Interactions between a minimal protein serine/threonine phosphatase and its phosphopeptide substrate sequence

Interaction of PINK1 with nucleotides and kinetin

The ubiquitin kinase PINK1 accumulates on damaged mitochondria to trigger mitophagy, and PINK1 loss-of-function mutations cause early onset Parkinson's disease. Nucleotide analogs such as kinetin triphosphate (KTP) were reported to enhance PINK1 activity and may represent a therapeutic strategy for the treatment of Parkinson's disease. Here, we investigate the interaction of PINK1 with nucleotides, including KTP. We establish a cryo-EM platform exploiting the dodecamer assembly of Pediculus humanus corporis (Ph) PINK1 and determine PINK1 structures bound to AMP-PNP and ADP, revealing conformational changes in the kinase N-lobe that help establish PINK1's ubiquitin binding site. Notably, we find that KTP is unable to bind PhPINK1 or human (Hs) PINK1 due to a steric clash with the kinase "gatekeeper" methionine residue, and mutation to Ala or Gly is required for PINK1 to bind and use KTP as a phosphate donor in ubiquitin phosphorylation and mitophagy. HsPINK1 M318G can be used to conditionally uncouple PINK1 stabilization and activity on mitochondria.

A PPP-type pseudophosphatase is required for the maintenance of basal complex integrity in Plasmodium falciparum

During its asexual blood stage, P. falciparum replicates via schizogony, wherein dozens of daughter cells are formed within a single parent. The basal complex, a contractile ring that separates daughter cells, is critical for schizogony. In this study, we identify a Plasmodium basal complex protein essential for basal complex maintenance. Using multiple microscopy techniques, we demonstrate that PfPPP8 is required for uniform basal complex expansion and maintenance of its integrity. We characterize PfPPP8 as the founding member of a novel family of pseudophosphatases with homologs in other Apicomplexan parasites. By co-immunoprecipitation, we identify two additional new basal complex proteins. We characterize the unique temporal localizations of these new basal complex proteins (late-arriving) and of PfPPP8 (early-departing). In this work, we identify a novel basal complex protein, determine its specific role in segmentation, identify a new pseudophosphatase family, and establish that the P. falciparum basal complex is a dynamic structure.

Towards Dissecting the Mechanism of Protein Phosphatase-1 Inhibition by Its C-Terminal Phosphorylation

Phosphoprotein phosphatase‐1 (PP1) is a key player in the regulation of phospho‐serine (pSer) and phospho‐threonine (pThr) dephosphorylation and is involved in a large fraction of cellular signaling pathways. Aberrant activity of PP1 has been linked to many diseases, including cancer and heart failure. Besides a well‐established activity control by regulatory proteins, an inhibitory function for phosphorylation (p) of a Thr residue in the C‐terminal intrinsically disordered tail of PP1 has been demonstrated. The associated phenotype of cell‐cycle arrest was repeatedly proposed to be due to autoinhibition of PP1 through either conformational changes or substrate competition. Here, we use PP1 variants created by mutations and protein semisynthesis to differentiate between these hypotheses. Our data support the hypothesis that pThr exerts its inhibitory function by mediating protein complex formation rather than by a direct mechanism of structural changes or substrate competition.

De novo missense variants in PPP1CB are associated with intellectual disability and congenital heart disease

Intellectual disabilities are genetically heterogeneous and can be associated with congenital anomalies. Using whole-exome sequencing (WES), we identified five different de novo missense variants in the protein phosphatase-1 catalytic subunit beta (PPP1CB) gene in eight unrelated individuals who share an overlapping phenotype of dysmorphic features, macrocephaly, developmental delay or intellectual disability (ID), congenital heart disease, short stature, and skeletal and connective tissue abnormalities. Protein phosphatase-1 (PP1) is a serine/threonine-specific protein phosphatase involved in the dephosphorylation of a variety of proteins. The PPP1CB gene encodes a PP1 subunit that regulates the level of protein phosphorylation. All five altered amino acids we observed are highly conserved among the PP1 subunit family, and all are predicted to disrupt PP1 subunit binding and impair dephosphorylation. Our data suggest that our heterozygous de novo PPP1CB pathogenic variants are associated with syndromic intellectual disability.

Enzymatic characteristics of an ApaH-like phosphatase, PrpA, and a diadenosine tetraphosphate hydrolase, ApaH, from Myxococcus xanthus

We characterized the activities of the Myxococcus xanthus ApaH-like phosphatases PrpA and ApaH, which share homologies with both phosphoprotein phosphatases and diadenosine tetraphosphate (Ap4A) hydrolases. PrpA exhibited a phosphatase activity towards p-nitrophenyl phosphate (pNPP), tyrosine phosphopeptide and tyrosine-phosphorylated protein, and a weak hydrolase activity towards ApnA and ATP. In the presence of Mn(2+), PrpA hydrolyzed Ap4A into AMP and ATP, whereas in the presence of Co(2+) PrpA hydrolyzed Ap4A into two molecules of ADP. ApaH exhibited high phosphatase activity towards pNPP, and hydrolase activity towards ApnA and ATP. Mn(2+) was required for ApaH-mediated pNPP dephosphorylation and ATP hydrolysis, whereas Co(2+) was required for ApnA hydrolysis. Thus, PrpA and ApaH may function mainly as a tyrosine protein phosphatase and an ApnA hydrolase, respectively.

Characterization of protein phosphatase 5 from three lepidopteran insects: Helicoverpa armigera, Mythimna separata and Plutella xylostella

Protein phosphatase 5 (PP5), a unique member of serine/threonine phosphatases, regulates a variety of biological processes. We obtained full-length PP5 cDNAs from three lepidopteran insects, Helicoverpa armigera, Mythimna separata and Plutella xylostella, encoding predicted proteins of 490 (55.98 kDa), 490 (55.82 kDa) and 491 (56.07 kDa) amino acids, respectively. These sequences shared a high identity with other insect PP5s and contained the TPR (tetratricopeptide repeat) domains at N-terminal regions and highly conserved C-terminal catalytic domains. Tissue- and stage-specific expression pattern analyses revealed these three PP5 genes were constitutively expressed in all stages and in tested tissues with predominant transcription occurring at the egg and adult stages. Activities of Escherichia coli-produced recombinant PP5 proteins could be enhanced by almost 2-fold by a known PP5 activator: arachidonic acid. Kinetic parameters of three recombinant proteins against substrate pNPP were similar both in the absence or presence of arachidonic acid. Protein phosphatases inhibitors, okadaic acid, cantharidin, and endothall strongly impeded the activities of the three recombinant PP5 proteins, as well as exerted an inhibitory effect on crude protein phosphatases extractions from these three insects. In summary, lepidopteran PP5s share similar characteristics and are all sensitive to the protein phosphatases inhibitors. Our results also imply protein phosphatase inhibitors might be used in the management of lepidopteran pests.

Identification and biochemical characterization of protein phosphatase 5 from the cantharidin-producing blister beetle, Epicauta chinensis

Protein phosphatase 5 (PP5) is a unique member of serine/threonine phosphatases which has been recognized in regulation of diverse cellular processes. A cDNA fragment encoding PP5 (EcPP5) was cloned and characterized from the cantharidin-producing blister beetle, E. chinensis. EcPP5 contains an open reading frame of 1500 bp that encodes a protein of 56.89 kDa. The deduced amino acid sequence shares 88% and 68% identities to the PP5 of Tribolium castaneum and humans, respectively. Analysis of the primary sequence shows that EcPP5 has three TPR (tetratricopeptide repeat) motifs at its N-terminal region and contains a highly conserved C-terminal catalytic domain. RT-PCR reveals that EcPP5 is expressed in all developmental stages and in different tissues. The recombinant EcPP5 (rEcPP5) was produced in Escherichia coli and purified to homogeneity. The purified protein exhibited phosphatase activity towards pNPP (p-nitrophenyl phosphate) and phosphopeptides, and its activity can be enhanced by arachidonic acid. In vitro inhibition study revealed that protein phosphatase inhibitors, okadaic acid, cantharidin, norcantharidin and endothall, inhibited its activity. Further, protein phosphatase activity of total soluble protein extract from E. chinensis adults could be impeded by these inhibitors suggesting there might be some mechanism to protect this beetle from being damaged by its self-produced cantharidin.

Importance of a Surface Hydrophobic Pocket on Protein Phosphatase-1 Catalytic Subunit in Recognizing Cellular Regulators

Cellular functions of protein phosphatase-1 (PP1), a major eukaryotic serine/threonine phosphatase, are defined by the association of PP1 catalytic subunits with endogenous protein inhibitors and regulatory subunits. Many PP1 regulators share a consensus RVXF motif, which docks within a hydrophobic pocket on the surface of the PP1 catalytic subunit. Although these regulatory proteins also possess additional PP1-binding sites, mutations of the RVXF sequence established a key role of this PP1-binding sequence in the function of PP1 regulators. WT PP1alpha, the C-terminal truncated PP1alpha-(1-306), a chimeric PP1alpha containing C-terminal sequences from PP2A, another phosphatase, PP1alpha-(1-306) with the RVXF-binding pocket substitutions L289R, M290K, and C291R, and PP2A were analyzed for their regulation by several mammalian proteins. These studies established that modifications of the RVXF-binding pocket had modest effects on the catalytic activity of PP1, as judged by recognition of substrates and sensitivity to toxins. However, the selected modifications impaired the sensitivity of PP1 to the inhibitor proteins, inhibitor-1 and inhibitor-2. In addition, they impaired the ability of PP1 to bind neurabin-I, the neuronal regulatory subunit, and G(M), the skeletal muscle glycogen-targeting subunit. These data suggested that differences in RVXF interactions with the hydrophobic pocket dictate the affinity of PP1 for cellular regulators. Substitution of a distinct RVXF sequence in inhibitor-1 that enhanced its binding and potency as a PP1 inhibitor emphasized the importance of the RVXF sequence in defining the function of this and other PP1 regulators. Our studies suggest that the diversity of RVXF sequences provides for dynamic physiological regulation of PP1 functions in eukaryotic cells.

Identification of a novel shrimp protein phosphatase and its association with latency-related ORF427 of white spot syndrome virus

To characterize the role of latency-associated ORF427 of white spot syndrome virus (WSSV), a shrimp cDNA library was constructed to screen interacting proteins of ORF427. Employing the yeast two-hybrid system, a novel shrimp protein phosphatase (named PPs), sharing 93% homology with human protein phosphatase 1, has been identified able to bind ORF427 in yeast. Through co-immunoprecipitation assays, the interaction between PPs and ORF427 was further confirmed both in vitro and in vivo. Interestingly, the novel shrimp protein phosphatase consists of only 199 aa and contains almost all the functional catalytic domains of human protein phosphatase, while it lacks the corresponding C-terminal non-catalytic sequence. Transcription and translation products of the identified cDNA can be detected in both normal and WSSV-infected shrimps; and PPs was found to localize mainly in the lysosome of shrimp cells. To characterize its function, the PPs cDNA was highly expressed in bacteria and the purified protein showed phosphatase activity when tested against pNPP in a standard phosphatase assay. Our results suggest that the identified protein phosphatase, PPs, may represent a novel member of protein phosphatase family and might be involved in the regulation of WSSV's life cycle through interaction with latency-related ORF427 of WSSV.

When proteome meets genome: the alpha helix and the beta strand of proteins are eschewed by mRNA splice junctions and may define the minimal indivisible modules of protein architecture

The significance of the intron-exon structure of genes is a mystery. As eukaryotic proteins are made up of modular functional domains, each exon was suspected to encode some form of module; however, the definition of a module remained vague. Comparison of pre-mRNA splice junctions with the three-dimensional architecture of its protein product from different eukaryotes revealed that the junctions were far less likely to occur inside the alpha-helices and beta-strands of proteins than within the more flexible linker regions ('turns' and 'loops') connecting them. The splice junctions were equally distributed in the different types of linkers and throughout the linker sequence, although a slight preference for the central region of the linker was observed. The avoidance of the alpha-helix and the beta-strand by splice junctions suggests the existence of a selection pressure against their disruption, perhaps underscoring the investment made by nature in building these intricate secondary structures. A corollary is that the helix and the strand are the smallest integral architectural units of a protein and represent the minimal modules in the evolution of protein structure. These results should find use in comparative genomics, designing of cloning strategies, and in the mutual verification of genome sequences with protein structures.

Post-translational generation of constitutively active cores from larger phosphatases in the malaria parasite, Plasmodium falciparum: implications for proteomics

Background

Although the complete genome sequences of a large number of organisms have been determined, the exact proteomes need to be characterized. More specifically, the extent to which post-translational processes such as proteolysis affect the synthesized proteins has remained unappreciated. We examined this issue in selected protein phosphatases of the protease-rich malaria parasite, Plasmodium falciparum.

Results

P. falciparum encodes a number of Ser/Thr protein phosphatases (PP) whose catalytic subunits are composed of a catalytic core and accessory domains essential for regulation of the catalytic activity. Two examples of such regulatory domains are found in the Ca⁺²-regulated phosphatases, PP7 and PP2B (calcineurin). The EF-hand domains of PP7 and the calmodulin-binding domain of PP2B are essential for stimulation of the phosphatase activity by Ca⁺². We present biochemical evidence that P. falciparum generates these full-length phosphatases as well as their catalytic cores, most likely as intermediates of a proteolytic degradation pathway. While the full-length phosphatases are activated by Ca⁺², the processed cores are constitutively active and either less responsive or unresponsive to Ca⁺². The processing is extremely rapid, specific, and occurs in vivo.

Conclusions

Post-translational cleavage efficiently degrades complex full-length phosphatases in P. falciparum. In the course of such degradation, enzymatically active catalytic cores are produced as relatively stable intermediates. The universality of such proteolysis in other phosphatases or other multi-domain proteins and its potential impact on the overall proteome of a cell merits further investigation.

Binding of the concave surface of the Sds22 superhelix to the alpha 4/alpha 5/alpha 6-triangle of protein phosphatase-1

Functional studies of the protein phosphatase-1 (PP1) regulator Sds22 suggest that it is indirectly and/or directly involved in one of the most ancient functions of PP1, i.e. reversing phosphorylation by the Aurora-related protein kinases. We predict that the conserved portion of Sds22 folds into a curved superhelix and demonstrate that mutation to alanine of any of eight residues (Asp(148), Phe(170), Glu(192), Phe(214), Asp(280), Glu(300), Trp(302), or Tyr(327)) at the concave surface of this superhelix thwarts the interaction with PP1. Furthermore, we show that all mammalian isoforms of PP1 have the potential to bind Sds22. Interaction studies with truncated versions of PP1 and with chimeric proteins comprising fragments of PP1 and the yeast PP1-like protein phosphatase Ppz1 suggest that the site(s) required for the binding of Sds22 reside between residues 43 and 173 of PP1gamma(1). Within this region, a major interaction site was mapped to a triangular region delineated by the alpha4-, alpha5-, and alpha6-helices. Our data also show that well known regulatory binding sites of PP1, such as the RVXF-binding channel, the beta12/beta13-loop, and the acidic groove, are not essential for the interaction with Sds22.

Characterisation and expression of a PP1 serine/threonine protein phosphatase (PfPP1) from the malaria parasite, Plasmodium falciparum: demonstration of its essential role using RNA interference

BACKGROUND

Reversible protein phosphorylation is relatively unexplored in the intracellular protozoa of the Apicomplexa family that includes the genus Plasmodium, to which belong the causative agents of malaria. Members of the PP1 family represent the most highly conserved protein phosphatase sequences in phylogeny and play essential regulatory roles in various cellular pathways. Previous evidence suggested a PP1-like activity in Plasmodium falciparum, not yet identified at the molecular level.

RESULTS

We have identified a PP1 catalytic subunit from P. falciparum and named it PfPP1. The predicted primary structure of the 304-amino acid long protein was highly similar to PP1 sequences of other species, and showed conservation of all the signature motifs. The purified recombinant protein exhibited potent phosphatase activity in vitro. Its sensitivity to specific phosphatase inhibitors was characteristic of the PP1 class. The authenticity of the PfPP1 cDNA was further confirmed by mutational analysis of strategic amino acid residues important in catalysis. The protein was expressed in all erythrocytic stages of the parasite. Abrogation of PP1 expression by synthetic short interfering RNA (siRNA) led to inhibition of parasite DNA synthesis.

CONCLUSIONS

The high sequence similarity of PfPP1 with other PP1 members suggests conservation of function. Phenotypic gene knockdown studies using siRNA confirmed its essential role in the parasite. Detailed studies of PfPP1 and its regulation may unravel the role of reversible protein phosphorylation in the signalling pathways of the parasite, including glucose metabolism and parasitic cell division. The use of siRNA could be an important tool in the functional analysis of Apicomplexan genes.

Identification and biochemical characterisation of a protein phosphatase 5 homologue from Plasmodium falciparum

We report the identification of a new serine/threonine phosphatase from Plasmodium falciparum at the DNA and protein levels. A 1.8 kb cDNA fragment encoding the protein phosphatase was identified via PCR amplification. The sequence has a coding capacity of 594 amino acids. Immunoblot analysis of P. falciparum extracts showed that antibodies generated against the His(6)-fusion protein recognise a protein of approximately 80 kDa. The deduced amino acid sequence shares 55% identity with a mouse protein, identified as Protein Phosphatase 5 (PP5). We show that the P. falciparum PP5 homologue (PfPP5) has all structural and functional characteristics of this class of enzymes. It contains three tetratricopeptide repeats (TPR) and a nuclear targeting sequence at its N-terminus and a highly conserved C-terminal catalytic domain. Southern blot results are compatible with the existence of PfPP5 as a single copy gene. Purified recombinant protein, like the native protein enriched from P. falciparum extracts exhibited phosphatase activity that can be enhanced by both arachidonic and oleic acids, but not by myristic or stearic acid. In addition, the activity is inhibited by okadaic acid (OA) with an IC(50) of 4 nM. Immunofluorescence microscopy has localised PfPP5 preferentially to the nucleus. The function of PfPP5 is presently unclear, but like other PP5s of many eukaryotic organisms, it may have important regulatory functions in the parasite cell cycle.

Kinetic analysis of PPi-dependent phosphofructokinase from Porphyromonas gingivalis

We have previously cloned the gene encoding a pyrophosphate-dependent phosphofructokinase (PFK), designated PgPFK, from Porphyromonas gingivalis, an oral anaerobic bacterium implicated in advanced periodontal disease. In this study, recombinant PgPFK was purified to homogeneity, and biochemically characterized. The apparent K(m) value for fructose 6-phosphate was 2.2 mM, which was approximately 20 times higher than that for fructose 1,6-bisphosphate. The value was significantly greater than any other described PFKs, except for Amycolatopsis methanolica PFK which is proposed to function as a fructose 1,6 bisphosphatase (FBPase). The PgPFK appears to serves as FBPase in this organism. We postulate that this may lead to the gluconeogenic pathways to synthesize the lipopolysaccharides and/or glycoconjugates essential for cell viability.

Normal light response, photoreceptor integrity, and rhodopsin dephosphorylation in mice lacking both protein phosphatases with EF hands (PPEF-1 and PPEF-2)

Rhodopsin dephosphorylation in Drosophila is a calcium-dependent process that appears to be catalyzed by the protein product of the rdgC gene. Two vertebrate rdgC homologs, PPEF-1 and PPEF-2, have been identified. PPEF-1 transcripts are present at low levels in the retina, while PPEF-2 transcripts and PPEF-2 protein are abundant in photoreceptors. To determine if PPEF-2 alone or in combination with PPEF-1 plays a role in rhodopsin dephosphorylation and to determine if retinal degeneration accompanies mutation of PPEF-1 and/or PPEF-2, we have produced mice carrying targeted disruptions in the PPEF-1 and PPEF-2 genes. Loss of either or both PPEFs has little or no effect on rod function, as mice lacking both PPEF-1 and PPEF-2 show little or no changes in the electroretinogram and PPEF-2-/- mice show normal single-cell responses to light in suction pipette recordings. Light-dependent rhodopsin phosphorylation and dephosphorylation are also normal or nearly normal as determined by (i) immunostaining of PPEF-2-/- retinas with the phosphorhodopsin-specific antibody RT-97 and (ii) mass spectrometry of C-terminal rhodopsin peptides from mice lacking both PPEF-1 and PPEF-2. Finally, PPEF-2-/- retinas show normal histology at 1 year of age, and retinas from mice lacking both PPEF-1 and PPEF-2 show normal histology at 3 months of age, the latest time examined. These data indicate that, in contrast to loss of rdgC function in Drosophila, elimination of PPEF function does not cause retinal degeneration in vertebrates.

A novel tetratricopeptide repeat (TPR) containing PP5 serine/threonine protein phosphatase in the malaria parasite, Plasmodium falciparum

BACKGROUND

The malarial parasite, Plasmodium falciparum (Pf), is responsible for nearly 2 million deaths worldwide. However, the mechanisms of cellular signaling in the parasite remain largely unknown. Recent discovery of a few protein kinases and phosphatases point to a thriving reversible phosphorylation system in the parasite, although their function and regulation need to be determined.

RESULTS

We provide biochemical and sequence evidence for a protein serine/threonine phosphatase type PP5 in Plasmodium falciparum, and named it PfPP5. The 594-amino acid polypeptide was encoded by a 1785 nucleotide long intronless gene in the parasite. The recombinant protein, expressed in bacteria, was indistinguishable from native PfPP5. Sequencing comparison indicated that the extra-long N-terminus of PfPP5 outside the catalytic core contained four tetratricopeptide repeats (TPRs), compared to three such repeats in other PP5 phosphatases. The PfPP5 N-terminus was required for stimulation of the phosphatase activity by polyunsaturated fatty acids. Co-immunoprecipitation demonstrated an interaction between native PfPP5 and Pf heat shock protein 90 (hsp90). PfPP5 was expressed in all the asexual erythrocytic stages of the parasite, and was moderately sensitive to okadaic acid.

CONCLUSIONS

This is the first example of a TPR-domain protein in the Apicomplexa family of parasites. Since TPR domains play important roles in protein-protein interaction, especially relevant to the regulation of PP5 phosphatases, PfPP5 is destined to have a definitive role in parasitic growth and signaling pathways. This is exemplified by the interaction between PfPP5 and the cognate chaperone hsp90.

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.

Characterization of a novel serine/threonine protein phosphatase (PfPPJ) from the malaria parasite, Plasmodium falciparum

A novel protein phosphatase cDNA of the PPP superfamily was identified from the malaria parasite, Plasmodium falciparum (Pf), and tentatively named PfPPJ. The predicted primary structure of the phosphatase contained all the known conserved motifs of the PPP superfamily essential for catalytic activity. The enzyme was specific for dephosphorylation of phosphoserine and phosphothreonine residues with very little activity against phosphotyrosine residues. However, the sequence at its C-terminal end was unique, and was consistent with its resistance to the classical PP2A-specific inhibitors such as okadaic acid and microcystin-LR, and the PP1-specific inhibitor, mammalian heat-stable inhibitor-2 (I-2). Even the catalytic core of PfPPJ had a sequence substantially different from the other PPPs such that PfPPJ could be placed in an apparently separate phylogenetic branch. At 294 amino acids residues, PfPPJ was one of the smallest okadaic acid-resistant PPP phosphatases known. By Northern blot analysis, the expression of the PfPPJ mRNA showed the following pattern: schizont > ring > trophozoite, which closely paralleled the expression of the protein, as determined by immunofluorescence. Together, these results suggested a parasitic stage-specific transcriptional regulation of this novel and potentially unique protozoan phosphatase.

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.

Sequencing, expression and biochemical characterization of the Porphyromonas gingivalis pepO gene encoding a protein homologous to human endothelin-converting enzyme

We have determined the nucleotide sequence of the clone pAL2 obtained from Porphyromonas gingivalis 381 in the previous study [Ansai et al. (1995) Microbiology 141, 2047-20521. The DNA sequence analysis of this fragment revealed one complete ORF and one incomplete ORF. The ORF encoded a protein (PgPepO) of 690 amino acids with a calculated molecular weight of 78796. The deduced amino acid sequence exhibited a significant homology with human endothelin-converting enzyme (ECE)-1. Recombinant PgPepO was purified to homogeneity and characterized. The purified enzyme was strongly inhibited by phosphoramidon, and converted big endothelin-1 to endothelin-1. Furthermore, the purified PgPepO strongly cross-reacted with a monoclonal antibody against rat ECE-1. These results indicate that PgPepO has striking similarity to mammalian ECE in structure and function.

Casein kinase 2-mediated phosphorylation of respiratory syncytial virus phosphoprotein P is essential for the transcription elongation activity of the viral polymerase; phosphorylation by casein kinase 1 occurs mainly at Ser(215) and is without effect

The major site of in vitro phosphorylation by casein kinase 2 (CK2) was the conserved Ser(232) in the P proteins of human, bovine, and ovine strains of respiratory syncytial virus (RSV). Enzymatic removal of this phosphate group from the P protein instantly halted transcription elongation in vitro. Transcription reconstituted in the absence of P protein or in the presence of phosphate-free P protein produced abortive initiation products but no full-length transcripts. A recombinant P protein in which Ser(232) was mutated to Asp exhibited about half of the transcriptional activity of the wild-type phosphorylated protein, suggesting that the negative charge of the phosphate groups is an important contributor to P protein function. Use of a temperature-sensitive CK2 mutant yeast revealed that in yeast, phosphorylation of recombinant P by non-CK2 kinase(s) occurs mainly at Ser(215). In vitro, P protein could be phosphorylated by purified CK1 at Ser(215) but this phosphorylation did not result in transcriptionally active P protein. A triple mutant P protein in which Ser(215), Ser(232), and Ser(237) were all mutated to Ala was completely defective in phosphorylation in vitro as well as ex vivo. The xanthate compound D609 inhibited CK2 but not CK1 in vitro and had a very modest effect on P protein phosphorylation and RSV yield ex vivo. Together, these results suggest a role for CK2-mediated phosphorylation of the P protein in the promoter clearance and elongation properties of the viral RNA-dependent RNA polymerase.

Importance of the beta12-beta13 loop in protein phosphatase-1 catalytic subunit for inhibition by toxins and mammalian protein inhibitors

Type-1 protein serine/threonine phosphatases (PP1) are uniquely inhibited by the mammalian proteins, inhibitor-1 (I-1), inhibitor-2 (I-2), and nuclear inhibitor of PP1 (NIPP-1). In addition, several natural compounds inhibit both PP1 and the type-2 phosphatase, PP2A. Deletion of C-terminal sequences that included the beta12-beta13 loop attenuated the inhibition of the resulting PP1alpha catalytic core by I-1, I-2, NIPP-1, and several toxins, including tautomycin, microcystin-LR, calyculin A, and okadaic acid. Substitution of C-terminal sequences from the PP2A catalytic subunit produced a chimeric enzyme, CRHM2, that was inhibited by toxins with dose-response characteristics of PP1 and not PP2A. However, CRHM2 was insensitive to the PP1-specific inhibitors, I-1, I-2, and NIPP-1. The anticancer compound, fostriecin, differed from other phosphatase inhibitors in that it inhibited wild-type PP1alpha, the PP1alpha catalytic core, and CRHM2 with identical IC(50). Binding of wild-type and mutant phosphatases to immobilized microcystin-LR, NIPP-1, and I-2 established that the beta12-beta13 loop was essential for the association of PP1 with toxins and the protein inhibitors. These studies point to the importance of the beta12-beta13 loop structure and conformation for the control of PP1 functions by toxins and endogenous proteins.

A protein phosphatase methylesterase (PME-1) is one of several novel proteins stably associating with two inactive mutants of protein phosphatase 2A

Carboxymethylation of proteins is a highly conserved means of regulation in eukaryotic cells. The protein phosphatase 2A (PP2A) catalytic (C) subunit is reversibly methylated at its carboxyl terminus by specific methyltransferase and methylesterase enzymes which have been purified, but not cloned. Carboxymethylation affects PP2A activity and varies during the cell cycle. Here, we report that substitution of glutamine for either of two putative active site histidines in the PP2A C subunit results in inactivation of PP2A and formation of stable complexes between PP2A and several cellular proteins. One of these cellular proteins, herein named protein phosphatase methylesterase-1 (PME-1), was purified and microsequenced, and its cDNA was cloned. PME-1 is conserved from yeast to human and contains a motif found in lipases having a catalytic triad-activated serine as their active site nucleophile. Bacterially expressed PME-1 demethylated PP2A C subunit in vitro, and okadaic acid, a known inhibitor of the PP2A methylesterase, inhibited this reaction. To our knowledge, PME-1 represents the first mammalian protein methylesterase to be cloned. Several lines of evidence indicate that, although there appears to be a role for C subunit carboxyl-terminal amino acids in PME-1 binding, amino acids other than those at the extreme carboxyl terminus of the C subunit also play an important role in PME-1 binding to a catalytically inactive mutant.

Characterization of protein Ser/Thr phosphatases of the malaria parasite, Plasmodium falciparum: inhibition of the parasitic calcineurin by cyclophilin-cyclosporin complex

Two major protein phosphatase (PP) activities were purified from cytosolic extracts of the erythrocytic stage of the malaria parasite, Plasmodium falciparum. Both enzymes were specific for phosphoserine and phosphothreonine residues with very little activity against phosphotyrosine residues. The biochemical properties of the enzymes suggested their strong similarity with eukaryotic PP2A and PP2B protein phosphatases. Both enzymes preferentially dephosphorylated the alpha subunit of phosphorylase kinase, and were resistant to inhibitor-1. The PP2A-like enzyme required Mn2+ for activity and was inhibited by nanomolar concentrations of okadaic acid (OA). The cDNA sequence of the PP2A-like enzyme was identified through a match of its predicted amino acid sequence with the N-terminal sequence of the catalytic subunit. The PP2B-like (calcineurin) enzyme was stimulated by calmodulin and Ca2+ or Ni2+, but was resistant to OA. Malarial calcineurin was strongly and specifically inhibited by cyclosporin A (CsA) only in the presence of wild type P. falciparum cyclophilin but not a mutant cyclophilin. The inhibition was noncompetitive, and provides a potential explanation for the cyclosporin-sensitivity of the parasite. There was no significant quantitative difference in the total protein Ser/Thr phosphatase activity among the ring, trophozoite, and schizont stages.

Effects of microcystins on phosphorylase-a binding to phosphatase-2A: kinetic analysis by surface plasmon resonance biosensor

Cyclic heptapeptide microcystins are a group of hepatoxicants which exert the cytotoxic effects by inhibiting the catalytic activities of phosphatase-2A (PP-2A) and phosphatase-1 (PP-1) and thus disrupt the normal signal transduction pathways. Microcystins interact with PP-2A and PP-1 by a two-step mechanism involving rapid binding and inactivation of protein phosphatase catalytic subunit, followed by a slower covalent interaction. It was proposed that inactivation of PP-2A/PP-1 catalytic activity by microcystins precedes covalent adduct formation. In this study, we used a biosensor based on surface plasmon resonance (SPR) to examine the effects of three microcystins, MCLR, MCRR and MCYR, on the binding between PP-2A and its substrate, phosphorylase-a (PL-a), during the first step of the interaction. The SPR biosensor provides real-time information on the association and dissociation kinetics of PL-a with immobilized PP-2A in the absence and presence of microcystins. It was found that the affinity of PL-a to microcystin-bound PP-2A was four times smaller compared to unbound PP-2A, due to 50% decreases in the association rates and two-fold increases in dissociation rates of PL-a binding to PP-2A. The results suggest that the rapid binding of microcystins to the PP-2A catalytic site leads to the formation of a noncovalent microcystin/PP-2A adduct. While the adduct formation fully inhibits the catalytic activity of PP-2A, it only results in partial inhibition of the substrate binding. The similar effects of the three microcystins on PP-2A suggest that the toxins bind to PP-2A at the same site and cause similar conformational changes. The present work also demonstrates the potential application of biosensor technology in environmental toxicological research.

Role of cellular actin in the gene expression and morphogenesis of human respiratory syncytial virus

Cytoskeletal protein actin and nonactin cellular proteins were essential for human respiratory syncytial virus (RSV) gene expression. In vitro, specific antibodies against actin inhibited RSV transcription, whereas antibodies against other cytoskeletal proteins had little or no effect. Affinity purified cellular actin or bacterially expressed recombinant actin activated RSV transcription. However, optimal transcription required additional cellular protein(s) that appeared to function as accessory factor(s) for actin. In the absence of actin, these proteins did not activate viral transcription. Purified viral nucleocapsids contained actin, but no cytokeratin, tubulin, or vimentin. Cytochalasin D or DNasel--agents that destabilize actin polymers--had little effect on RSV transcription. RSV infection itself seemed to alter the structure of the cellular actin filaments. Treatment of infected cells with cytochalasin D produced a more severe disruption of the filaments and drastically reduced the production of infectious virus particles but still had little effect on intracellular synthesis of viral macromolecules. Thus actin seems to serve a dual role in RSV life cycle: its monomeric form as well as polymeric form activate viral transcription, while only the microfilament form may take part in viral morphogenesis and/or budding.

Inhibitor-1 interaction domain that mediates the inhibition of protein phosphatase-1

Inhibitor-1 (I-1), a cyclic AMP-regulated phosphoprotein, inhibits protein phosphatase-1 (PP1) activity in response to hormones. The molecular mechanism for PP1 inhibition by I-1 remains unknown. Mutation of nine acidic residues lining a proposed I-1-binding channel in rabbit PP1alpha yielded one mutant (E256A) slightly impaired in its inhibition by I-1, with the IC50 increased by 3-fold, and one mutant (E275R) located in the beta12-beta13 loop that showed 4-fold enhanced inhibition by I-1. Substituting Tyr-272, a proposed binding site for the toxins okadaic acid and microcystin-LR, in the beta12-beta13 loop with Trp, Phe, Asp, Arg, or Ala impaired PP1alpha inhibition by I-1 by 8-10-fold. Chemical mutagenesis of the Saccharomyces cerevisiae PP1 gene (GLC7) yielded 20 point mutations in the PP1 coding region. Two-hybrid analyses and biochemical assays of these yeast enzymes identified four additional residues in the beta12-beta13 loop that were required for PP1 binding and inhibition by I-1. Ten-fold higher concentrations of I-1 were required to inhibit these mutants. Finally, deletion of the beta12-beta13 loop from PP1alpha maintained full enzyme activity, but attenuated inhibition by I-1 by >100-fold. These data identified the beta12-beta13 loop in the PP1 catalytic subunit as a domain that mediates binding and enzyme inhibition by I-1.

Persistent activation of RelA by respiratory syncytial virus involves protein kinase C, underphosphorylated IkappaBbeta, and sequestration of protein phosphatase 2A by the viral phosphoprotein

Respiratory syncytial virus (RSV) activated the RelA (p65) subunit of nuclear factor kappa B (NF-kappaB) over many hours postinfection. The initial activation coincided with phosphorylation and degradation of IkappaBalpha, the cytoplasmic inhibitor of RelA. During persistent activation of NF-kappaB at later times in infection, syntheses of inhibitors IkappaBalpha as well as IkappaBbeta were restored. However, the resynthesized IkappaBbeta was in an underphosphorylated state, which apparently prevented inhibition of NF-kappaB. Use of specific inhibitors suggested that the pathway leading to the persistent-but not the initial-activation of NF-kappaB involved signaling through protein kinase C (PKC) and reactive oxygen intermediates of nonmitochondrial origin, whereas phospholipase C or D played little or no role. Thus, RSV infection led to the activation of NF-kappaB by a biphasic mechanism: a transient or early activation involving phosphorylation of the inhibitor IkappaB polypeptides, and a persistent or long-term activation requiring PKC and the generation of hypophosphorylated IkappaBbeta. At least a part of the activation was through a novel mechanism in which the viral phosphoprotein P associated with but was not dephosphorylated by protein phosphatase 2A and thus sequestered and inhibited the latter. We postulate that this led to a net increase in the phosphorylation state of signaling proteins that are responsible for RelA activation.

Purification and characterization of alkaline phosphatase containing phosphotyrosyl phosphatase activity from the bacterium Prevotella intermedia

A novel alkaline phosphatase, designated PiALP, has been purified and characterized from Prevotella intermedia ATCC 25611, an anaerobe implicated in progressive periodontal disease. The enzyme was a homodimer of apparently identical subunits of Mr 54 kDa. Thiol-reducing agents completely inhibited the purified enzyme. The enzyme was highly stable even at 80 degrees C. It exhibited substantial activity against tyrosine-phosphate-containing Raytide. The phosphatase activity was sensitive to orthovanadate and Zn2+ but highly resistant to okadaic acid. The amino acid sequence of peptides derived from PiALP showed a high degree of identity (65%) with alkaline phosphatases from Zymomonas mobilis and Synechococcus. The present results imply that PiALP might represent a new family of alkaline phosphotyrosyl phosphatases which has not been described previously.

Identification, cloning, and mutational analysis of the casein kinase 1 cDNA of the malaria parasite, Plasmodium falciparum. Stage-specific expression of the gene

The cDNA for casein kinase 1 (CK1) of Plasmodium falciparum was cloned, sequenced, and expressed in bacteria. The single major open reading frame of the 1.2-kilobase pair cDNA coded for a 324-amino acid polypeptide of approximately 37 kDa, the predicted sequence of which showed strong identity with known CK1 isoforms. The purified recombinant enzyme exhibited properties characteristic of CK1, such as inhibition by CK1-7, the ability to phosphorylate a highly specific peptide substrate, and a strong preference for ATP over GTP. A casein kinase activity, partially purified from soluble extracts of P. falciparum by affinity chromatography through CK1-7 columns displayed identical properties. The activity showed a stage-specific expression in the parasite, in the order trophozoite > ring >> schizont. Northern analysis indicated the existence of two major CK1 mRNAs, 2.4 and 3.2 kilobase pairs long, the levels of which were in the order ring > schizont > trophozoite. Mutagenesis of recombinant CK1 defined important amino acid residues and their potential role in the conformation of the enzyme. The malarial CK1 appeared to be the one of the smallest and perhaps the most primitive CK1 enzymes known, containing little sequence information beyond the minimal catalytic domain.

Purification of a fatty acid-stimulated protein-serine/threonine phosphatase from bovine brain and its identification as a homolog of protein phosphatase 5

An arachidonic acid-stimulated Ser/Thr phosphatase activity was detected in soluble extracts prepared from rat pituitary clonal GH4C1 cells, rat or bovine brain, and bovine heart. The enzyme activity was purified to homogeneity from bovine brain as a monomer with a Mr of 63,000 and a specific activity of 32 nmol of Pi released per min/mg of protein when assayed in the presence of 10 microM phosphocasein in the absence of lipid. Arachidonic acid stimulated activity 4-14-fold, with half-maximal stimulation at 50-100 microM, when assayed in the presence of a variety of phosphosubstrates including casein, reduced carboxamidomethylated and maleylated lysozyme, myelin basic protein, and histone. Oleic acid, linoleic acid, and palmitoleic acid also stimulated activity; however, saturated fatty acids and alcohol or methyl ester derivatives of fatty acids did not significantly affect activity. The lipid-stimulated phosphatase was identified as the bovine equivalent of protein phosphatase 5 or a closely related homolog by sequence analysis of proteolytic fragments generated from the purified enzyme. When recombinant rat protein phosphatase 5 was expressed as a cleavable glutathione S-transferase fusion protein, the affinity-purified thrombin-cleaved enzyme exhibited a specific activity and sensitivity to arachidonic acid similar to those of the purified bovine brain enzyme. These results suggest that protein phosphatase 5 may be regulated in vivo by a lipid second messenger or another endogenous activator.