Heterologous expression of human prostatic acid phosphatase and site-directed mutagenesis of the enzyme active site

Purification, identification and Cryo-EM structure of prostatic acid phosphatase in human semen

Prostatic acid phosphatase (PAP) is a glycoprotein that plays a crucial role in the hydrolysis of phosphate ester present in prostatic exudates. It is a well-established indicator for prostate cancer due to its elevated serum levels in disease progression. Despite its abundance in semen, PAP's influence on male fertility has not been extensively studied. In our study, we report a significantly optimized method for purifying human endogenous PAP, achieving remarkably high efficiency and active protein recovery rate. This achievement allowed us to better analyze and understand the PAP protein. We determined the cryo-electron microscopic (Cryo-EM) structure of prostatic acid phosphatase in its physiological state for the first time. Our structural and gel filtration analysis confirmed the formation of a tight homodimer structure of human PAP. This functional homodimer displayed an elongated conformation in the cryo-EM structure compared to the previously reported crystal structure. Additionally, there was a notable 5-degree rotation in the angle between the α domain and α/β domain of each monomer. Through structural analysis, we revealed three potential glycosylation sites: Asn94, Asn220, and Asn333. These sites contained varying numbers and forms of glycosyl units, suggesting sugar moieties influence PAP function. Furthermore, we found that the active sites of PAP, His44 and Asp290, are located between the two protein domains. Overall, our study not only provide an optimized approach for PAP purification, but also offer crucial insights into its structural characteristics. These findings lay the groundwork for further investigations into the physiological function and potential therapeutic applications of this important protein.

Structural insights into a new substrate binding mode of a histidine acid phosphatase from Legionella pneumophila

MapA is a histidine acid phosphatase (HAP) from Legionella pneumophila that catalyzes the hydroxylation of a phosphoryl group from phosphomonoesters by an active-site histidine. Several structures of HAPs, including MapA, in complex with the inhibitor tartrate have been solved and the substrate binding tunnel identified; however, the substrate recognition mechanism remains unknown. To gain insight into the mechanism of substrate recognition, the crystal structures of apo-MapA and the MapA^D281A mutant in complex with 5'-AMP were solved at 2.2 and 2.6 Å resolution, respectively. The structure of the MapA^D281A/5'-AMP complex reveals that the 5'-AMP fits fully into the substrate binding tunnel, with the 2'-hydroxyl group of the ribose moiety stabilized by Glu201 and the adenine moiety sandwiched between His205 and Phe237. This is the second structure of a HAP/AMP complex solved with 5'-AMP binding in a unique manner in the active site. The structure presents a new substrate recognition mechanism of HAPs.

Crystal structure and tartrate inhibition of Legionella pneumophila histidine acid phosphatase

Histidine acid phosphatases (HAPs) utilize a nucleophilic histidine residue to catalyze the transfer of a phosphoryl group from phosphomonoesters to water. HAPs function as protein phosphatases and pain suppressors in mammals, are essential for Giardia lamblia excystation, and contribute to virulence of the category A pathogen Francisella tularensis. Herein we report the first crystal structure and steady-state kinetics measurements of the HAP from Legionella pneumophila (LpHAP), also known as Legionella major acid phosphatase. The structure of LpHAP complexed with the inhibitor l(+)-tartrate was determined at 2.0 Å resolution. Kinetics assays show that l(+)-tartrate is a 50-fold more potent inhibitor of LpHAP than of other HAPs. Electrostatic potential calculations provide insight into the basis for the enhanced tartrate potency: the tartrate pocket of LpHAP is more positive than other HAPs because of the absence of an ion pair partner for the second Arg of the conserved RHGXRXP HAP signature sequence. The structure also reveals that LpHAP has an atypically expansive active site entrance and lacks the nucleotide substrate base clamp found in other HAPs. These features imply that nucleoside monophosphates may not be preferred substrates. Kinetics measurements confirm that AMP is a relatively inefficient in vitro substrate of LpHAP.

Cellular prostatic acid phosphatase, a PTEN-functional homologue in prostate epithelia, functions as a prostate-specific tumor suppressor

The inactivation of tumor suppressor genes (TSGs) plays a vital role in the progression of human cancers. Nevertheless, those ubiquitous TSGs have been shown with limited roles in various stages of diverse carcinogenesis. Investigation on identifying unique TSG, especially for early stage of carcinogenesis, is imperative. As such, the search for organ-specific TSGs has emerged as a major strategy in cancer research. Prostate cancer (PCa) has the highest incidence in solid tumors in US males. Cellular prostatic acid phosphatase (cPAcP) is a prostate-specific differentiation antigen. Despite intensive studies over the past several decades on PAcP as a PCa biomarker, the role of cPAcP as a PCa-specific tumor suppressor has only recently been emerged and validated. The mechanism underlying the pivotal role of cPAcP as a prostate-specific TSG is, in part, due to its function as a protein tyrosine phosphatase (PTP) as well as a phosphoinositide phosphatase (PIP), an apparent functional homologue to phosphatase and tensin homolog (PTEN) in PCa cells. This review is focused on discussing the function of this authentic prostate-specific tumor suppressor and the mechanism behind the loss of cPAcP expression leading to prostate carcinogenesis. We review other phosphatases' roles as TSGs which regulate oncogenic PI3K signaling in PCa and discuss the functional similarity between cPAcP and PTEN in prostate carcinogenesis.

Emerging roles of human prostatic Acid phosphatase

Prostate cancer is one of the most prevalent non-skin related cancers. It is the second leading cause of cancer deaths among males in most Western countries. If prostate cancer is diagnosed in its early stages, there is a higher probability that it will be completely cured. Prostatic acid phosphatase (PAP) is a non-specific phosphomonoesterase synthesized in prostate epithelial cells and its level proportionally increases with prostate cancer progression. PAP was the biochemical diagnostic mainstay for prostate cancer until the introduction of prostate-specific antigen (PSA) which improved the detection of early-stage prostate cancer and largely displaced PAP. Recently, however, there is a renewed interest in PAP because of its usefulness in prognosticating intermediate to high-risk prostate cancers and its success in the immunotherapy of prostate cancer. Although PAP is believed to be a key regulator of prostate cell growth, its exact role in normal prostate as well as detailed molecular mechanism of PAP regulation is still unclear. Here, many different aspects of PAP in prostate cancer are revisited and its emerging roles in other environment are discussed.

Crystal structures and biochemical studies of human lysophosphatidic acid phosphatase type 6

Lysophosphatidic acid (LPA) is an important bioactive phospholipid involved in cell signaling through Gprotein-coupled receptors pathways. It is also involved in balancing the lipid composition inside the cell, and modulates the function of lipid rafts as an intermediate in phospholipid metabolism. Because of its involvement in these important processes, LPA degradation needs to be regulated as precisely as its production. Lysophosphatidic acid phosphatase type 6 (ACP6) is an LPA-specific acid phosphatase that hydrolyzes LPA to monoacylglycerol (MAG) and phosphate. Here, we report three crystal structures of human ACP6 in complex with malonate, L-(+)-tartrate and tris, respectively. Our analyses revealed that ACP6 possesses a highly conserved Rossmann-foldlike body domain as well as a less conserved cap domain. The vast hydrophobic substrate-binding pocket, which is located between those two domains, is suitable for accommodating LPA, and its shape is different from that of other histidine acid phosphatases, a fact that is consistent with the observed difference in substrate preferences. Our analysis of the binding of three molecules in the active site reveals the involvement of six conserved and crucial residues in binding of the LPA phosphate group and its catalysis. The structure also indicates a water-supplying channel for substrate hydrolysis. Our structural data are consistent with the fact that the enzyme is active as a monomer. In combination with additional mutagenesis and enzyme activity studies, our structural data provide important insights into substrate recognition and the mechanism for catalytic activity of ACP6.

Isolation and enzymatic properties of a nonspecific acid phosphatase from Vigna aconitifolia seeds

Acid phosphatase (EC 3.1.3.2) from Vigna aconitifolia seeds was purified to apparent homogeneity by using ammonium sulfate fractionation and cation-exchange chromatography [carboxymethyl (CM) cellulose]. The enzyme was 228-fold purified with 14.6% recovery. Analytical gel filtration chromatography on Sephadex G-200 column showed that Mr of native enzyme was 58 kDa and denaturing PAGE demonstrated that it was made up of two subunits of 24 and 27 kDa. The enzyme showed its optimum activity at pH 5.0 and 60°C. It exhibited broad substrate specificity and showed a higher specificity constant for para-nitrophenyl phosphate, Na β-naphthyl phosphate, and adenosine monophosphate (AMP). Cu²⁺, Mo⁶⁺, Fe³⁺, phosphate, and fluoride ions were reported as strong inhibitors for the enzyme. Active site study for the enzyme demonstrated that tryptophan and aspartic acid may be important for the catalysis.

Human prostatic acid phosphatase: structure, function and regulation

Human prostatic acid phosphatase (PAcP) is a 100 kDa glycoprotein composed of two subunits. Recent advances demonstrate that cellular PAcP (cPAcP) functions as a protein tyrosine phosphatase by dephosphorylating ErbB-2/Neu/HER-2 at the phosphotyrosine residues in prostate cancer (PCa) cells, which results in reduced tumorigenicity. Further, the interaction of cPAcP and ErbB-2 regulates androgen sensitivity of PCa cells. Knockdown of cPAcP expression allows androgen-sensitive PCa cells to develop the castration-resistant phenotype, where cells proliferate under an androgen-reduced condition. Thus, cPAcP has a significant influence on PCa cell growth. Interestingly, promoter analysis suggests that PAcP expression can be regulated by NF-κB, via a novel binding sequence in an androgen-independent manner. Further understanding of PAcP function and regulation of expression will have a significant impact on understanding PCa progression and therapy.

Secretion and N-linked glycosylation are required for prostatic acid phosphatase catalytic and antinociceptive activity

Secretory human prostatic acid phosphatase (hPAP) is glycosylated at three asparagine residues (N62, N188, N301) and has potent antinociceptive effects when administered to mice. Currently, it is unknown if these N-linked residues are required for hPAP protein stability and activity in vitro or in animal models of chronic pain. Here, we expressed wild-type hPAP and a series of Asn to Gln point mutations in the yeast Pichia pastoris X33 then analyzed protein levels and enzyme activity in cell lysates and in conditioned media. Pichia secreted wild-type recombinant (r)-hPAP into the media (6-7 mg protein/L). This protein was as active as native hPAP in biochemical assays and in mouse models of inflammatory pain and neuropathic pain. In contrast, the N62Q and N188Q single mutants and the N62Q, N188Q double mutant were expressed at lower levels and were less active than wild-type r-hPAP. The purified N62Q, N188Q double mutant protein was also 1.9 fold less active in vivo. The N301Q mutant was not expressed, suggesting a critical role for this residue in protein stability. To explicitly test the importance of secretion, a construct lacking the signal peptide of hPAP was expressed in Pichia and assayed. This "cellular" construct was not expressed at levels detectable by western blotting. Taken together, these data indicate that secretion and post-translational carbohydrate modifications are required for PAP protein stability and catalytic activity. Moreover, our findings indicate that recombinant hPAP can be produced in Pichia--a yeast strain that is used to generate biologics for therapeutic purposes.

Structural and functional analysis of human prostatic acid phosphatase

Prostatic acid phosphatase (PAP) is the most abundant phosphatase in human prostate tissue/secretions. It is a clinically important protein for its relevance as a biomarker of prostate carcinoma. Furthermore, it has a potential role in fertilization. We describe here most of the features of PAP including gene regulation, gene/protein structure, functions, its role in tumor progression and evolutionary features. PAP has phosphatase activity and is an extensively studied biomarker of prostate cancer. The major action of PAP is to dephosphorylate macromolecules with the help of catalytic residues (His(12) and Asp(258)) that are located in the cleft between two domains. This article will be of great interest to all those scientists who are working in the area of prostate pathophysiology.

Acid phosphatases do not contribute to the pathogenesis of type A Francisella tularensis

The intracellular pathogen Francisella tularensis is the causative agent of tularemia, a zoonosis that can affect humans with potentially lethal consequences. Essential to Francisella virulence is its ability to survive and proliferate within phagocytes through phagosomal escape and cytosolic replication. Francisella spp. encode a variety of acid phosphatases, whose roles in phagosomal escape and virulence have been documented yet remain controversial. Here we have examined in the highly virulent (type A) F. tularensis strain Schu S4 the pathogenic roles of three distinct acid phosphatases, AcpA, AcpB, and AcpC, that are most conserved between Francisella subspecies. Neither the deletion of acpA nor the combination of acpA, acpB, and acpC deletions affected the phagosomal escape or cytosolic growth of Schu S4 in murine and human macrophages, despite decreases in acid phosphatase activities by as much as 95%. Furthermore, none of these mutants were affected in their ability to cause lethality in mice upon intranasal inoculation. Hence, the acid phosphatases AcpA, AcpB, and AcpC do not contribute to intracellular pathogenesis and do not play a major role in the virulence of type A Francisella strains.

Crystal Structures of the histidine acid phosphatase from Francisella tularensis provide insight into substrate recognition

Histidine acid phosphatases catalyze the transfer of a phosphoryl group from phosphomonoesters to water at acidic pH using an active-site histidine. The histidine acid phosphatase from the category A pathogen Francisella tularensis (FtHAP) has been implicated in intramacrophage survival and virulence, motivating interest in understanding the structure and mechanism of this enzyme. Here, we report a structure-based study of ligand recognition by FtHAP. The 1.70-A-resolution structure of FtHAP complexed with the competitive inhibitor l(+)-tartrate was solved using single-wavelength anomalous diffraction phasing. Structures of the ligand-free enzyme and the complex with inorganic phosphate were determined at resolutions of 1.85 and 1.70 A, respectively. The structure of the Asp261Ala mutant enzyme complexed with the substrate 3'-AMP was determined at 1.50 A resolution to gain insight into substrate recognition. FtHAP exhibits a two-domain fold similar to that of human prostatic acid phosphatase, consisting of an alpha/beta core domain and a smaller domain that caps the core domain. The structures show that the core domain supplies the phosphoryl binding site, catalytic histidine (His17), and an aspartic acid residue (Asp261) that protonates the leaving group, while the cap domain contributes residues that enforce substrate preference. FtHAP and human prostatic acid phosphatase differ in the orientation of the crucial first helix of the cap domain, implying differences in the substrate preferences of the two enzymes. 3'-AMP binds in one end of a 15-A-long tunnel, with the adenine clamped between Phe23 and Tyr135, and the ribose 2'-hydroxyl interacting with Gln132. The importance of the clamp is confirmed with site-directed mutagenesis; mutation of Phe23 and Tyr135 individually to Ala increases K(m) by factors of 7 and 10, respectively. The structural data are consistent with a role for FtHAP in scavenging phosphate from small molecules present in host macrophage cells.

Revisiting histidine-dependent acid phosphatases: a distinct group of tyrosine phosphatases

Although classical protein tyrosine phosphatase (PTP) superfamily members are cysteine-dependent, emerging evidence shows that many acid phosphatases (AcPs) function as histidine-dependent PTPs in vivo. These AcPs dephosphorylate phospho-tyrosine substrates intracellularly and could have roles in development and disease. In contrast to cysteine-dependent PTPs, they utilize histidine, rather than cysteine, for substrate dephosphorylation. Structural analyses reveal that active site histidine, but not cysteine, faces towards the substrate and functions as the phosphate acceptor. Nonetheless, during dephosphorylation, both histidine-dependent and cysteine-dependent PTPs use their active site arginine and aspartate for substrate binding and proton donation, respectively. Thus, we propose that they should be referred to as a distinct group of 'histidine-dependent PTPs' within the PTP superfamily.

Recombinant mouse PAP has pH-dependent ectonucleotidase activity and acts through A(1)-adenosine receptors to mediate antinociception

Prostatic acid phosphatase (PAP) is expressed in nociceptive neurons and functions as an ectonucleotidase. When injected intraspinally, the secretory isoforms of human and bovine PAP protein have potent and long-lasting antinociceptive effects that are dependent on A₁-adenosine receptor (A₁R) activation. In this study, we purified the secretory isoform of mouse (m)PAP using the baculovirus expression system to determine if recombinant mPAP also had antinociceptive properties. We found that mPAP dephosphorylated AMP, and to a much lesser extent, ADP at neutral pH (pH 7.0). In contrast, mPAP dephosphorylated all purine nucleotides (AMP, ADP, ATP) at an acidic pH (pH 5.6). The transmembrane isoform of mPAP had similar pH-dependent ectonucleotidase activity. A single intraspinal injection of mPAP protein had long-lasting (three day) antinociceptive properties, including antihyperalgesic and antiallodynic effects in the Complete Freund's Adjuvant (CFA) inflammatory pain model. These antinociceptive effects were transiently blocked by the A₁R antagonist 8-cyclopentyl-1, 3-dipropylxanthine (CPX), suggesting mPAP dephosphorylates nucleotides to adenosine to mediate antinociception just like human and bovine PAP. Our studies indicate that PAP has species-conserved antinociceptive effects and has pH-dependent ectonucleotidase activity. The ability to metabolize nucleotides in a pH-dependent manner could be relevant to conditions like inflammation where tissue acidosis and nucleotide release occur. Lastly, our studies demonstrate that recombinant PAP protein can be used to treat chronic pain in animal models.

The histidine phosphatase superfamily: structure and function

The histidine phosphatase superfamily is a large functionally diverse group of proteins. They share a conserved catalytic core centred on a histidine which becomes phosphorylated during the course of the reaction. Although the superfamily is overwhelmingly composed of phosphatases, the earliest known and arguably best-studied member is dPGM (cofactor-dependent phosphoglycerate mutase). The superfamily contains two branches sharing very limited sequence similarity: the first containing dPGM, fructose-2,6-bisphosphatase, PhoE, SixA, TIGAR [TP53 (tumour protein 53)-induced glycolysis and apoptosis regulator], Sts-1 and many other activities, and the second, smaller, branch composed mainly of acid phosphatases and phytases. Human representatives of both branches are of considerable medical interest, and various parasites contain superfamily members whose inhibition might have therapeutic value. Additionally, several phosphatases, notably the phytases, have current or potential applications in agriculture. The present review aims to draw together what is known about structure and function in the superfamily. With the benefit of an expanding set of histidine phosphatase superfamily structures, a clearer picture of the conserved elements is obtained, along with, conversely, a view of the sometimes surprising variation in substrate-binding and proton donor residues across the superfamily. This analysis should contribute to correcting a history of over- and mis-annotation in the superfamily, but also suggests that structural knowledge, from models or experimental structures, in conjunction with experimental assays, will prove vital for the future description of function in the superfamily.

Cloning and characterization of two human VIP1-like inositol hexakisphosphate and diphosphoinositol pentakisphosphate kinases

Eukaryotes possess numerous inositol phosphate (IP) and diphosphoinositol phosphate (PP-IPs or inositol pyrophosphates) species that act as chemical codes important for intracellular signaling pathways. Production of IP and PP-IP molecules occurs through several classes of evolutionarily conserved inositol phosphate kinases. Here we report the characterization of a human inositol hexakisphosphate (IP6) and diphosphoinositol pentakisphosphate (PP-IP5 or IP7) kinase with similarity to the yeast enzyme Vip1, a recently identified IP6/IP7 kinase (Mulugu, S., Bai, W., Fridy, P. C., Bastidas, R. J., Otto, J. C., Dollins, D. E., Haystead, T. A., Ribeiro, A. A., and York, J. D. (2007) Science 316, 106-109). Recombinant human VIP1 exhibits in vitro IP6 and IP7 kinase activities and restores IP7 synthesis when expressed in mutant yeast. Expression of human VIP1 in HEK293T cells engineered to produce high levels of IP7 results in dramatic increases in bisdiphosphoinositol tetrakisphosphate (PP2-IP4 or IP8). Northern blot analysis indicates that human VIP1 is expressed in a variety of tissues and is enriched in skeletal muscle, heart, and brain. The subcellular distribution of tagged human VIP1 is indicative of a cytoplasmic non-membrane localization pattern. We also characterized human and mouse VIP2, an additional gene product with nearly 90% similarity to VIP1 in the kinase domain, and observed both IP6 and IP7 kinase activities. Our data demonstrate that human VIP1 and VIP2 function as IP6 and IP7 kinases that act along with the IP6K/Kcs1-class of kinases to convert IP6 to IP8 in mammalian cells, a process that has been found to occur in response to various stimuli and signaling events.

Lily pollen alkaline phytase is a histidine phosphatase similar to mammalian multiple inositol polyphosphate phosphatase (MINPP)

Phytic acid is the most abundant inositol phosphate in cells; it constitutes 1-5% of the dry weight of cereal grains and legumes. Phytases are the primary enzymes responsible for the hydrolysis of phytic acid and thus play important roles in inositol phosphate metabolism. A novel alkaline phytase in lily pollen (LlALP) was recently purified in our laboratory. In this paper, we describe the cloning and characterization of LlALP cDNA from lily pollen. Two isoforms of alkaline phytase cDNAs, LlAlp1 and LlAlp2, which are 1467 and 1533 bp long and encode proteins of 487 and 511 amino acids, respectively, were identified. The deduced amino acid sequences contains the signature heptapeptide of histidine phosphatases, -RHGXRXP-, but shares < 25% identity to fungal histidine acid phytases. Phylogenetic analysis reveals that LlALP is most closely related to multiple inositol polyphosphate phosphatase (MINPP) from humans (25%) and rats (23%). mRNA corresponding to LlAlp1 and LlAlp2 were expressed in leaves, stem, petals and pollen grains. The expression profiles of LlAlp isoforms in anthers indicated that mRNA corresponding to both isoforms were present at all stages of flower development. The expression of LlAlp2 cDNA in Escherichia coli revealed the accumulation of the active enzyme in inclusion bodies and confirmed that the cDNA encodes an alkaline phytase. In summary, plant alkaline phytase is a member of the histidine phosphatase family that includes MINPP and exhibits properties distinct from bacterial and fungal phytases.

Transcriptional control and patterning of the pho-1 gene, an essential acid phosphatase expressed in the C. elegans intestine

We have previously described an acid phosphatase enzyme, PHO-1, present at the lumenal surface of all but the anterior six cells of the Caenorhabditis elegans intestine. In the present paper, we identify the pho-1 structural gene, which encodes a histidine acid phosphatase showing highest similarity to human prostatic acid phosphatase. The pho-1 5'-flanking DNA is capable of directing reporter gene expression that is both gut specific, correctly timed and correctly "patterned", that is, not expressed in the gut anterior. Furthermore, this anterior-posterior patterning of pho-1 expression responds to the C. elegans Wnt pathway as if pho-1 is repressed (directly or indirectly) by high levels of the HMG effector protein POP-1. Transgenic analysis of the pho-1 promoter shows that gut expression is critically dependent on a single WGATAR site. The gut-specific GATA factor ELT-2 binds to this site in vitro and removal of ELT-2 from the embryo destroys expression of the pho-1 reporter. Thus, all our results indicate that pho-1 is a direct downstream target of ELT-2. Finally, the pho-1 loss-of-function mutation shows an interesting and unexpected phenotype for a somatically-expressed hydrolytic enzyme: loss of pho-1 causes arrest of the majority of embryos but this lethality is a maternal effect. We suggest that pho-1 is required by the maternal intestine to assimilate some nutrient or cleavage product that is subsequently provided to the next generation of embryos.

From gene to protein: Prostatic acid phosphatase: Structure and expression of gene and protein*

A set of classes for medical students is designed to reinforce an understanding of the basic laboratory methods of molecular biology and protein biochemistry in the context of a clinically important problem, prostate gland pathology. Students examine the gene coding for prostatic acid phosphatase and they assay expression of the gene in different lines of prostate cancer cell cultures (LNCaP and PC-3). The three-dimensional structure of the expressed protein is also investigated, in relation to its catalytic function. Students are encouraged to collect data for their experiments and to perform laboratory exercises on their own. The theory and practice should stimulate the students' discussion of various fields of biochemistry and molecular biology.

Regulation of ErbB4 phosphorylation and cleavage by a novel histidine acid phosphatase

Signaling by a variety of ligands including epidermal growth factor, betacellulin and neuregulin is mediated by the ErbB family of receptor tyrosine kinases. Studies on the prostate have shown that ErbB2 phosphorylation and signaling can be regulated by prostatic acid phosphatase, a histidine acid phosphatase which can dephosphorylate phospho-tyrosine residues in the ErbB2 receptor. Here we report that the histidine acid phosphatase ACPT (testicular acid phosphatase), which is highly homologous to the prostatic acid phosphatase, can dephosphorylate the ErbB4 receptor, which is known to play important roles in neuronal differentiation and synaptogenesis. ACPT and ErbB4 are both expressed in the brain where they are enriched at post-synaptic sites, and furthermore they can be co-immunoprecipitated from brain. We demonstrate that ACPT can inhibit basal and neuregulin-induced tyrosine phosphorylation of ErbB4. We also show that ACPT-dependent dephosphorylation can regulate the proteolytic cleavage of ErbB4, and this process can be reversed by applying the tyrosine phosphatase inhibitor, pervanadate. Furthermore, neuregulin-dependent differentiation of PC12 cells expressing ErbB4 is prevented by co-expressing ACPT. These results indicate that ACPT acts as a tyrosine phosphatase to modulate signals mediated by ErbB4 that are important for neuronal development and synaptic plasticity.

Equilibrium unfolding of dimeric human prostatic acid phosphatase involves an inactive monomeric intermediate

Guanidine hydrochloride (GdnHCl)-induced unfolding of human prostatic acid phosphatase (hPAP), a homodimer of 50 kDa subunit molecular weight, was investigated with activity measurements, size exclusion HPLC, tryptophan fluorescence, 1-anilinonaphtalene-8-sulfonate (ANS) binding and reactivity with 2-(4'-maleimidoanilino)naphthalene-6-sulfonate (MIANS). Equilibrium analysis was performed to shed light on the role of dimerization in the folding and stability of the catalytically active oligomeric protein. Unfolding was reversible, as verified by activity measurements and tryptophan fluorescence. The noncoincidence of the unfolding curves obtained by different techniques suggests the occurrence of a multiphasic process. The reaction of hPAP inactivation is accompanied by dissociation of the dimer into two monomers. The midpoint of this transition is at 0.65 M GdnHCl with 4.24+/-0.12 kcalmol(-1) free energy change. Binding of ANS to the inactive phosphatase monomer, especially remarkable in the region from 0.8 to 1.25M GdnHCl, suggests that the hydrophobic probe indicates exposition of the intersubunit hydrophobic surface and a loosening of the monomer's tertiary structure. Strong fluorescence of thiol group derivatives, the products of their reaction with MIANS, appears in a limited range of GdnHCl concentrations (1.2-1.6M). This shows that in the relaxed structure of the intermediate, the reagent is allowed to penetrate into the hydrophobic environment of the partially hidden thiol groups. The equilibrium unfolding reaction of hPAP, as monitored by tryptophan fluorescence, does not depend on the protein concentration and displays a single transition curve with a midpoint at 1.7 M GdnHCl and value of DeltaG(unf)(H(2)O)=3.38+/-0.08 kcalmol(-1) per monomer, a result implying that this transition is related to the conformational change of the earlier dissociated and already inactive subunit of the protein.

Crystal structures of human prostatic acid phosphatase in complex with a phosphate ion and alpha-benzylaminobenzylphosphonic acid update the mechanistic picture and offer new insights into inhibitor design

The X-ray crystal structure of human prostatic acid phosphatase (PAP) in complex with a phosphate ion has been determined at 2.4 A resolution. This structure offers a snapshot of the final intermediate in the catalytic mechanism and does not support the role of Asp 258 as a proton donor in catalysis. A total of eight hydrogen bonds serve to strongly bind the phosphate ion within the active site. Bound PEG molecules from the crystallization matrix have allowed the identification of a channel within the molecule that likely plays a role in molecular recognition and in macromolecular substrate selectivity. Additionally, the structure of PAP in complex with a phosphate derivative, alpha-benzylaminobenzylphosphonic acid, a potent inhibitor (IC(50) = 4 nM), has been determined to 2.9 A resolution. This structure gives new insight into the determinants of binding hydrophobic ligands within the active site and allows us to explain PAP's preference for aromatic substrates.

Inositol phosphatase activity of the Escherichia coli agp-encoded acid glucose-1-phosphatase

When screening an Escherichia coli gene library for myo-inositol hexakisphosphate (InsP6) phosphatases (phytases), we discovered that the agp-encoded acid glucose-1-phosphatase also possesses this activity. Purified Agp hydrolyzes glucose-1-phosphate, p-nitrophenyl phosphate, and InsP6 with pH optima, 6.5, 3.5, and 4.5, respectively, and was stable when incubated at pH values ranging from 3 to 10. Glucose-1-phosphate was hydrolyzed most efficiently at 55 degrees C. while InsP6 and p-nitrophenyl phosphate were hydrolyzed maximally at 60 degrees C. The Agp exhibited Km values of (0.39 mM, 13 mM, and 0.54 mM for the hydrolysis of glucose-1-phosphate, p-nitrophenyl phosphate, and InsP6, respectively. High-pressure liquid chromatography (HPLC) analysis of inositol phosphate hydrolysis products of Agp demonstrated that the enzyme catalyzes the hydrolysis of phosphate from each of InsP6, D-Ins(1,2,3,4,5)P5, Ins(1,3,4,5,6)P5, and Ins(1,2,3,4,6)P5, producing D/L-Ins(1,2,4,5,6)P5. D-Ins(1,2,4,5)P4, D/L-Ins(1,4,5,6)P4 and D/L-Ins(1,2,4,6)P4, respectively. These data support the contention that Agp is a 3-phosphatase.

Human XPA and XRCC1 DNA repair proteins expressed in yeast, Saccharomyces cerevisiae

Human XPA and XRCC1 DNA repair proteins have been expressed in a series of novel yeast episomal vectors. Expression of XPA cDNA resulted in synthesis of anti-XPA crossreacting polypeptides of 40 and 42 kDa, the status of the native protein found in human cells. Likewise, the majority of the recombinant XRCC1 found in the yeast intracellular fraction corresponded to the molecular mass of the full-length human protein. Recombinant XPA protein expressed as an NH(2)-terminal polyhistidine fusion could be affinity purified using Ni(2+) agarose.

A cofactor-dependent phosphoglycerate mutase homolog from Bacillus stearothermophilus is actually a broad specificity phosphatase

The distribution of phosphoglycerate mutase (PGM) activity in bacteria is complex, with some organisms possessing both a cofactor-dependent and a cofactor-independent PGM and others having only one of these enzymes. Although Bacillus species contain only a cofactor-independent PGM, genes homologous to those encoding cofactor-dependent PGMs have been detected in this group of bacteria, but in at least one case the encoded protein lacks significant PGM activity. Here we apply sequence analysis, molecular modeling, and enzymatic assays to the cofactor-dependent PGM homologs from B. stearothermophilus and B. subtilis, and show that these enzymes are phosphatases with broad substrate specificity. Homologs from other gram-positive bacteria are also likely to possess phosphatase activity. These studies clearly show that the exploration of genomic sequences through three-dimensional modeling is capable of producing useful predictions regarding function. However, significant methodological improvements will be needed before such analysis can be carried out automatically.

Cooperative kinetics of human prostatic acid phosphatase

The steady-state kinetics of hydrolysis reaction catalysed by human prostatic acid phosphatase (PAP) by using 1-naphthyl phosphate, phenyl phosphate and phosphotyrosine as substrates has been studied at pH 5.5. The substrate binding curves were sigmoidal and Hill cooperation coefficient h was higher than 1 for each of the examined compounds. Thus, human prostatic acid phosphatase kinetics exhibits positive cooperativity towards the studied substrates. The extent of cooperativity was found to depend on the substrate used and on enzyme concentration. The highest cooperativity of PAP was observed for 1-naphthyl phosphate and the lowest for phosphotyrosine. When prostatic phosphatase concentration increased, Hill cooperation coefficient (h) and half saturation constant (K(0.5)) both grew, but the catalytic constant (k(cat)) remained constant, for each of the substrates studied. Ligand-induced association-dissociation equilibrium of the active oligomeric species (monomer-dimer-tetramer-oligomers) is suggested.

Characterization of a prostate-specific tyrosine phosphatase by mutagenesis and expression in human prostate cancer cells

The cellular form of human prostatic acid phosphatase (PAcP) is a neutral protein-tyrosine phosphatase (PTP) and may play a key role in regulating the growth and androgen responsiveness of prostate cancer cells. The functional role of the enzyme is at least due in part to its dephosphorylation of c-ErbB-2, an in vivo substrate of the enzyme. In this study, we investigated the molecular mechanism of phosphotyrosine dephosphorylation by cellular PAcP. We mutated several amino acid residues including one cysteine residue that was proposed to be involved in the PTP activity of the enzyme by serving as the phosphate acceptor. The cDNA constructs of mutant enzymes were transiently transfected into C-81 LNCaP and PC-3 human prostate cancer cells that lack the endogenous PAcP expression. The phosphotyrosine level of ErbB-2 in these transfected cells was subsequently analyzed. Our results demonstrated that the phosphotyrosine level of ErbB-2 in cells expressing H12A or D258A mutant PAcP is similar to that in control cells without PAcP expression, suggesting that these mutants are incapable of dephosphorylating ErbB-2. In contrast, cells expressing C183A, C281A, or wild-type PAcP had a decreased phosphotyrosine level of ErbB-2, compared with the control cells. Similar results were obtained from in vitro dephosphorylation of immunoprecipitated ErbB-2 by these mutant enzymes. Furthermore, transient expression of C183A, C281A, or the wild-type enzyme, but not H12A or D258A, decreased the growth rate of C-81 LNCaP cells. The data collectively indicate that His-12 and Asp-258, but not Cys-183 or Cys-281, are required for the PTP activity of PAcP.

Active site residue 297 of Aspergillus niger phytase critically affects the catalytic properties

The wild-type phytases from the Aspergillus niger strains NRRL 3135 and T213 display a three-fold difference in specific activity (103 versus 32 U/mg protein), despite only 12 amino acid differences that are distributed all over the sequence of the protein. Of the 12 divergent positions, three are located in or close to the substrate binding site. Site-directed mutagenesis of these residues in A. niger T213 phytase showed that the R297Q mutation (R in T213, Q in NRRL 3135) fully accounts for the differences in catalytic properties observed. Molecular modelling revealed that R297 may directly interact with a phosphate group of phytic acid. The fact that this presumed ionic interaction - causing stronger binding of substrates and products - correlates with a lower specific activity indicates that product (myo-inositol pentakisphosphate) release is the rate-limiting step of the reaction.

Multiple inositol polyphosphate phosphatase: evolution as a distinct group within the histidine phosphatase family and chromosomal localization of the human and mouse genes to chromosomes 10q23 and 19

Multiple inositol polyphosphate phosphatase is the only enzyme known to hydrolyze the abundant metabolites inositol pentakisphosphate and inositol hexakisphosphate. We have previously demonstrated that the chick homolog of multiple inositol polyphosphate phosphatase, designated HiPER1, has a role in growth plate chondrocyte differentiation. The relationship of these enzymes to intracellular signaling is obscure, and as part of our investigation we have examined the murine ((MMU)Minpp1) and human ((HSA)MINPP1) homologs. Northern blot analysis demonstrated expression of ((MMU)Minpp1 in a variety of mouse tissues, comparable to the expression of other mammalian homologs, but less restricted than the expression of HiPER1 in chick. A purified (MMU)Minpp1 fusion protein cleaved phosphate from inositol (1,3,4,5)-tetrakisphosphate and para-nitrophenyl phosphate. When the presumptive active site histidine was altered to alanine by site-directed mutagenesis, enzyme activity was abolished, confirming the classification of (MMU)Minpp1 as a histidine phosphatase. The amino acid sequences of the murine and human MINPP proteins share >80% identity with the rat enzyme and >56% identity with HiPER1, with conservation of the C-terminal consensus sequence that retains proteins in the endoplasmic reticulum. The intron/exon structure of the mammalian (MMU)Minpp1 and (HSA)MINPP1 genes is also conserved compared to the chick HiPER1 gene. Sequence analysis of plant and fruit fly MINPP homologs supports the hypothesis that the MINPP enzymes constitute a distinct evolutionary group within the histidine phosphatase family. We have mapped (HSA)MINPP1 to human chromosome 10q23 by fluorescence in situ hybridization, YAC screening, and radiation hybrid mapping. This assignment places (HSA)MINPP1 in a region of chromosome 10 that is frequently mutated in human cancers and places (HSA)MINPP1 proximal to the tumor suppressor PTEN, which maps to 10q23.3. Using a radiation hybrid panel, we localized (MMU)Minpp1 to a region of mouse chromosome 19 that includes the murine homolog of Pten. The evolutionary conservation of this novel enzyme within the inositol polyphosphate pathway suggests a significant role for multiple inositol polyphosphate phosphatase throughout higher eukaryotes.

The human and rat forms of multiple inositol polyphosphate phosphatase: functional homology with a histidine acid phosphatase up-regulated during endochondral ossification

We have derived the full-length sequences of the human and rat forms of the multiple inositol polyphosphate phosphatase (MIPP); their structural and functional comparison with a chick histidine acid phosphatase (HiPER1) has revealed new information: (1) MIPP is approximately 50% identical to HiPER1, but the ER-targeting domains are divergent; (2) MIPP appears to share the catalytic requirement of histidine acid phosphatases, namely, a C-terminal His residue remote from the RHGxRxP catalytic motif; (3) rat MIPP mRNA is up-regulated during chondrocyte hypertrophy. The latter observation provides a context for proposing that MIPP may aid bone mineralization and salvage the inositol moiety prior to apoptosis.

Tyrosine phosphorylation of c-ErbB-2 is regulated by the cellular form of prostatic acid phosphatase in human prostate cancer cells

Human prostatic acid phosphatase (PAcP) is a prostate epithelium-specific differentiation antigen. In prostate carcinomas, the cellular PAcP is decreased. We investigated its functional role in these cells. Several lines of evidence support the hypothesis that cellular PAcP functions as a neutral protein-tyrosine phosphatase and is involved in regulating prostate cell growth. In this study, we identify its in vivo substrate. Our results demonstrated that, in different human prostate cancer cell lines, the phosphotyrosine (Tyr(P)) level of a 185-kDa phosphoprotein (pp185) inversely correlates with the cellular activity of PAcP. On SDS-PAGE, this pp185 co-migrates with the c-ErbB-2 oncoprotein. Immunodepletion experiments revealed that c-ErbB-2 protein is the major pp185 in cells. Results from subclones of LNCaP cells indicated the lower the cellular PAcP activity, the higher the Tyr(P) levels of c-ErbB-2. This inverse correlation was further observed in PAcP cDNA-transfected cells. In clone 33 LNCaP cells, L-(+)-tartrate suppresses the cellular PAcP activity and causes an elevated Tyr(P) level of c-ErbB-2 protein. Epidermal growth factor stimulates the proliferation of LNCaP cells, which concurs with a decreased cellular PAcP activity as well as an increased Tyr(P) level of c-ErbB-2. Biochemically, PAcP dephosphorylates c-ErbB-2 at pH 7.0. The results thus suggest that cellular PAcP down-regulates prostate cell growth by dephosphorylating Tyr(P) on c-ErbB-2 oncoprotein in those cells.

HiPER1, a phosphatase of the endoplasmic reticulum with a role in chondrocyte maturation

We have previously identified and partially cloned Band 17, a gene expressed in growth plate chondrocytes transiting from proliferation to hypertrophy. We now rename this gene HiPER1, Histidine Phosphatase of the Endoplasmic Reticulum-1, based on the results reported here. HiPER1 encodes two proteins of 318 (HiPER1(318)) and 449 (HiPER1(449)) amino acids, which are 20–21% identical to a group of yeast acid phosphatases that are in the histidine phosphatase family. HiPER1(449) is significantly more abundant than HiPER1(318), correlating with the abundance of the alternatively spliced messages encoding HiPER449 and HiPER318. Anti-HiPER1 antibodies detect two proteins of 53 and 55 kDa in growth plate chondrocytes that are absent in articular chondrocytes. We confirm that the 53 and 55 kDa proteins are HiPER1(449) by heterologous expression of the HiPER1(449) coding sequence in chick embryo fibroblasts. The 53 and 55 kDa proteins are glycosylated forms of HiPER1(449), as N-glycosidase F digestion reduces these proteins to 48 kDa, the predicted size of HiPER1(449) without the N-terminal signal sequence. Immunocytochemistry demonstrates that HiPER1(449) is found in chondrocytes maturing from proliferation to hypertrophy, but is not detectable in resting zone, deep hypertrophic zone or articular chondrocytes, a distribution that is consistent with the message distribution. HiPER1(449) was predicted to localize to the lumen of endoplasmic reticulum by an N-terminal signal sequence and by the C-terminal sequence Ala-Asp-Glu-Leu, which closely matches the consensus signal for ER retention, Lys-Asp-Glu-Leu. We confirm this prediction by demonstrating colocalization of HiPER1(449) with the ER protein HSP47 using dual-label immunofluorescence. PTHrP, a peptide that prevents hypertrophy in chondrocytes, suppressed HiPER1 and HiPER1(449) expression in vitro, an observation that further supports a role for HiPER1 in chondrocyte maturation. The yeast phosphatase homology, localization to the endoplasmic reticulum and pattern of expression suggest that HiPER1 represents a previously unrecognized intracellular pathway, involved in differentiation of chondrocytes.

Expression of human prostatic acid phosphatase correlates with androgen-stimulated cell proliferation in prostate cancer cell lines

Androgen plays a critical role in regulating the growth and differentiation of normal prostate epithelia, as well as the initial growth of prostate cancer cells. Nevertheless, prostate carcinomas eventually become androgen-unresponsive, and the cancer is refractory to hormonal therapy. To gain insight into the mechanism involved in this hormone-refractory phenomenon, we have examined the potential role of the androgen receptor (AR) in that process. We have investigated the expression of AR and two prostate-specific androgen-responsive antigens, prostatic acid phosphatase (PAcP) and prostate-specific antigen (PSA), for the functional activity of AR in LNCaP and PC-3 human prostate carcinoma cells. Our results are as follows. (i) Clone 33 LNCaP cells express AR, PAcP, and PSA, and cell growth is stimulated by 5alpha-dihydrotestosterone (DHT). Stimulation of cell growth correlates with decreased cellular PAcP activity. (ii) In clone 81 LNCaP cells, the expression of PAcP decreases with a concurrent decrease in the degree of androgen stimulation of cell growth, whereas the expression of PSA mRNA level is up-regulated by DHT, as in clone 33 cells. Conversely, in PAcP cDNA-transfected clone 81 cells, an additional expression of cellular PAcP correlates with an increased stimulation by androgen, higher than the corresponding control cells. (iii) PC-3 cells express a low level of functional AR with no detectable PAcP or PSA, and the growth of PC-3 cells is not affected by DHT treatment. Nevertheless, in two PAcP cDNA-transfected PC-3 sublines, the expression of exogenous cellular PAcP correlates with androgen stimulation. This androgen stimulation of cell growth concurs with an increased tyrosine phosphorylation of a phosphoprotein of 185 kDa. In summary, the data indicate that the expression of AR alone is not sufficient for androgen stimulation of cell growth. Furthermore, in AR-expressing prostate cancer cells, the expression of cellular PAcP correlates with androgen stimulation of cell proliferation.

Molecular cloning and functional expression of mannitol-1-phosphatase from the apicomplexan parasite Eimeria tenella

A metabolic pathway responsible for the biosynthesis and utilization of mannitol is present in the seven species of Eimeria that infect chickens, but is not in the avian host. Mannitol-1-phosphatase (M1Pase), a key enzyme for mannitol biosynthesis, is a highly substrate-specific phosphatase and, accordingly, represents an attractive chemotherapeutic target. Amino acid sequence of tryptic peptides obtained from biochemically purified Eimeria tenella M1Pase was used to synthesize degenerate oligonucleotide hybridization probes. Using these reagents, a partial genomic clone and full-length cDNA clones have been isolated and characterized. The deduced amino acid sequence of E. tenella M1Pase shows limited overall homology to members of the phosphohistidine family of phosphatases. This limited homology to other histidine phosphatases does, however, include several conserved residues that have been shown to be essential for their catalytic activity. Kinetic parameters of recombinant M1Pase expressed in bacteria are essentially identical to those of the biochemically purified preparation from E. tenella. Moreover, recombinant M1Pase is subject to active site-directed, hydroxylamine-reversible inhibition by the histidine-selective acylating reagent diethyl pyrocarbonate. These results indicate the presence of an essential histidine residue(s) at the M1Pase active site, as predicted for a histidine phosphatase.

Cloning of the phytases from Emericella nidulans and the thermophilic fungus Talaromyces thermophilus

Phytases (EC 3.1.3.8) belong to the family of histidine acid phosphatases. We have cloned the phytases of the fungi Emericella nidulans and Talaromyces thermophilus. The putative enzyme encoded by the E. nidulans sequence consists of 463 amino acids and has a Mr of 51785. The protein deduced from the T. thermophilus sequence consists of 466 amino acids corresponding to a Mr of 51450. Both predicted amino acid sequences exhibited high identity (48% to 67%) to known phytases. This high level of identity allowed the modelling of all available fungal phytases based on the three-dimensional structure coordinates of the Aspergillus niger phytase. By this approach we identified 21 amino acids which are conserved in fungal phyA phytases and are part of the residues forming the substrate pocket. Furthermore, potential glycosylation sites were identified and compared between the aforementioned phytases and the A. niger phytase.

From phosphatases to vanadium peroxidases: a similar architecture of the active site

We show here that the amino acid residues contributing to the active sites of the vanadate containing haloperoxidases are conserved within three families of acid phosphatases; this suggests that the active sites of these enzymes are very similar. This is confirmed by activity measurements showing that apochloroperoxidase exhibits phosphatase activity. These observations not only reveal interesting evolutionary relationships between these groups of enzymes but may also have important implications for the research on acid phosphatases, especially glucose-6-phosphatase-the enzyme affected in von Gierke disease-of which the predicted membrane topology may have to be reconsidered.

Identification of a novel phosphatase sequence motif

We have identified a novel, conserved phosphatase sequence motif, KXXXXXXRP-(X12-54)-PSGH-(X31-54)-SRXXXXX HXXXD, that is shared among several lipid phosphatases, the mammalian glucose-6-phosphatases, and a collection of bacterial nonspecific acid phosphatases. This sequence was also found in the vanadium-containing chloroperoxidase of Curvularia inaequalis. Several lines of evidence support this phosphatase motif identification. Crystal structure data on chloroperoxidase revealed that all three domains are in close proximity and several of the conserved residues are involved in the binding of the cofactor, vanadate, a compound structurally similar to phosphate. Structure-function analysis of the human glucose-6-phosphatase has shown that two of the conserved residues (the first domain arginine and the central domain histidine) are essential for enzyme activity. This conserved sequence motif was used to identify nine additional putative phosphatases from sequence databases, one of which has been determined to be a lipid phosphatase in yeast.

Clinical and biological aspects of acid phosphatase

The identity and genetic origins of the nonspecific orthophosphate monoesterases with an acid pH optimum--the acid phosphatases--are now becoming clear. They form a family of genetically distinct isoenzymes, many of which show significant posttranslational modification. Four true isoenzymes exist. The erythrocytic and lysosomal forms show widespread distribution and are expressed in most cells; in contrast, the prostatic and macrophagic forms have a more limited expression. The erythrocytic and macrophagic forms are distinguished from the others in resisting inhibition by dextrorotatory tartrate. The prostatic form has long been used as a marker for prostatic cancer and the macrophagic forms have been linked with miscellaneous disorders, notably increased osteolysis, Gaucher's disease of spleen, and hairy cell leukemia, whereas the normal levels of intravesical lysosomal acid phosphatase in I cell disease pointed the way toward the mechanisms underlying its intracellular processing.