A protein-tyrosine/serine phosphatase encoded by the genome of the cyanobacterium Nostoc commune UTEX 584

An essential tyrosine phosphatase homolog regulates cell separation, outer membrane integrity, and morphology in Caulobacter crescentus

Although reversible phosphorylation on tyrosine residues regulates the activity of many eukaryotic proteins, there are few examples of this type of regulation in bacteria. We have identified the first essential tyrosine phosphatase homolog in a bacterium, Caulobacter crescentus CtpA. ctpA mutants with altered active-site residues are nonviable, and depletion of CtpA yields chains of cells with blebbed outer membranes, linked by unresolved peptidoglycan. CtpA overexpression reduces cell curvature in a manner similar to deleting the intermediate filament protein crescentin, but it does not disrupt crescentin localization or membrane attachment. Although it has no obvious signal sequence or transmembrane-spanning domains, CtpA associates with the Caulobacter inner membrane. Immunolocalization experiments suggest that CtpA accumulates at the division site during the last quarter of the cell cycle. We propose that CtpA dephosphorylates one or more proteins involved in peptidoglycan biosynthesis or remodeling, which in turn affect cell separation, cell envelope integrity, and vibrioid morphology.

Protein phosphorylation on Ser, Thr and Tyr residues in cyanobacteria

Cyanobacteria belong to an extremely diverse group of gram-negative prokaryotes. They are all able to perform oxygen-evolving photosynthesis, but differ in morphology, ecological habitats, and physiology. This diversity is also reflected in the complexity of regulatory proteins involved in protein phosphorylation on Ser, Thr and Tyr residues. For those strains whose genomes are completely sequenced, for example, the number of genes identified so far that encode Ser/Thr and Tyr kinases range from none to 52. Genetic, molecular as well as functional genomic analyses demonstrate that Ser/Thr and Tyr kinases and phosphatases are involved in the regulation of a variety of activities according to changes in growth conditions or cell metabolism, such as cell motility, carbon and nitrogen metabolism, photosynthesis and stress response. The major challenge in the near future is to integrate these components into signaling pathways and identify their targets. Some of the Ser/Thr and Tyr kinases and phosphatases are expected to interact with classical two-component signaling pathways.

The nitrate/nitrite ABC transporter of Phormidium laminosum: Phosphorylation state of NrtA is not involved in its substrate binding activity

Most cyanobacteria take up nitrate or nitrite through a multisubunit ABC transporter (ATP-binding cassette) located in the cytoplasmic membrane. Nitrate and nitrite transport activity is instantaneously blocked by the presence of ammonium in the medium. Previous biochemical studies reported the existence of phosphorylation/dephosphorylation events of the nitrate transporter (NRT) related to the presence of ammonium-sensitive kinase/phosphatase activities in plasma membranes of the cyanobacterium Synechococcus elongatus PCC 6301. In this work, we have analyzed the biochemical properties of the periplasmic nitrate/nitrite-binding subunit (NrtA) of NRT from the thermophilic nondiazotrophic cyanobacterium Phormidium laminosum. Our results show that cyanobacterial NrtA is phosphorylated in vivo. However, substrate binding activity in vitro is not affected by the phosphorylation state of the protein, ruling out the possibility that phosphorylation/dephosphorylation of NrtA is involved in the regulation of the nitrate/nitrite uptake by NRT transporter. Moreover, NrtA is present as multiple isoforms showing the same molecular mass but different isoelectric points ranging from pI 5 to 6. Mass spectrometric characterization of NrtA isoforms shows that the protein is phosphorylated at residue Tyr203, and contains several methionine sulphoxide residues which account for the observed isoforms. Both phosphorylated and non-phosphorylated forms of NrtA are active in vitro, showing comparable binding affinity for nitrate and nitrite. Both substrates behave as pure competitive inhibitors with a binding stoichiometry of one molecule of anion per NrtA monomer.

Low-molecular-weight protein tyrosine phosphatases of Bacillus subtilis

In gram-negative organisms, enzymes belonging to the low-molecular-weight protein tyrosine phosphatase (LMPTP) family are involved in the regulation of important physiological functions, including stress resistance and synthesis of the polysaccharide capsule. LMPTPs have been identified also in gram-positive bacteria, but their functions in these organisms are presently unknown. We cloned two putative LMPTPs from Bacillus subtilis, YfkJ and YwlE, which are highly similar to each other in primary structure as well as to LMPTPs from gram-negative bacteria. When purified from overexpressing Escherichia coli strains, both enzymes were able to dephosphorylate p-nitrophenyl-phosphate and phosphotyrosine-containing substrates in vitro but showed significant differences in kinetic parameters and sensitivity to inhibitors. Transcriptional analyses showed that yfkJ was transcribed at a low level throughout the growth cycle and underwent a sigma(B)-dependent transcriptional upregulation in response to ethanol stress. The transcription of ywlE was growth dependent but stress insensitive. Genomic deletion of each phosphatase-encoding gene led to a phenotype of reduced bacterial resistance to ethanol stress, which was more marked in the ywlE deletion strain. Our study suggests that YfkJ and YwlE play roles in B. subtilis stress resistance.

Comparative analysis of eukaryotic-type protein phosphatases in two streptomycete genomes

Inspection of the genomes of Streptomyces coelicolor A3(2) and Streptomyces avermitilis reveals that each contains 55 putative eukaryotic-type protein phosphatases (PPs), the largest number ever identified from any single prokaryotic organism. Unlike most other prokaryotic genomes that have only one or two superfamilies of eukaryotic-type PPs, the streptomycete genomes possess the eukaryotic-type PPs that belong to four superfamilies: 2 phosphoprotein phosphatases and 2 low-molecular-mass protein tyrosine phosphatases in each species, 49 Mg(2+)- or Mn(2+)-dependent protein phosphatases (PPMs) and 2 conventional protein tyrosine phosphatases (CPTPs) in S. coelicolor A3(2), and 48 PPMs and 3 CPTPs in S. avermitilis. Sixty-four percent of the PPs found in S. coelicolor A3(2) have orthologues in S. avermitilis, indicating that they originated from a common ancestor and might be involved in the regulation of more conserved metabolic activities. The genes of eukaryotic-type PP unique to each surveyed streptomycete genome are mainly located in two arms of the linear chromosomes and their evolution might be involved in gene acquisition or duplication to adapt to the extremely variable soil environments where these organisms live. In addition, 56 % of the PPs from S. coelicolor A3(2) and 65 % of the PPs from S. avermitilis possess at least one additional domain having a putative biological function. These include the domains involved in the detection of redox potential, the binding of cyclic nucleotides, mRNA, DNA and ATP, and the catalysis of phosphorylation reactions. Because they contained multiple functional domains, most of them were assigned functions other than PPs in previous annotations. Although few studies have been conducted on the physiological functions of the PPs in streptomycetes, the existence of large numbers of putative PPs in these two streptomycete genomes strongly suggests that eukaryotic-type PPs play important regulatory roles in primary or secondary metabolic pathways. The identification and analysis of such a large number of putative eukaryotic-type PPs from S. coelicolor A3(2) and S. avermitilis constitute a basis for further exploration of the signal transduction pathways mediated by these phosphatases in industrially important strains of streptomycetes.

An arsenate reductase from Synechocystis sp. strain PCC 6803 exhibits a novel combination of catalytic characteristics

The deduced protein product of open reading frame slr0946 from Synechocystis sp. strain PCC 6803, SynArsC, contains the conserved sequence features of the enzyme superfamily that includes the low-molecular-weight protein-tyrosine phosphatases and the Staphylococcus aureus pI258 ArsC arsenate reductase. The recombinant protein product of slr0946, rSynArsC, exhibited vigorous arsenate reductase activity (V(max) = 3.1 micro mol/min. mg), as well as weak phosphatase activity toward p-nitrophenyl phosphate (V(max) = 0.08 micro mol/min. mg) indicative of its phosphohydrolytic ancestry. pI258 ArsC from S. aureus is the prototype of one of three distinct families of detoxifying arsenate reductases. The prototypes of the others are Acr2p from Saccharomyces cerevisiae and R773 ArsC from Escherichia coli. All three have converged upon catalytic mechanisms involving an arsenocysteine intermediate. While SynArsC is homologous to pI258 ArsC, its catalytic mechanism exhibited a unique combination of features. rSynArsC employed glutathione and glutaredoxin as the source of reducing equivalents, like Acr2p and R773 ArsC, rather than thioredoxin, as does the S. aureus enzyme. As postulated for Acr2p and R773 ArsC, rSynArsC formed a covalent complex with glutathione in an arsenate-dependent manner. rSynArsC contains three essential cysteine residues like pI258 ArsC, whereas the yeast and E. coli enzymes require only one cysteine for catalysis. As in the S. aureus enzyme, these "extra" cysteines apparently shuttle a disulfide bond to the enzyme's surface to render it accessible for reduction. SynArsC and pI258 ArsC thus appear to represent alternative branches in the evolution of their shared phosphohydrolytic ancestor into an agent of arsenic detoxification.

Autophosphorylation of the Escherichia coli protein kinase Wzc regulates tyrosine phosphorylation of Ugd, a UDP-glucose dehydrogenase

Autophosphorylation of protein-tyrosine kinases (PTKs) involved in exopolysaccharide and capsular polysaccharide biosynthesis and transport has been observed in a number of Gram-negative and Gram-positive bacteria. However, besides their own phosphorylation, little is known about other substrates targeted by these protein-modifying enzymes. Here, we present evidence that the protein-tyrosine kinase Wzc of Escherichia coli is able to phosphorylate an endogenous enzyme, UDP-glucose dehydrogenase (Ugd), which participates in the synthesis of the exopolysaccharide colanic acid. The process of phosphorylation of Ugd by Wzc was shown to be stimulated by previous autophosphorylation of Wzc on tyrosine 569. The phosphorylation of Ugd was demonstrated to actually occur on tyrosine and result in a significant increase of its dehydrogenase activity. In addition, the phosphotyrosine-protein phosphatase Wzb, which is known to effectively dephosphorylate Wzc, exhibited only a low effect, if any, on the dephosphorylation of Ugd. These data were related to the recent observation that two other UDP-glucose dehydrogenases have been also shown to be phosphorylated by a PTK in the Gram-positive bacterium Bacillus subtilis. Comparative analysis of the activities of PTKs from Gram-negative and Gram-positive bacteria showed that they are regulated by different mechanisms that involve, respectively, either the autophosphorylation of kinases or their interaction with a membrane protein activator.

Tk-PTP, protein tyrosine/serine phosphatase from hyperthermophilic archaeon Thermococcus kodakaraensis KOD1: enzymatic characteristics and identification of its substrate proteins

The Tk-ptp gene encoding a protein tyrosine phosphatase from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 was cloned and biochemical characteristics of the recombinant protein (Tk-PTP) were examined. A series of mutants, D63A (replacing Asp-63 with Ala), C93S, C93A, R99K, and R99M, were also constructed and analyzed. Two unique features were found. First, the Tk-PTP showed the phosphatase activity not only toward phosphotyrosine but also toward phosphoserine. Second, the conserved Asp-63, which corresponds to a critical residue among other known PTPs, was not essential for catalysis. Cys-93 and Arg-99 residues played a crucial role in substrate binding and catalysis. To know a specific substrate for Tk-PTP, C93S mutant was used to trap substrate proteins from cell extract of KOD1. Phenylalanyl-tRNA synthetase subunit beta-chain, one of the gene products of RNA terminal phosphate cyclase operon and phosphomannomutase, was identified, suggesting that they functioned for phosphate donation.

Inhibition of bacteriophage lambda protein phosphatase by organic and oxoanion inhibitors

Bacteriophage lambda protein phosphatase (lambdaPP) with Mn(2+) as the activating metal cofactor was studied using phosphatase inhibition kinetics and electron paramagnetic resonance (EPR) spectroscopy. Orthophosphate and the oxoanion analogues orthovanadate, tungstate, molybdate, arsenate, and sulfate were shown to inhibit the phosphomonoesterase activity of lambdaPP, albeit with inhibition constants (K(i)) that range over 5 orders of magnitude. In addition, small organic anions were tested as inhibitors. Phosphonoacetohydroxamic acid (PhAH) was found to be a strong competitive inhibitor (K(i) = 5.1 +/- 1.6 microM) whereas phosphonoacetic acid (K(i) = 380 +/- 45 microM) and acetohydroxamic acid (K(i) > 75 mM) modestly inhibited lambdaPP. Low-temperature EPR spectra of Mn(2+)-reconstituted lambdaPP in the presence of oxoanions and PhAH demonstrate that inhibitor binding decreases the spin-coupling constant, J, compared to the native enzyme. This suggests a change in the bridging interaction between Mn(2+) ions of the dimer due to protonation or replacement of a bridging ligand. Inhibitor binding also induces several spectral shifts. Hyperfine splitting characteristic of a spin-coupled (Mn(2+))(2) dimer is most prominent upon the addition of orthovanadate (K(i) = 0.70 +/- 0.20 microM) and PhAH, indicating that these inhibitors tightly interact with the (Mn(2+))(2) form of lambdaPP. These EPR and inhibition kinetic results are discussed in the context of establishing a common mechanism for the hydrolysis of phosphate esters by lambdaPP and other serine/threonine protein phosphatases.

Identification of the high affinity Mn2+ binding site of bacteriophage lambda phosphoprotein phosphatase: effects of metal ligand mutations on electron paramagnetic resonance spectra and phosphatase activities

Bacteriophage lambda phosphoprotein phosphatase (lambdaPP) has structural similarity to the mammalian Ser/Thr phosphoprotein phosphatases (PPPs) including the immunosuppressant drug target calcineurin. PPPs possess a conserved active site containing a dinuclear metal cluster, with metal ligands provided by a phosphoesterase motif plus two additional histidine residues at the C-terminus. Multiple sequence alignment of lambdaPP with 28 eubacterial and archeal phosphoesterases identified active site residues from the phosphoesterase motif and in many cases 2 additional C-terminal His metal ligands. Most highly similar to lambdaPP are E. coli PrpA and PrpB. Using the crystal structure of lambdaPP [Voegtli, W. C., et al. (2000) Biochemistry 39, 15365-15374] as a structural and active site model for PPPs and related bacterial phosphoesterases, we have studied mutant forms of lambdaPP reconstituted with Mn(2+) by electron paramagnetic resonance (EPR) spectroscopy, Mn(2+) binding analysis, and phosphatase kinetics. Analysis of Mn(2+)-bound active site mutant lambdaPP proteins shows that H22N, N75H, and H186N mutations decrease phosphatase activity but still allow mononuclear Mn(2+) and [(Mn(2+))(2)] binding. The high affinity Mn(2+) binding site is shown to consist of M2 site ligands H186 and Asn75, but not H22 from the M1 site which is ascribed as the lower affinity site.

Cloning and characterization of secretory tyrosine phosphatases of Mycobacterium tuberculosis

Two genes with sequence homology to those encoding protein tyrosine phosphatases were cloned from genomic DNA of Mycobacterium tuberculosis H(37)Rv. The calculated molecular masses of these two putative tyrosine phosphatases, designated MPtpA and MPtpB, were 17. 5 and 30 kDa, respectively. MPtpA and MPtpB were expressed as glutathione S-transferase fusion proteins in Escherichia coli. The affinity-purified proteins dephosphorylated the phosphotyrosine residue of myelin basic protein (MBP), but they failed to dephosphorylate serine/threonine residues of MBP. The activity of these phosphatases was inhibited by sodium orthovanadate, a specific inhibitor of tyrosine phosphatases, but not by okadaic acid, an inhibitor of serine/threonine phosphatases. Mutations at the catalytic site motif, cysteine 11 of MPtpA and cysteine 160 of MPtpB, abolished enzyme activity. Southern blot analysis revealed that, while mptpA is present in slow-growing mycobacterial species as well as fast-growing saprophytes, mptpB was restricted to members of the M. tuberculosis complex. These phosphatases were present in both whole-cell lysates and culture filtrates of M. tuberculosis, suggesting that these proteins are secreted into the extracellular medium. Since tyrosine phosphatases are essential for the virulence of several pathogenic bacteria, the restricted distribution of mptpB makes it a good candidate for a virulence gene of M. tuberculosis.

The eukaryotic-like Ser/Thr protein kinases of Mycobacterium tuberculosis

In bacteria, extracellular signals are generally transduced into cellular responses via a two-component system. However, genome sequence data have now revealed the presence of 'eukaryotic-like' protein kinases and phosphatases. Mycobacterium tuberculosis appears to be unique among bacteria in that its genome contains 11 members of a newly identified protein kinase family. These M. tuberculosis eukaryotic-like protein kinases could be key regulators of metabolic processes, including transcription, cell development and interactions with host cells.

Identification of novel VirR/VirS-regulated genes in Clostridium perfringens

Novel genes that are regulated in Clostridium perfringens by the two-component regulatory system, VirR/VirS, were identified using a differential display method. A plasmid library was constructed from C. perfringens chromosomal DNA, and the plasmids were hybridized with cDNA probes prepared from total RNA of wild-type strain 13 and its virR mutant derivative TS133. Three clones were identified that carry newly identified VirR/VirS-regulated genes, two of which were positively regulated and one of which was negatively regulated. Genes located on the identified clones were deduced by nucleotide sequencing, and the target genes of the VirR/VirS system were identified with a set of Northern hybridizations. A 4.9 kb mRNA transcribing the metB (cystathionine gamma-synthase), cysK (cysteine synthase) and ygaG (hypothetical protein) genes was negatively regulated, whereas 1.6 and 6.0 kb transcripts encoding ptp (protein tyrosine phosphatase) and cpd (2',3'-cyclic nucleotide 2'-phosphodiesterase) respectively, were shown to be positively regulated by the VirR/VirS system. The other gene, hyp7, whose transcript was positively regulated by the VirR/VirS system, was shown to activate the transcription of the colA (kappa-toxin) and plc (alpha-toxin) genes, but not the pfoA (theta-toxin) gene in C. perfringens. These results suggested that the global regulatory system VirR/VirS could regulate various genes, other than toxin genes, both positively and negatively and that the hyp7 gene might encode a novel regulatory factor for toxin production in C. perfringens.

No longer an exclusive club: eukaryotic signalling domains in bacteria

Reversible phosphorylation of serine, threonine and tyrosine residues by the interplay of protein kinases and phosphatases plays a key role in regulating many different cellular processes in eukaryotic organisms. A diversity of control mechanisms exists to influence the activity of these enzymes and choreograph the correct concert of protein modifications to achieve distinct biological responses. Such enzymes and their adaptor molecules were long thought to be specific to eukaryotic cellular processes. However, there is increasing evidence that many prokaryotes achieve regulation of key components of cellular function through similar mechanisms.

Isolation, cloning, and expression of an acid phosphatase containing phosphotyrosyl phosphatase activity from Prevotella intermedia

A novel acid phosphatase containing phosphotyrosyl phosphatase (PTPase) activity, designated PiACP, from Prevotella intermedia ATCC 25611, an anaerobe implicated in progressive periodontal disease, has been purified and characterized. PiACP, a monomer with an apparent molecular mass of 30 kDa, did not require divalent metal cations for activity and was sensitive to orthovanadate but highly resistant to okadaic acid. The enzyme exhibited substantial activity against tyrosine phosphate-containing peptides derived from the epidermal growth factor receptor. On the basis of N-terminal and internal amino acid sequences of purified PiACP, the gene coding for PiACP was isolated and sequenced. The PiACP gene consisted of 792 bp and coded for a basic protein with an M(r) of 29,164. The deduced amino acid sequence exhibited striking similarity (25 to 64%) to those of members of class A bacterial acid phosphatases, including PhoC of Morganella morganii, and involved a conserved phosphatase sequence motif that is shared among several lipid phosphatases and the mammalian glucose-6-phosphatases. The highly conservative motif HCXAGXXR in the active domain of PTPase was not found in PiACP. Mutagenesis of recombinant PiACP showed that His-170 and His-209 were essential for activity. Thus, the class A bacterial acid phosphatases including PiACP may function as atypical PTPases, the biological functions of which remain to be determined.

Expression and characterization of the Mycobacterium tuberculosis serine/threonine protein kinase PknB

PknB is a member of the newly discovered eukaryotic-like protein serine/threonine kinase (PSTK) family of proteins. The pknB gene was cloned and expressed in Escherichia coli. The active recombinant protein was purified and shown to be reactive with antiphosphoserine antibodies, as well as with antibodies to the phosphorylated eukaryotic Ser/Thr kinases mitogen-activated protein kinase kinase 3 and 6, P38, and Creb. In vitro kinase assays demonstrated that PknB is a functional kinase that is autophosphorylated on serine/threonine residues and is also able to phosphorylate the peptide substrate myelin basic protein. Analysis of pknB expression in Mycobacterium tuberculosis indicates the presence of pknB mRNA in (i) organisms grown in vitro in bacteriological media, (ii) a murine macrophage in vitro infection model, and (iii) in vivo alveolar macrophages from a patient with tuberculosis.

Studies on the cytoplasmic protein tyrosine kinase activity of the Antarctic psychrotrophic bacterium Pseudomonas syringae

The Antarctic psychrotrophic bacterium Pseudomonas syringae contains a 66-kDa cytoplasmic protein which was found to by phosphorylated on a tyrosine residue [Ray, M.K. et al. (1994) FEMS Microbiol. Lett. 122, pp. 49-54]. To investigate the nature of the cytoplasmic protein tyrosine kinase and its role in the bacterial physiology, we carried out some biochemical studies of the enzyme in vitro in the presence of exogenous peptide substrates and expression studies in vivo at low and high temperature during various phases of growth. The results suggest that the protein tyrosine kinase associated with the cytoplasmic fraction of the bacterium has certain similarities and dissimilarities with the known eukaryotic tyrosine kinases. The protein tyrosine kinase could phosphorylate exogenous substrate corresponding to the N-terminal peptide of p34cdc2 kinase but could not do so on poly(Glu:Tyr). The enzyme could not be inhibited by genistein, staurosporine and dimethyl aminopurine, but could be inhibited by piceatannol which is a known competitive inhibitor of the peptide binding site of mammalian protein tyrosine kinases. The enzyme activity in the cytoplasm is uniquely inhibited by sodium orthovanadate (IC50 = 20 microM) which is a known protein tyrosine phosphatase inhibitor. The expression studies show that the enzyme is produced more at a higher temperature (22 degrees C) of growth than at lower temperature (4 degrees C) and during the stationary phase of growth of P. syringae.

The serine, threonine, and/or tyrosine-specific protein kinases and protein phosphatases of prokaryotic organisms: a family portrait

Inspection of the genomes for the bacteria Bacillus subtilis 168, Borrelia burgdorferi B31, Escherichia coli K-12, Haemophilus influenzae KW20, Helicobacter pylori 26695, Mycoplasma genitalium G-37, and Synechocystis sp PCC 6803 and for the archaeons Archaeoglobus fulgidus VC-16 DSM4304, Methanobacterium thermoautotrophicum delta H, and Methanococcus jannaschii DSM2661 revealed that each contains at least one ORF whose predicted product displays sequence features characteristic of eukaryote-like protein-serine/threonine/tyrosine kinases and protein-serine/threonine/tyrosine phosphatases. Orthologs for all four major protein phosphatase families (PPP, PPM, conventional PTP, and low molecular weight PTP) were present in the bacteria surveyed, but not all strains contained all types. The three archaeons surveyed lacked recognizable homologs of the PPM family of eukaryotic protein-serine/threonine phosphatases; and only two prokaryotes were found to contain ORFs for potential phosphatases from all four major families. Intriguingly, our searches revealed a potential ancestral link between the catalytic subunits of microbial arsenate reductases and the protein-tyrosine phosphatases; they share similar ligands (arsenate versus phosphate) and features of their catalytic mechanism (formation of arseno-versus phospho-cysteinyl intermediates). It appears that all prokaryotic organisms, at one time, contained the genetic information necessary to construct protein phosphorylation-dephosphorylation networks that target serine, threonine, and/or tyrosine residues on proteins. However, the potential for functional redundancy among the four protein phosphatase families has led many prokaryotic organisms to discard one, two, or three of the four.

Tyrosine phosphorylation in Escherichia coli

The phosphorylation on tyrosine of a protein in Escherichia coli both in vivo and in vitro was revealed by recognition by anti-phosphotyrosine antibodies, labelling with [gamma-32P]ATP, and phosphoamino acid analysis. This protein, which we name TypA, is the product of the o591 reading frame as revealed by N-terminal sequencing and antibody cross-reactivity. Inactivation of typA altered the patterns of protein synthesis during both exponential growth and carbon starvation. These alterations included the disappearance of an acidic isoform of both the universal stress protein UspA and carbon starvation protein Csp15, and increased synthesis of the histone-like protein H-NS. The sequence of TypA from strain K-12 differs from that of an enteropathogenic strain in six amino acid residues and the protein is three residues shorter. We propose that TypA interacts with global regulatory networks and that its phosphorylation may be relevant to pathogenesis.

Structure and function of the low Mr phosphotyrosine protein phosphatases

Phosphotyrosine protein phosphatases (PTPases) catalyse the hydrolysis of phosphotyrosine residues in proteins and are hence implicated in the complex mechanism of the control of cell proliferation and differentiation. The low Mr PTPases are a group of soluble PTPases displaying a reduced molecular mass; in addition, a group of low molecular mass dual specificity (ds)PTPases which hydrolyse phosphotyrosine and phosphoserine/threonine residues in proteins are known. The enzymes belonging to the two groups are unrelated to each other and to other PTPase classes except for the presence of a CXXXXXRS/T sequence motif containing some of the catalytic residues (active site signature) and for the common catalytic mechanism, clearly indicating convergent evolution. The low Mr PTPases have a long evolutionary history since microbial (prokaryotic and eukaryotic) counterparts of both tyrosine-specific and dsPTPases have been described. Despite the relevant number of data reported on the structural and catalytic features of a number of low Mr PTPases, only limited information is presently available on the substrate specificity and the true biological roles of these enzymes, in prokaryotic, yeast and eukaryotic cells.

Protein-tyrosine phosphorylation in the Archaea

Sulfolobus sulfataricus ATCC 35091, Haloferax volcanii, and Methanosarcina thermophila TM-1, representing the Euryarchaeota and Crenarchaeota subdomains of the Archaea, contain proteins which are phosphorylated on tyrosine. These data raise fundamental questions as to the origin and evolution of tyrosine phosphorylation, a protein modification that is of pivotal importance in the regulation of the physiology of eukaryotic cells.

Protein tyrosine phosphorylation in the cyanobacterium Anabaena sp. strain PCC 7120

Components of a protein tyrosine phosphorylation/dephosphorylation network were identified in the cyanobacterium Anabaena sp. strain PCC 7120. Three phosphotyrosine (P-Tyr) proteins of 27, 36, and 52 kDa were identified through their conspicuous immunoreactions with RC20H monoclonal antibodies specific for P-Tyr. These immunoreactions were outcompeted completely by free P-Tyr (5 mM) but not by phosphoserine or phosphothreonine. The P-Tyr content of the three major P-Tyr proteins and several minor proteins increased with their time of incubation in the presence of Mg-ATP and the protein phosphatase inhibitors sodium orthovanadate and sodium fluoride. Incubation of the same extracts with [gamma-32P]ATP but not [alpha-32P]ATP led to the phosphorylation of five polypeptides with molecular masses of 20, 27, 52, 85, and 100 kDa. Human placental protein tyrosine phosphatase 1B, with absolute specificity for P-Tyr, liberated significant quantities of 32Pi from four of the polypeptides, confirming that a portion of the protein-bound phosphate was present as 32P-Tyr. Alkaline phosphatase and the dual-specificity protein phosphatase IphP from the cyanobacterium Nostoc commune UTEX 584 also dephosphorylated these proteins and did so with greater apparent efficiency. Two of the polypeptides were partially purified, and phosphoamino analysis identified 32P-Tyr, [32P]phosphoserine, and [32P]phosphothreonine. Anabaena sp. strain PCC 7120 cell extracts contained a protein tyrosine phosphatase activity that was abolished in the presence of sodium orthovanadate and inhibited significantly by the sulfhydryl-modifying agents p-hydroxymercuriphenylsulfonic acid and p-hydroxymercuribenzoate as well as by heparin. In Anabaena sp. strain PCC 7120 the presence and/or phosphorylation status of P-Tyr proteins was influenced by incident photon flux density.

Structural, catalytic, and functional properties of low M(r), phosphotyrosine protein phosphatases. Evidence of a long evolutionary history

The PTPase family comprises a number of classes of functionally and structurally unrelated enzymes; it represents an important component of the protein-tyrosine phosphorylation/dephosphorylation machinery, which regulates the level of tyrosine phosphorylation of a number of intracellular proteins. A wealth of recently reported data indicates growing interest in a group of PTPases characterized by low (near 20 kDa) molecular weight and high sequence homology (low M(r), PTPases). These enzymes are present in organisms spanning the philogenetic scale, from prokaryotes to yeast and mammals. The sequence homology of the low M(r), PTPases with other classes of PTPases is limited to the active site sequence CXXXXXRS/T, containing the Cys and Arg residues involved in enzyme catalysis found in all PTPases. The X-ray structural data of three enzymes belonging to different classes of PTPases, a bovine liver low M(r), PTPase isoenzyme, PTP1B, and Yersinia PTPase, show that all these enzymes maintain the same active site and overall catalytic mechanism, though displaying different chain foldings and topologies, supporting convergent evolution. Limited findings on the in vivo function of the low M(r), PTPases are presently available; however, an involvement of the mammalian enzymes in the membrane growth factor receptor signal transduction is emerging. The distribution of these enzymes in philogenetically distant unicellular and multicellular organisms supports their participation in important cell functions.

Structure and function of the protein tyrosine phosphatases

The tyrosine and dual-specificity phosphatases are involved in signaling, cell growth and differentiation, and the cell cycle. The enzymes share a common catalytic mechanism mediated by an active site cysteine, arginine and aspartic acid. Supplementary domains assist in targeting and substrate specificity.

A possible tyrosine phosphorylation of phytochrome

Red/far-red light signal transduction by the phytochrome family of photoreceptors regulates plant growth and development. We investigated the possibility that tyrosine kinases and/or phosphatases are involved in phytochrome-mediated signal transduction using crude extracts of oat seedlings that are grown in the dark. We found that a 124 kDa protein was tyrosine-phosphorylated as determined by Western blotting with a phosphotyrosine-specific monoclonal antibody. The 124 kDa protein was recognized by the anti-phosphotyrosine antibody in anti-phytochrome A immunoprecipitates. The level of anti-phosphotyrosine antibody binding to the 124 kDa protein(s) in phytochrome immunoprecipitates that had been treated with red light prior to immunoprecipitation decreased relative to dark controls. These results suggest that either phytochrome from dark-grown seedlings is tyrosine phosphorylated or that it co-immunoprecipitates with a phosphotyrosine-containing protein of the same molecular weight. The implications of these results in the regulation of (a) the putative Ser/Thr kinase activity of the photoreceptor and (b) the binding of signaling molecules, such as phospholipase C to phytochrome, are discussed.

Fancy meeting you here! A fresh look at "prokaryotic" protein phosphorylation

Bacteria play host to a wide range of protein phosphorylation-dephosphorylation systems (Fig. 1). As little as five years ago the known systems were thought to be late-emerging and absolutely prokaryote specific. Today we know that most protein kinases and protein phosphatases are descended from a set of common, and possibly quite ancient, prototypes. Prokaryote- and eukaryote-specific protein kinases and protein phosphatases are rare and represent exceptions, not the rule as previously thought. Commonality suggests that a dynamic and versatile regulatory mechanism was first adapted to the modulation of protein function as early if not earlier than more "basic" mechanisms such as allosterism, etc. The existence of common molecular themes confirms that the microbial world offers a unique, largely untapped library and a powerful set of tools for the understanding of a regulatory mechanism which is crucial to all organisms, tools whose diversity and experimental malleability will provide new avenues for exploring and understanding key modes of cellular regulation.

The protein phosphatase 2C (PP2C) superfamily: detection of bacterial homologues

A thorough sequence analysis of the various members of the eukaryotic protein serine/threonine phosphatase 2C (PP2C) family revealed the conservation of 11 motifs. These motifs could be identified in numerous other sequences, including fungal adenylate cyclases that are predicted to contain a functionally active PP2C domain, and a family of prokaryotic serine/threonine phosphatases including SpoIIE. Phylogenetic analysis of all the proteins indicates a widespread sequence family for which a considerable number of isoenzymes can be inferred.

Substrate specificity of IphP, a cyanobacterial dual-specificity protein phosphatase with MAP kinase phosphatase activity

The substrate specificity of the cyanobacterial dual-specificity protein phosphatase, IphP, was explored using a variety of potential substrates. The enzyme displayed phosphomonoesterase activity toward a broad range of peptide, protein, and low molecular weight organophosphate compounds. It displayed little or no hydrolase activity toward phosphodiesters, phosphoramides, carboxyl esters, or sulfoesters. However, it did display measurable pyrophosphatase activity, especially toward ADP and ATP. Among the low molecular weight phosphomonoesters, the presence of an aromatic ring either as part of the leaving group alcohol or immediately adjacent thereto, as in 5'-AMP, was a strong positive determinant for hydrolysis. Among peptide and protein substrates, a rough, but imperfect, correlation between charge character and hydrolysis was noted in which proteins and phosphorylation sites of an acidic nature seemed favored. Heparin affected IphP activity in a substrate-dependent manner. Toward small organophosphates, heparin had no significant effect, but it was inhibitory toward most protein and peptide substrates. However, toward phosphoseryl casein and MAP kinase, it enhanced activity as much as 10-fold. This enhancement was attributed to the ability of heparin to bind to these substrate proteins, as well as IphP, and recruit them to the same microenvironment.

Bacterial signalling involving eukaryotic-type protein kinases

Protein Ser, Thr and Tyr kinases play essential roles in signal transduction in organisms ranging from yeast to mammals, where they regulate a variety of cellular activities. During the last few years, a number of genes that encode eukaryotic-type protein kinases have also been identified in four different bacterial species, suggesting that such enzymes are also widespread in prokaryotes. Although many of them have yet to be fully characterized, several studies indicate that eukaryotic-type protein kinases play important roles in regulating cellular activities of these bacteria, such as cell differentiation, pathogenicity and secondary metabolism. A model based on the possible coupling between two-component systems and eukaryotic-type protein kinases is proposed to explain the function of eukaryotic-type protein kinases in bacterial signalling in the light of studies in bacteria, as well as in plants and yeast. These two groups of eukaryotes possess signal-transduction pathways involving both two-component systems and eukaryotic protein kinases.

Cloning, purification, and properties of a phosphotyrosine protein phosphatase from Streptomyces coelicolor A3(2)

We describe the isolation and characterization of a gene (ptpA) from Streptomyces coelicolor A3(2) that codes for a protein with a deduced M(r) of 17,690 containing significant amino acid sequence identity with mammalian and prokaryotic small, acidic phosphotyrosine protein phosphatases (PTPases). After expression of S. coelicolor ptpA in Escherichia coli with a pT7-7-based vector system, PtpA was purified to homogeneity as a fusion protein containing five extra amino acids. The purified fusion enzyme catalyzed the removal of phosphate from p-nitrophenylphosphate (PNPP), phosphotyrosine (PY), and a commercial phosphopeptide containing a single phosphotyrosine residue but did not cleave phosphoserine or phosphothreonine. The pH optima for PNPP and PY hydrolysis by PtpA were 6.0 and 6.5, respectively. The Km values for hydrolysis of PNPP and PY by PtpA were 0.75 mM (pH 6.0, 37 degrees C) and 2.7 mM (pH 6.5, 37 degrees C), respectively. Hydrolysis of PNPP by S. coelicolor PtpA were 0.75 mM (pH 6.0, 37 degrees C) and 2.7 mM (pH 6.5, 37 degrees C), respectively. Hydrolysis of PNPP by S. coelicolor PtpA was competitively inhibited by dephostatin with a Ki of 1.64 microM; the known PTPase inhibitors phenylarsine oxide, sodium vanadate, and iodoacetate also inhibited enzyme activity. Apparent homologs of ptpA were detected in other streptomycetes by Southern hybridization; the biological functions of PtpA and its putative homologs in streptomycetes are not yet known.

Isolation and cloning of a protein-serine/threonine phosphatase from an archaeon

A divalent metal ion-stimulated protein-serine/threonine phosphatase, PP1-arch, was purified approximately 1,000-fold from the extreme acidothermophilic archaeon Sulfolobus solfataricus (ATCC 35091). Purified preparations contained 40 to 70% of total protein as PP1-arch, as determined by assay-ing sodium dodecyl sulfate-polyacrylamide gels for protein phosphatase activity. The first 25 amino acids of the protein's sequence were identified, as well as an internal sequence spanning some 20 amino acids. Using this information, we cloned the gene for PP1-arch via the application of PCR and conventional cloning techniques. The gene for PP1-arch predicted a protein of 293 amino acids that bore striking resemblance to the members of the major family of protein-serine/threonine phosphatases from members of the domain Eucarya, the PP1/2A/2B superfamily. The core of the protein, spanning residues 4 to 275, possessed 29 to 31% identity with these eucaryal protein phosphatases. Of the 42 residues found to be absolutely conserved among the known eucaryal members of the PP1/2A/2B superfamily, 33 were present in PP1-arch. If highly conservative substitutions are included, this total reached 37. The great degree of sequence conservation between molecules from two distinct phylogenetic domains implies that the members of this enzyme superfamily had evolved as specialized, dedicated protein phosphatases prior to the divergence of members of the Archaea and Eucarya from one another.

Properties of a baculovirus mutant defective in the protein phosphatase gene

Autographa california nuclear polyhedrosis virus (AcMNPV) contains a gene, ptp, encoding a protein tyrosine/serine phosphatase, BV-PTP. To investigate the biological function of ptp in the baculoviral replication cycle, we constructed a recombinant baculovirus, vPTPdel, in which the catalytically active site of BV-PTP was deleted. Although the vPTPdel mutant was viable in cell culture, it was partially defective in occluded virus production in SF-21 but not TN-368 cell lines. SF-21 cells infected with vPTPdel were heterogeneous in their ability to support occluded virus production. These results suggest that BV-PTP functions in a cell line-specific and possibly a cell cycle-specific fashion. The yield of occlusion bodies, infectivity (concentration of virus causing 50% mortality) and virulence (the time at which 50% of the cells died) of vPTPdel appeared to be normal in insect larvae. We identified a 35-kDa phosphoprotein as a potential target of the BV-PTP in SF-21 cells.

Ubiquitin in the prokaryote Anabaena variabilis

The ubiquitin-dependent pathway for protein degradation has been found to play a major role in controlling protein turnover in the cell. Ubiquitin is one of the most conserved proteins yet identified, and up until now it has been thought to be present only in eukaryotes and archaebacteria. This is the first report on the detection and purification of ubiquitin from a eubacterium, the cyanobacterium Anabaena variabilis. The purification procedure included a heat denaturing step, fractionated ammonium sulfate precipitation, two gel filtration runs (Sephadex G-50 and Superose 12), and a final hydroxylapatite chromatography. Comparisons with bovine ubiquitin showed a high similarity with respect to antigenicity to anti-ubiquitin (bovine), molecular mass (M(r) = 6,000), isoelectric point (pI 6.5), and NH2-terminal sequence. The existence of ubiquitin in A. variabilis was confirmed by Southern hybridization. In in vitro experiments both cyanobacterial and bovine ubiquitin were covalently attached to several target proteins from A. variabilis, respectively. Data are presented which suggest ubiquitination of dinitrogenase reductase, the Fe-protein subunit of nitrogenase. Our findings imply that ubiquitination equivalent to the eukaryotic system is instrumental in this organism.

Expression and functional analysis of a baculovirus gene encoding a truncated protein kinase homolog

Autographa californica nuclear polyhedrosis virus (AcMNPV) potentially encodes a 215-amino acid polypeptide containing 6 out of 11 motifs conserved among eukaryotic protein kinases (Morris et al., Virology 200, 360-369, 1994). We examined the expression of this gene, named pk2, at the transcriptional and translational levels and the possible role of the gene during baculovirus replication in cell culture and insect larvae. Northern (RNA) blot analysis revealed that pk2 was transcribed primarily as an early 1.2-kb RNA. Western blot analysis showed that pk2 was expressed as a 25-kDa protein, PK2, which was present both early and late during virus infection. To examine the function(s) of pk2, we constructed a mutant baculovirus, vKINdel, in which one-third of the PK2-coding region was deleted and then compared the characteristics of vKINdel with wild-type AcMNPV and a marker-rescued revertant. The pk2 deletion mutation had no discernable effect on the number, size, or appearance of plaques, the kinetics of protein synthesis or protein phosphorylation profiles during virus infection of cultured SF-21 cells. Deletion of pk2 also had no significant influence on the infectivity or virulence of the baculovirus in larval bioassays and the level of occluded virus production was normal. Thus, pk2 does not appear to have a significant influence on virus replication in the host systems examined.

The phosphotyrosyl phosphatase activator of protein phosphatase 2A. A novel purification method, immunological and enzymic characterization

A simple, improved procedure for the isolation of the phosphotyrosyl phosphatase activator (PTPA) from rabbit skeletal muscle has been developed. The majority of the protein phosphatase 2A (PP2A) was separated from PTPA at an early stage in the procedure. The procedure yields approximately 1 mg essentially pure PTPA/kg rabbit skeletal muscle; it was also applied to porcine brain and the yeast Saccharomyces cerevisiae. The physico-chemical properties of PTPA obtained from all sources are very similar. The pure rabbit skeletal muscle protein was used to raise polyclonal goat antibodies and to affinity purify these antibodies. Immunological studies revealed the presence of PTPA in all mammalian tissues and cell lines examined with differences in tissue distribution, brain showing the highest concentration. PTPA could only be detected in cytosolic fractions. Using a semi-quantitative immunological assay (Western blot), the in vivo concentration could be estimated to be micromolar, which is in the same range as the PP2A target. The purified Xenopus oocyte PTPA showed only a weak cross reactivity, whereas yeast PTPA was not recognised by the antibody indicating some evolutionary diversity of the protein. In a PTPA-affinity column chromatography, the weak interaction with PP2A was independent of the presence of ATP.Mg, a necessary cofactor in the activation process. Interaction of PTPA with PP2A in a 1:1 ratio induces a low (kcat = 3 min-1) ATPase activity that is inhibited by okadaic acid, ADP and non-hydrolysable ATP analogues.

Posttranslational modification of nitrogenase. Differences between the purple bacterium Rhodospirillum rubrum and the cyanobacterium Anabaena variabilis

In the photosynthetic bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulatus post-translational regulation of nitrogenase is due to ADP-ribosylation of the Fe-protein, the dinitrogenase reductase [Burris, R. H. (1991) J. Biol. Chem. 266, 9339-9342]. This mechanism has been assumed to be responsible for nitrogenase modification in a variety of organisms. In the present study, we examined whether ADP-ribosylation holds true for the filamentous cyanobacterium Anabaena variabilis. Genes coding for the nitrogenase-modifying enzymes dinitrogenase reductase-activating glycohydrolase (DRAG) and dinitrogenase reductase ADP-ribosyl transferase (DRAT) from R. rubrum have been subcloned and overexpressed in Escherichia coli. After isolation under anaerobic conditions, both proteins were functional as determined by in-vitro assays using nitrogenase from R. rubrum as substrate. In contrast to the R. rubrum enzyme, nitrogenase from A. variabilis was not affected by DRAG or DRAT. Neither could inactive nitrogenase be restored by DRAG, nor nitrogenase activity suppressed by DRAT. Using specific antibodies against arginine-bound ADP-ribose [Meyer, T. & Hilz, H. (1986) Eur. J. Biochem. 155, 157-165], immunoblotting of the inactive, modified form of the Fe-protein from R. rubrum but not that from A. variabilis showed a strong cross reaction. Furthermore, differently to R. rubrum no ADP-ribosylated proteins could be detected at all, indicating the absence of this posttranslational modification in A. variabilis.

Inhibition of an archaeal protein phosphatase activity by okadaic acid, microcystin-LR, or calyculin A

Soluble extracts of the methanogenic archaeon, Methanosarcina thermophila TM-1, contained a divalent metal ion-stimulated protein-serine phosphatase activity. This activity was sensitive to micromolar concentrations of okadaic acid, microcystin-LR, or calyculin A, three compounds thought to be highly specific inhibitors of the type 1/2A/2B genetic superfamily of eukaryotic protein-serine/threonine phosphatases. The observation that each of these three chemically unrelated compounds inhibited this archaeal protein phosphatase activity suggests the existence of structural homology, and perhaps even common genetic ancestry, with the type 1/2A/2B superfamily of protein-serine/threonine phosphatases found in eukaryotic organisms.