. Many prokaryotic Ser/Thr kinases exhibit a wide variety of modular organization which indicates a degree of complexity and protein-protein interactions in the signaling pathways in these microbes.

Archaeal eukaryote-like serine/threonine protein kinase interacts with and phosphorylates a forkhead-associated-domain-containing protein

Protein phosphorylation plays an important role in cell signaling. However, in the Archaea, little is known about which proteins are phosphorylated and which kinases are involved. In this study, we identified, for the first time, a typical eukaryote-like Ser/Thr protein kinase and its protein partner, a forkhead-associated (FHA)-domain-containing protein, from the archaeon Sulfolobus tokodaii strain 7. This protein kinase, ST1565, physically interacted with the FHA-domain-containing protein, ST0829, both in vivo and in vitro. ST1565 preferred Mn(2+) as a cofactor for autophosphorylation and for substrate phosphorylation; the optimal temperature for this was 45 degrees C, and the optimal pH was 5.5 to 7.5. The critical amino acid residues of the conserved FHA and kinase domain sites were identified by performing a series of mutation assays. Thr329 was part of a major activation site in the kinase, while Thr326 was a negative regulation site. Several mutants with amino acid substitutions in the conserved FHA domain sites of ST0829 did not physically interact with ST1565. A structural model for the FHA domain demonstrated that the mutation sites were located at the edge of the protein and thus were in the domain that potentially interacts with ST1565. This report describes pioneering work on the third domain of life, the Archaea, showing that a protein kinase interacts with and phosphorylates an FHA-domain-containing protein. Our data provide critical information on the structural or functional characteristics of archaeal proteins and could help increase our understanding of fundamental signaling mechanisms in all three domains of life.

Something old, something new, something borrowed; how the thermoacidophilic archaeon Sulfolobus solfataricus responds to oxidative stress

To avoid molecular damage of biomolecules due to oxidation, all cells have evolved constitutive and responsive systems to mitigate and repair chemical modifications. Archaea have adapted to some of the most extreme environments known to support life, including highly oxidizing conditions. However, in comparison to bacteria and eukaryotes, relatively little is known about the biology and biochemistry of archaea in response to changing conditions and repair of oxidative damage. In this study transcriptome, proteome, and chemical reactivity analyses of hydrogen peroxide (H(2)O(2)) induced oxidative stress in Sulfolobus solfataricus (P2) were conducted. Microarray analysis of mRNA expression showed that 102 transcripts were regulated by at least 1.5 fold, 30 minutes after exposure to 30 microM H(2)O(2). Parallel proteomic analyses using two-dimensional differential gel electrophoresis (2D-DIGE), monitored more than 800 proteins 30 and 105 minutes after exposure and found that 18 had significant changes in abundance. A recently characterized ferritin-like antioxidant protein, DPSL, was the most highly regulated species of mRNA and protein, in addition to being post-translationally modified. As expected, a number of antioxidant related mRNAs and proteins were differentially regulated. Three of these, DPSL, superoxide dismutase, and peroxiredoxin were shown to interact and likely form a novel supramolecular complex for mitigating oxidative damage. A scheme for the ability of this complex to perform multi-step reactions is presented. Despite the central role played by DPSL, cells maintained a lower level of protection after disruption of the dpsl gene, indicating a level of redundancy in the oxidative stress pathways of S. solfataricus. This work provides the first "omics" scale assessment of the oxidative stress response for an archeal organism and together with a network analysis using data from previous studies on bacteria and eukaryotes reveals evolutionarily conserved pathways where complex and overlapping defense mechanisms protect against oxygen toxicity.

Posttranslational protein modification in Archaea

One of the first hurdles to be negotiated in the postgenomic era involves the description of the entire protein content of the cell, the proteome. Such efforts are presently complicated by the various posttranslational modifications that proteins can experience, including glycosylation, lipid attachment, phosphorylation, methylation, disulfide bond formation, and proteolytic cleavage. Whereas these and other posttranslational protein modifications have been well characterized in Eucarya and Bacteria, posttranslational modification in Archaea has received far less attention. Although archaeal proteins can undergo posttranslational modifications reminiscent of what their eucaryal and bacterial counterparts experience, examination of archaeal posttranslational modification often reveals aspects not previously observed in the other two domains of life. In some cases, posttranslational modification allows a protein to survive the extreme conditions often encountered by Archaea. The various posttranslational modifications experienced by archaeal proteins, the molecular steps leading to these modifications, and the role played by posttranslational modification in Archaea form the focus of this review.

Identification and characterization of Sulfolobus solfataricus D-gluconate dehydratase: a key enzyme in the non-phosphorylated Entner-Doudoroff pathway

The extremely thermoacidophilic archaeon Sulfolobus solfataricus utilizes D-glucose as a sole carbon and energy source through the non-phosphorylated Entner-Doudoroff pathway. It has been suggested that this micro-organism metabolizes D-gluconate, the oxidized form of D-glucose, to pyruvate and D-glyceraldehyde by using two unique enzymes, D-gluconate dehydratase and 2-keto-3-deoxy-D-gluconate aldolase. In the present study, we report the purification and characterization of D-gluconate dehydratase from S. solfataricus, which catalyses the conversion of D-gluconate into 2-keto-3-deoxy-D-gluconate. D-Gluconate dehydratase was purified 400-fold from extracts of S. solfataricus by ammonium sulphate fractionation and chromatography on DEAE-Sepharose, Q-Sepharose, phenyl-Sepharose and Mono Q. The native protein showed a molecular mass of 350 kDa by gel filtration, whereas SDS/PAGE analysis provided a molecular mass of 44 kDa, indicating that D-gluconate dehydratase is an octameric protein. The enzyme showed maximal activity at temperatures between 80 and 90 degrees C and pH values between 6.5 and 7.5, and a half-life of 40 min at 100 degrees C. Bivalent metal ions such as Co2+, Mg2+, Mn2+ and Ni2+ activated, whereas EDTA inhibited the enzyme. A metal analysis of the purified protein revealed the presence of one Co2+ ion per enzyme monomer. Of the 22 aldonic acids tested, only D-gluconate served as a substrate, with K(m)=0.45 mM and V(max)=0.15 unit/mg of enzyme. From N-terminal sequences of the purified enzyme, it was found that the gene product of SSO3198 in the S. solfataricus genome database corresponded to D-gluconate dehydratase (gnaD). We also found that the D-gluconate dehydratase of S. solfataricus is a phosphoprotein and that its catalytic activity is regulated by a phosphorylation-dephosphorylation mechanism. This is the first report on biochemical and genetic characterization of D-gluconate dehydratase involved in the non-phosphorylated Entner-Doudoroff pathway.

A phosphohexomutase from the archaeon Sulfolobus solfataricus is covalently modified by phosphorylation on serine

A phosphoserine-containing peptide was identified from tryptic digests from Sulfolobus solfataricus P1 by liquid chromatography-tandem mass spectrometry. Its amino acid sequence closely matched that bracketing Ser-309 in the predicted protein product of open reading frame sso0207, a putative phosphohexomutase, in the genome of S. solfataricus P2. Open reading frame sso0207 was cloned, and its protein product expressed in Escherichia coli. The recombinant protein proved capable of interconverting mannose 1-phosphate and mannose 6-phosphate, as well as glucose 1-phosphate and glucose 6-phosphate, in vitro. It displayed no catalytic activity toward glucosamine 6-phosphate or N-acetylglucosamine 6-phosphate. Models constructed using the X-ray crystal structure of a homologous phosphohexomutase from Pseudomonas aeruginosa predicted that Ser-309 of the archaeal protein lies within the substrate binding site. The presence of a phosphoryl group at this location would be expected to electrostatically interfere with the binding of negatively charged phosphohexose substrates, thus attenuating the catalytic efficiency of the enzyme. Using site-directed mutagenesis, Ser-309 was substituted by aspartic acid to mimic the presence of a phosphoryl group. The V(max) of the mutationally altered protein was only 4% that of the unmodified form. Substitution of Ser-309 with larger, but uncharged, amino acids, including threonine, also decreased catalytic efficiency, but to a lesser extent--three- to fivefold. We therefore predict that phosphorylation of the enzyme in vivo serves to regulate its catalytic activity.

Cloning, overexpression, and characterization of a serine/threonine protein kinase pknI from Mycobacterium tuberculosis H37Rv

Protein phosphorylation-dephosphorylation is the principal mechanism for translation of external signals into cellular responses. Eukaryotic-like serine/threonine kinases have been reported to play important roles in bacterial development and/or virulence. The PknI protein is one of the 11 eukaryotic-like serine/threonine kinases in Mycobacterium tuberculosis H37Rv. From the bioinformatic studies, PknI protein has been shown to have an N-terminal cytoplasmic domain followed by a transmembrane region and an extracellular C-terminus suggestive of a sensor molecule. In this study, we have cloned, overexpressed, and characterized the entire coding region and the cytoplasmic domain of PknI as a fusion protein with an N-terminal histidine tag, and used immobilized metal affinity chromatography for purification of recombinant proteins. The purified recombinant proteins were found to be functionally active through in vitro phosphorylation assay and phosphoamino acid analysis. In vitro kinase assay of both proteins revealed that PknI is capable of autophosphorylation and showed manganese-dependent activity. Phosphoamino acid analysis indicated phosphorylation at serine and threonine residues. Southern blot analysis with genomic DNA highlighted the conserved nature of pknI among the various mycobacterial species. In silico analysis revealed a close homology of PknI to Stk1 from Streptococcus agalactiae, shown to have a role in virulence and cell segregation of the organism.