Plasmin reduction by phosphoglycerate kinase is a thiol-independent process

Phosphoglycerate kinase: structural aspects and functions, with special emphasis on the enzyme from Kinetoplastea

Phosphoglycerate kinase (PGK) is a glycolytic enzyme that is well conserved among the three domains of life. PGK is usually a monomeric enzyme of about 45 kDa that catalyses one of the two ATP-producing reactions in the glycolytic pathway, through the conversion of 1,3-bisphosphoglycerate (1,3BPGA) to 3-phosphoglycerate (3PGA). It also participates in gluconeogenesis, catalysing the opposite reaction to produce 1,3BPGA and ADP. Like most other glycolytic enzymes, PGK has also been catalogued as a moonlighting protein, due to its involvement in different functions not associated with energy metabolism, which include pathogenesis, interaction with nucleic acids, tumorigenesis progression, cell death and viral replication. In this review, we have highlighted the overall aspects of this enzyme, such as its structure, reaction kinetics, activity regulation and possible moonlighting functions in different protistan organisms, especially both free-living and parasitic Kinetoplastea. Our analysis of the genomes of different kinetoplastids revealed the presence of open-reading frames (ORFs) for multiple PGK isoforms in several species. Some of these ORFs code for unusually large PGKs. The products appear to contain additional structural domains fused to the PGK domain. A striking aspect is that some of these PGK isoforms are predicted to be catalytically inactive enzymes or ‘dead’ enzymes. The roles of PGKs in kinetoplastid parasites are analysed, and the apparent significance of the PGK gene duplication that gave rise to the different isoforms and their expression in Trypanosoma cruzi is discussed.

Extracellular Pgk1 enhances neurite outgrowth of motoneurons through Nogo66/NgR-independent targeting of NogoA

NogoA inhibits neurite outgrowth of motoneurons (NOM) through interaction with its receptors, Nogo66/NgR. Inhibition of Nogo receptors rescues NOM, but not to the extent exhibited by NogoA-knockout mice, suggesting the presence of other pathways. We found that NogoA-overexpressing muscle cells reduced phosphoglycerate kinase 1 (Pgk1) secretion, resulting in inhibiting NOM. Apart from its glycolytic role and independent of the Nogo66 pathway, extracellular Pgk1 stimulated NOM by triggering a reduction of p-Cofilin-S3, a growth cone collapse marker, through decreasing a novel Rac1-GTP/p-Pak1-T423/p-P38-T180/p-MK2-T334/p-Limk1-S323/p-Cofilin-S3 molecular pathway. Not only did supplementary Pgk1 enhance NOM in defective cells, but injection of Pgk1 rescued denervation in muscle-specific NogoA-overexpression of zebrafish and an Amyotrophic Lateral Sclerosis mouse model, SOD1 G93A. Thus, Pgk1 secreted from muscle is detrimental to motoneuron neurite outgrowth and maintenance.

Protein S-sulfenylation is a fleeting molecular switch that regulates non-enzymatic oxidative folding

The post-translational modification S-sulfenylation functions as a key sensor of oxidative stress. Yet the dynamics of sulfenic acid in proteins remains largely elusive due to its fleeting nature. Here we use single-molecule force-clamp spectroscopy and mass spectrometry to directly capture the reactivity of an individual sulfenic acid embedded within the core of a single Ig domain of the titin protein. Our results demonstrate that sulfenic acid is a crucial short-lived intermediate that dictates the protein's fate in a conformation-dependent manner. When exposed to the solution, sulfenic acid rapidly undergoes further chemical modification, leading to irreversible protein misfolding; when cryptic in the protein's microenvironment, it readily condenses with a neighbouring thiol to create a protective disulfide bond, which assists the functional folding of the protein. This mechanism for non-enzymatic oxidative folding provides a plausible explanation for redox-modulated stiffness of proteins that are physiologically exposed to mechanical forces, such as cardiac titin.

Protein S-sulfenylation is a posttranslational modification that can act as a sensor of redox oxidative stress. Here the authors show that, following mechanical unfolding, sulfenic acid drives disulfide bond reformation and guides non-enzymatic oxidative folding.

Methylthioadenosine (MTA) Regulates Liver Cells Proteome and Methylproteome: Implications in Liver Biology and Disease

Methylthioadenosine phosphorylase (MTAP), a key enzyme in the adenine and methionine salvage pathways, catalyzes the hydrolysis of methylthioadenosine (MTA), a compound suggested to affect pivotal cellular processes in part through the regulation of protein methylation. MTAP is expressed in a wide range of cell types and tissues, and its deletion is common to cancer cells and in liver injury. The aim of this study was to investigate the proteome and methyl proteome alterations triggered by MTAP deficiency in liver cells to define novel regulatory mechanisms that may explain the pathogenic processes of liver diseases. iTRAQ analysis resulted in the identification of 216 differential proteins (p < 0.05) that suggest deregulation of cellular pathways as those mediated by ERK or NFκB. R-methyl proteome analysis led to the identification of 74 differentially methylated proteins between SK-Hep1 and SK-Hep1+ cells, including 116 new methylation sites. Restoring normal MTA levels in SK-Hep1+ cells parallels the specific methylation of 56 proteins, including KRT8, TGF, and CTF8A, which provides a novel regulatory mechanism of their activity with potential implications in carcinogenesis. Inhibition of RNA-binding proteins methylation is especially relevant upon accumulation of MTA. As an example, methylation of quaking protein in Arg(242) and Arg(256) in SK-Hep1+ cells may play a pivotal role in the regulation of its activity as indicated by the up-regulation of its target protein p27(kip1) The phenotype associated with a MTAP deficiency was further verified in the liver of MTAP± mice. Our data support that MTAP deficiency leads to MTA accumulation and deregulation of central cellular pathways, increasing proliferation and decreasing the susceptibility to chemotherapeutic drugs, which involves differential protein methylation. Data are available via ProteomeXchange with identifier PXD002957 (http://www.ebi.ac.uk/pride/archive/projects/PXD002957).

Ectopic expression of Pokkali phosphoglycerate kinase-2 (OsPGK2-P) improves yield in tobacco plants under salinity stress

KEY MESSAGE

Our results indicate that OsPGK2a-P gene is differentially regulated in contrasting rice cultivars under stress and its overexpression confers salt stress tolerance in transgenic tobacco. Phosphoglycerate kinase (PGK; EC = 2.7.2.3) plays a major role for ATP production during glycolysis and 1, 3-bisphosphoglycerate production to participate in the Calvin cycle for carbon fixation in plants. Whole genome analysis of rice reveals the presence of four PGK genes (OsPgks) on different chromosomes. Comparative expression analysis of OsPgks in rice revealed highest level of transcripts for OsPgk2 at most of its developmental stages. Detailed characterization of OsPgk2 transcript and protein showed that it is strongly induced by salinity stress in two contrasting genotypes of rice, i.e., cv IR64 (salt sensitive) and landrace Pokkali (salt tolerant). Ectopic expression of OsPgk2a-P (isolated from Pokkali) in transgenic tobacco improved its salinity stress tolerance by higher chlorophyll retention and enhanced proline accumulation, besides maintaining better ion homeostasis. Ectopically expressing OsPgk2a-P transgenic tobacco plants showed tall phenotype with more number of pods than wild-type plants. Therefore, OsPgk2a-P appears to be a potential candidate for increasing salinity stress tolerance and enhanced yield in crop plants.

Inhibition of thiol isomerase activity diminishes endothelial activation of plasminogen, but not of protein C

INTRODUCTION

Cell surface thiol isomerase enzymes, principally protein disulphide isomerase (PDI), have emerged as important regulators of platelet function and tissue factor activation via their action on allosteric disulphide bonds. Allosteric disulphides are present in other haemostasis-related proteins, and we have therefore investigated whether thiol isomerase inhibition has any influence on two endothelial activities relevant to haemostatic regulation, namely activation of protein C and activation of plasminogen, with subsequent fibrinolysis.

MATERIALS AND METHODS

The study was performed using the human microvascular endothelial cell line HMEC-1. Thiol isomerase gene expression was measured by RT-PCR and activation of protein C and plasminogen by cell-based assays using chromogenic substrates S2366 and S2251, respectively. Cell mediated fibrinolysis was measured by monitoring absorbance at 405 nm following fibrin clot formation on the surface of HMEC-1 monolayers.

RESULTS AND CONCLUSIONS

A variety of thiol isomerase enzymes, including PDI, were expressed by HMEC-1 cells and thiol reductase activity detectable on the cell surface was inhibited by both RL90 anti-PDI antibody and by the PDI inhibitor quercetin-3-rutinoside (rutin). In cell-based assays, activation of plasminogen, but not of protein C, was inhibited by RL90 antibody and, to a lesser extent, by rutin. Fibrin clot lysis occurring on a HMEC-1 monolayer was also significantly slowed by RL90 antibody and by rutin, but RL90-mediated inhibition was abolished in the presence of exogenous tissue plasminogen activator (tPA). We conclude that thiol isomerases, including PDI, are involved in fibrinolytic regulation at the endothelial surface, although not via a direct action on tPA. These findings broaden understanding of haemostatic regulation by PDI, and may aid in development of novel anti-thrombotic therapeutic strategies targeted via the fibrinolysis system.

Control of blood proteins by functional disulfide bonds

Most proteins in nature are chemically modified after they are made to control how, when, and where they function. The 3 core features of proteins are posttranslationally modified: amino acid side chains can be modified, peptide bonds can be cleaved or isomerized, and disulfide bonds can be cleaved. Cleavage of peptide bonds is a major mechanism of protein control in the circulation, as exemplified by activation of the blood coagulation and complement zymogens. Cleavage of disulfide bonds is emerging as another important mechanism of protein control in the circulation. Recent advances in our understanding of control of soluble blood proteins and blood cell receptors by functional disulfide bonds is discussed as is how these bonds are being identified and studied.

Characterization of a reduced form of plasma plasminogen as the precursor for angiostatin formation

Plasma plasminogen is the precursor of the tumor angiogenesis inhibitor, angiostatin. Generation of angiostatin in blood involves activation of plasminogen to the serine protease plasmin and facilitated cleavage of two disulfide bonds and up to three peptide bonds in the kringle 5 domain of the protein. The mechanism of reduction of the two allosteric disulfides has been explored in this study. Using thiol-alkylating agents, mass spectrometry, and an assay for angiostatin formation, we show that the Cys(462)-Cys(541) disulfide bond is already cleaved in a fraction of plasma plasminogen and that this reduced plasminogen is the precursor for angiostatin formation. From the crystal structure of plasminogen, we propose that plasmin ligands such as phosphoglycerate kinase induce a conformational change in reduced kringle 5 that leads to attack by the Cys(541) thiolate anion on the Cys(536) sulfur atom of the Cys(512)-Cys(536) disulfide bond, resulting in reduction of the bond by thiol/disulfide exchange. Cleavage of the Cys(512)-Cys(536) allosteric disulfide allows further conformational change and exposure of the peptide backbone to proteolysis and angiostatin release. The Cys(462)-Cys(541) and Cys(512)-Cys(536) disulfides have -/+RHHook and -LHHook configurations, respectively, which are two of the 20 different measures of the geometry of a disulfide bond. Analysis of the structures of the known allosteric disulfide bonds identified six other bonds that have these configurations, and they share some functional similarities with the plasminogen disulfides. This suggests that the -/+RHHook and -LHHook disulfides, along with the -RHStaple bond, are potential allosteric configurations.

Sulfenic acid chemistry, detection and cellular lifetime

BACKGROUND

Reactive oxygen species-mediated cysteine sulfenic acid modification has emerged as an important regulatory mechanism in cell signaling. The stability of sulfenic acid in proteins is dictated by the local microenvironment and ability of antioxidants to reduce this modification. Several techniques for detecting this cysteine modification have been developed, including direct and in situ methods.

SCOPE OF REVIEW

This review presents a historical discussion of sulfenic acid chemistry and highlights key examples of this modification in proteins. A comprehensive survey of available detection techniques with advantages and limitations is discussed. Finally, issues pertaining to rates of sulfenic acid formation, reduction, and chemical trapping methods are also covered.

MAJOR CONCLUSIONS

Early chemical models of sulfenic acid yielded important insights into the unique reactivity of this species. Subsequent pioneering studies led to the characterization of sulfenic acid formation in proteins. In parallel, the discovery of oxidant-mediated cell signaling pathways and pathological oxidative stress has led to significant interest in methods to detect these modifications. Advanced methods allow for direct chemical trapping of protein sulfenic acids directly in cells and tissues. At the same time, many sulfenic acids are short-lived and the reactivity of current probes must be improved to sample these species, while at the same time, preserving their chemical selectivity. Inhibitors with binding scaffolds can be rationally designed to target sulfenic acid modifications in specific proteins.

GENERAL SIGNIFICANCE

Ever increasing roles for protein sulfenic acids have been uncovered in physiology and pathology. A more complete understanding of sulfenic acid-mediated regulatory mechanisms will continue to require rigorous and new chemical insights. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Post-translational control of protein function by disulfide bond cleavage

Protein action in nature is largely controlled by the level of expression and by post-translational modifications. Post-translational modifications result in a proteome that is at least two orders of magnitude more diverse than the genome. There are three basic types of post-translational modifications: covalent modification of an amino acid side chain, hydrolytic cleavage or isomerization of a peptide bond, and reductive cleavage of a disulfide bond. This review addresses the modification of disulfide bonds. Protein disulfide bonds perform either a structural or a functional role, and there are two types of functional disulfide: the catalytic and allosteric bonds. The allosteric disulfide bonds control the function of the mature protein in which they reside by triggering a change when they are cleaved. The change can be in ligand binding, substrate hydrolysis, proteolysis, or oligomer formation. The allosteric disulfides are cleaved by oxidoreductases or by thiol/disulfide exchange, and the configurations of the disulfides and the secondary structures that they link share some recurring features. How these bonds are being identified using bioinformatics and experimental screens and what the future holds for this field of research are also discussed.

Theoretical and in vitro studies of a C-terminal peptide from PGKC of Leishmania mexicana mexicana

Trypanosomatids cause deadly diseases in humans. Of the various biochemical pathways in trypanosomatids, glycolysis, has received special attention because of being sequestered in peroxisome like organelles critical for the survival of the parasites. This study focuses on phosphoglycerate kinase (PGK) from Leishmania spp. which, exists in two isoforms, the cytoplasmic PGKB and glycosomal PGKC differing in their biochemical properties. Computational analysis predicted the likelihood of a transmembrane helix only in the glycosomal isoform PGKC, of approximate length 20 residues in the 62-residue extension, ending at, arginine residues R471 and R472. From experimental studies using circular dichroism and NMR with deuterated sodium dodecyl sulfate, we find that the transmembrane helix spans residues 448±2 to 476 in Leishmania mexicana PGKC. The significance of this observation is discussed in the context of glycosomal transport and substrate tunneling.

Natural and engineered plasmin inhibitors: applications and design strategies

The serine protease plasmin is ubiquitously expressed throughout the human body in the form of the zymogen plasminogen. Conversion to active plasmin occurs through enzymatic cleavage by plasminogen activators. The plasminogen activator/plasmin system has a well-established function in the removal of intravascular fibrin deposition through fibrinolysis and the inhibition of plasmin activity; this has found widespread clinical use in reducing perioperative bleeding. Increasing evidence also suggests diverse, although currently less defined, roles for plasmin in a number of physiological and pathological processes relating to extracellular matrix degradation, cell migration and tissue remodelling. In particular, dysregulation of plasmin has been linked to cancer invasion/metastasis and various chronic inflammatory conditions; this has prompted efforts to develop inhibitors of this protease. Although a number of plasmin inhibitors exist, they commonly suffer from poor potency and/or specificity of inhibition that either results in reduced efficacy or prevents clinical use. Consequently, there is a need for further development of high-affinity plasmin inhibitors that maintain selectivity over other serine proteases. This review summarises clearly defined and potential applications for plasmin inhibition. The properties of naturally occurring and engineered plasmin inhibitors are discussed in the context of current knowledge regarding plasmin structure, specificity and function. This includes design strategies to obtain the potency and specificity of inhibition in addition to controlled temporal and spatial distribution tailored for the intended use.

A spring-loaded release mechanism regulates domain movement and catalysis in phosphoglycerate kinase

Phosphoglycerate kinase (PGK) is the enzyme responsible for the first ATP-generating step of glycolysis and has been implicated extensively in oncogenesis and its development. Solution small angle x-ray scattering (SAXS) data, in combination with crystal structures of the enzyme in complex with substrate and product analogues, reveal a new conformation for the resting state of the enzyme and demonstrate the role of substrate binding in the preparation of the enzyme for domain closure. Comparison of the x-ray scattering curves of the enzyme in different states with crystal structures has allowed the complete reaction cycle to be resolved both structurally and temporally. The enzyme appears to spend most of its time in a fully open conformation with short periods of closure and catalysis, thereby allowing the rapid diffusion of substrates and products in and out of the binding sites. Analysis of the open apoenzyme structure, defined through deformable elastic network refinement against the SAXS data, suggests that interactions in a mostly buried hydrophobic region may favor the open conformation. This patch is exposed on domain closure, making the open conformation more thermodynamically stable. Ionic interactions act to maintain the closed conformation to allow catalysis. The short time PGK spends in the closed conformation and its strong tendency to rest in an open conformation imply a spring-loaded release mechanism to regulate domain movement, catalysis, and efficient product release.

Formation, reactivity, and detection of protein sulfenic acids

It has become clear in recent decades that the post-translational modification of protein cysteine residues is a crucial regulatory event in biology. Evidence supports the reversible oxidation of cysteine thiol groups as a mechanism of redox-based signal transduction, while the accumulation of proteins with irreversible thiol oxidations is a hallmark of stress-induced cellular damage. The initial formation of cysteine-sulfenic acid (SOH) derivatives, along with the reactive properties of this functional group, serves as a crossroads whereby the local redox environment may dictate the progression of either regulatory or pathological outcomes. Protein-SOH are established as transient intermediates in the formation of more stable cysteine oxidation products both under basal conditions and in response to several redox-active extrinsic compounds. This review details both direct and multistep chemical routes proposed to generate protein-SOH, the spectrum of secondary reactions that may follow their initial formation and the arsenal of experimental tools available for their detection. Pioneering studies that have provided a framework for our current understanding of protein-SOH as well as state-of-the-art proteomic strategies designed for global assessments of this post-translational modification are highlighted.

Exocytosis and protein secretion in Trypanosoma

Background

Human African trypanosomiasis is a lethal disease caused by the extracellular parasite Trypanosoma brucei. The proteins secreted by T. brucei inhibit the maturation of dendritic cells and their ability to induce lymphocytic allogenic responses. To better understand the pathogenic process, we combined different approaches to characterize these secreted proteins.

Results

Overall, 444 proteins were identified using mass spectrometry, the largest parasite secretome described to date. Functional analysis of these proteins revealed a strong bias toward folding and degradation processes and to a lesser extent toward nucleotide metabolism. These features were shared by different strains of T. brucei, but distinguished the secretome from published T. brucei whole proteome or glycosome. In addition, several proteins had not been previously described in Trypanosoma and some constitute novel potential therapeutic targets or diagnostic markers. Interestingly, a high proportion of these secreted proteins are known to have alternative roles once secreted. Furthermore, bioinformatic analysis showed that a significant proportion of proteins in the secretome lack transit peptide and are probably not secreted through the classical sorting pathway. Membrane vesicles from secretion buffer and infested rat serum were purified on sucrose gradient and electron microscopy pictures have shown 50- to 100-nm vesicles budding from the coated plasma membrane. Mass spectrometry confirmed the presence of Trypanosoma proteins in these microvesicles, showing that an active exocytosis might occur beyond the flagellar pocket.

Conclusions

This study brings out several unexpected features of the secreted proteins and opens novel perspectives concerning the survival strategy of Trypanosoma as well as possible ways to control the disease. In addition, concordant lines of evidence support the original hypothesis of the involvement of microvesicle-like bodies in the survival strategy allowing Trypanosoma to exchange proteins at least between parasites and/or to manipulate the host immune system.

Phosphoglycerate kinase 1 as a candidate of tumor-associated antigen identified from esophageal squamous cell carcinoma

AIM: To investigate and identify novel tumor-associated antigens in esophageal squamous cell carcinoma (ESCC).

METHODS: Modified serological proteome analysis (mSERPA) strategy was used to separate and identify the candidate proteins. The subcellular protein fractions (cytosolic, membrane and nuclear fractions) of ESCC cell lines and EC0156 cells were extracted first and then cytosolic proteins were separated using SDS-PAGE. The separated proteins were incubated with different serum of ESCC patients (29 cases) or healthy controls (28 cases) respectively, and then one of the positive bands in 43 kDa was excised followed by in-gel tryptic digestion. Separated peptides were identified using a high definition mass spectrometry (HDMS). Western blot and immunohistochemical staining (IHC) were used to validate possible candidates.

RESULTS: Successful compartmental protein extraction was demonstrated by specific organelle markers. Serum samples of ESCC patients bound EC0156 cytoplasmic protein, suggesting selective recognition of tumor-associated antigen. 43 kDa protein band showed significantly higher positive binding rate with serum of ESCC patients (41.4%, 12/29) than with serum of healthy individuals (3.6%, 1/28). Five high-confidence proteins were identified from the 43 kDa band using HDMS including phosphoglycerate kinase 1 (PGK1), β-actin, proteasome 26S subunit, S-adenosylhomocysteine hydrolase and hosphoribosylaminoimidazole carboxylase. Immunohistochemistry. Western blot analysis showed that PGK1 was located in both cytoplasm and nucleus, and had a higher expression in cancer tissues (69.23%, 18/26) than in normal esophageal epithelia.

CONCLUSION: The mSERPA strategy is useful for tumor-associated antigen identification. As a new candidate of tumor-associated antigen, PGK1 was over-expressed in ESCC which may play a role in tumorigenesis of ESCC.

Regulation of tissue factor procoagulant activity by post-translational modifications

Post-translational modification of amino acid residues is a common way to regulate localization, stability and ultimately the function of the protein. Tissue factor (TF), the major initiator of blood coagulation cascade, receives several post-translational modifications, like glycosylation, phosphorylation, palmitoylation and nitrosylation. Recent studies have demonstrated that these processes play important roles in modulating biological functions of TF. The present review highlights the mechanisms of several common protein post-translational modifications of TF with the special reference on the recent knowledge about their roles in regulation of trafficking, stability as well as procoagulant and signaling functions of TF.

Allosteric disulfide bonds in thrombosis and thrombolysis

Allosteric disulfide bonds control protein function by mediating conformational change when they undergo reduction or oxidation. The known allosteric disulfide bonds are characterized by a particular bond geometry, the -RHStaple. A number of thrombosis and thrombolysis proteins contain one or more disulfide bonds of this type. Tissue factor (TF) was the first hemostasis protein shown to be controlled by an allosteric disulfide bond, the Cys186-Cys209 bond in the membrane-proximal fibronectin type III domain. TF exists in three forms on the cell surface: a cryptic form that is inert, a coagulant form that rapidly binds factor VIIa to initiate coagulation, and a signaling form that binds FVIIa and cleaves protease-activated receptor 2, which functions in inflammation, tumor progression and angiogenesis. Reduction and oxidation of the Cys186-Cys209 disulfide bond is central to the transition between the three forms of TF. The redox state of the bond appears to be controlled by protein disulfide isomerase and NO. Plasmin(ogen), vitronectin, glycoprotein 1balpha, integrin beta(3) and thrombomodulin also contain -RHStaple disulfides, and there is circumstantial evidence that the function of these proteins may involve cleavage/formation of these disulfide bonds.

Tumor shedding of laminin binding protein modulates angiostatin productionin vitro and interferes with plasmin-derived inhibition of angiogenesis in aortic ring cultures

The growth of solid tumors is largely controlled by the process of angiogenesis. A 67 kDa protein, the laminin binding protein (LBP), is shed from malignant cells in significant amounts and binds to laminin-1 (Starkey et al., Cytometry 1999;35:37-47; Karpatová et al., J Cell Biochem 1996;60:226-34). However, the functions of shed LBP are not fully understood. We hypothesize that matrix-bound LBP could modulate local tumor angiogenesis. In support of this hypothesis, we demonstrate that shed LBP exhibits sulfhydryl oxidase-like activities, and modifies the production of angiostatins from plasmin in vitro. The molecular weights of the autocatalytic products of lys-plasmin incubated with LBP in vitro suggest that PMDs (plasmin A chains attached to degraded B chains) (Ohyama et al., Eur J Biochem 2004;271:809-20) are preferentially generated. Using rat aortic ring assays, we also show that shed LBP reverses plasmin-dependent inhibition of vascular outgrowth. To elucidate which LBP region(s) are active in modulating angiogenesis, limited proteolysis experiments were conducted to determine stable rLBP domains likely to fold correctly, and these were cloned, expressed and purified. The stable LBP fragments were tested for binding to laminin-1 and for competition with shed LBP activity in the aortic ring assay. Results of these studies suggest that the active LBP domains lie within the 137-230 amino acid sequence, a region known to contain 2 bioactive sequences. Since this fragment binds to laminin-1 and modulates angiogenesis, it appears likely that binding of shed LBP to matrix laminin-1 is related to its functions in tumor angiogenesis. The findings presented in this manuscript suggest that LBP shedding could provide a useful therapeutic target.

Angiostatin's molecular mechanism: aspects of specificity and regulation elucidated

Tumor growth requires the development of new vessels that sprout from pre-existing normal vessels in a process known as "angiogenesis" [Folkman (1971) N Engl J Med 285:1182-1186]. These new vessels arise from local capillaries, arteries, and veins in response to the release of soluble growth factors from the tumor mass, enabling these tumors to grow beyond the diffusion-limited size of approximately 2 mm diameter. Angiostatin, a naturally occurring inhibitor of angiogenesis, was discovered based on its ability to block tumor growth in vivo by inhibiting the formation of new tumor blood vessels [O'Reilly et al. (1994a) Cold Spring Harb Symp Quant Biol 59:471-482]. Angiostatin is a proteolytically derived internal fragment of plasminogen and may contain various members of the five plasminogen "kringle" domains, depending on the exact sites of proteolysis. Different forms of angiostatin have measurably different activities, suggesting that much remains to be elucidated about angiostatin biology. A number of groups have sought to identify the native cell surface binding site(s) for angiostatin, resulting in at least five different binding sites proposed for angiostatin on the surface of endothelial cells (EC). This review will consider the data supporting all of the various reported angiostatin binding sites and will focus particular attention on the angiostatin binding protein identified by our group: F(1)F(O) ATP synthase. There have been several developments in the quest to elucidate the mechanism of action of angiostatin and the regulation of its receptor. The purpose of this review is to describe the highlights of research on the mechanism of action of angiostatin, its' interaction with ATP synthase on the EC surface, modulators of its activity, and issues that should be explored in future research related to angiostatin and other anti-angiogenic agents.

Single-gene disorders: what role could moonlighting enzymes play?

Single-gene disorders with "simple" Mendelian inheritance do not always imply that there will be an easy prediction of the phenotype from the genotype, which has been shown for a number of metabolic disorders. We propose that moonlighting enzymes (i.e., metabolic enzymes with additional functional activities) could contribute to the complexity of such disorders. The lack of knowledge about the additional functional activities of proteins could result in a lack of correlation between genotype and phenotype. In this review, we highlight some notable and recent examples of moonlighting enzymes and their possible contributions to human disease. Because knowledge and cataloging of the moonlighting activities of proteins are essential for the study of cellular function and human physiology, we also review recently reported and recommended methods for the discovery of moonlighting activities.

Annexin A2-S100A10 heterotetramer, a novel substrate of thioredoxin

The binding of plasminogen activators and plasminogen to the cell surface results in the rapid generation of the serine protease plasmin. Plasmin is further degraded by an autoproteolytic reaction, resulting in the release of an angiostatin, A61 (Lys78-Lys468). Previously, we demonstrated that the annexin A2-S100A10 heterotetramer (AIIt) stimulates the release of A61 from plasmin by promoting the autoproteolytic cleavage of the Lys468-Gly469 bond and reduction of the plasmin Cys462-Cys541 disulfide (Kwon, M., Caplan, J. F., Filipenko, N. R., Choi, K. S., Fitzpatrick, S. L., Zhang, L., and Waisman, D. M. (2002) J. Biol. Chem. 277, 10903-10911). Mechanistically, it was unclear if AIIt promoted a conformational change in plasmin, resulting in contortion of the plasmin disulfide, or directly reduced the plasmin disulfide. In the present study, we show that AIIt thiols are oxidized during the reduction of plasmin disulfides, establishing that AIIt directly participates in the reduction reaction. Incubation of HT1080 cells with plasminogen resulted in the rapid loss of thiol-specific labeling of AIIt by 3-(N-maleimidopropionyl)biocytin. The plasminogen-dependent oxidation of AIIt could be attenuated by thioredoxin. Thioredoxin reductase catalyzed the transfer of electrons from NADPH to the oxidized thioredoxin, thus completing the flow of electrons from NADPH to AIIt. Therefore, we identify AIIt as a substrate of the thioredoxin system and propose a new model for the role of AIIt in the redox-dependent processing of plasminogen and generation of an angiostatin at the cell surface.

Secretion of phosphoglycerate kinase from tumour cells is controlled by oxygen-sensing hydroxylases

Solid tumour cells employ glycolytic enzymes including phosphoglycerate kinase (PGK) to make ATP when their supply of oxygen is limiting. PGK is also secreted by tumour cells and facilitates cleavage of disulfide bonds in plasmin, which triggers proteolytic release of the angiogenesis inhibitor, angiostatin. Although PGK production by tumour cells was enhanced by hypoxia, its secretion was inhibited. Inhibition of secretion correlated with decrease in angiostatin formation by the tumour cells. In contrast, hypoxia did not inhibit the secretion of the angiogenesis activator, vascular endothelial cell growth factor (VEGF). PGK secretion was reversed by normoxia and was under control of the oxygen-sensing protein hydroxylases, as inhibitors of this class of enzymes mimicked the effect of hypoxia on PGK secretion. Direct hydroxylation of PGK was not the mechanism by which the protein hydroxylases controlled its secretion. These findings show that production and secretion of PGK are regulated separately and indicate that oxygen and the protein hydroxylases can control not only gene expression but also protein secretion.

Nonlysine-analog plasminogen modulators promote autoproteolytic generation of plasmin(ogen) fragments with angiostatin-like activity

We recently discovered several nonlysine-analog conformational modulators for plasminogen. These include SMTP-6, thioplabin B and complestatin that are low molecular mass compounds of microbial origin. Unlike lysine-analog modulators, which increase plasminogen activation but inhibit its binding to fibrin, the nonlysine-analog modulators enhance both activation and fibrin binding of plasminogen. Here we show that some nonlysine-analog modulators promote autoproteolytic generation of plasmin(ogen) derivatives with its catalytic domain undergoing extensive fragmentation (PMDs), which have angiostatin-like anti-endothelial activity. The enhancement of urokinase-catalyzed plasminogen activation by SMTP-6 was followed by rapid inactivation of plasmin due to its degradation mainly in the catalytic domain, yielding PMD with a molecular mass ranging from 68 to 77 kDa. PMD generation was observed when plasmin alone was treated with SMTP-6 and was inhibited by the plasmin inhibitor aprotinin, indicating an autoproteolytic mechanism in PMD generation. Thioplabin B and complestatin, two other nonlysine-analog modulators, were also active in producing similar PMDs, whereas the lysine analog 6-aminohexanoic acid was inactive while it enhanced plasminogen activation. Peptide sequencing and mass spectrometric analyses suggested that plasmin fragmentation was due to cleavage at Lys615-Val616, Lys651-Leu652, Lys661-Val662, Lys698-Glu699, Lys708-Val709 and several other sites mostly in the catalytic domain. PMD was inhibitory to proliferation, migration and tube formation of endothelial cells at concentrations of 0.3-10 microg.mL(-1). These results suggest a possible application of nonlysine-analog modulators in the treatment of cancer through the enhancement of endogenous plasmin(ogen) fragment formation.

What the structure of angiostatin may tell us about its mechanism of action

Originally discovered in 1994 by Folkman and coworkers, angiostatin was identified through its antitumor effects in mice and later shown to be a potent inhibitor of angiogenesis. An internal fragment of plasminogen, angiostatin consists of kringle domains that are known to be lysine-binding. The crystal structure of angiostatin was the first multikringle domain-containing structure to be published. This review will focus on what is known about the structure of angiostatin and its implications in function from the current literature.

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Disulfide bonds as switches for protein function

The prevailing view is that disulfide bonds have been added during evolution to enhance the stability of proteins that function in a fluctuating cellular environment. However, recent evidence indicates that disulfide bonds can be more than inert structural motifs. The function of some secreted soluble proteins and cell-surface receptors is controlled by cleavage of one or more of their disulfide bonds; this cleavage is mediated by catalysts or facilitators that are specific for their substrate.

Structure-function studies of the adipocyte-secreted hormone Acrp30/adiponectin. Implications fpr metabolic regulation and bioactivity

Acrp30/adiponectin is an adipocyte-specific secretory protein that has recently been implicated as a mediator of systemic insulin sensitivity with liver and muscle as target organs. Acrp30 is found as two forms in serum, as a lower molecular weight trimer-dimer and a high molecular weight complex. Little is know about the regulation and significance of these Acrp30 complexes in serum and about the events that lead to the generation of the bioactive ligand. Here, we show that there is a profound sexual dimorphism of Acrp30 levels and complex distribution in serum. Female mice display significantly higher levels of the high molecular weight complex in serum than males. In both females and males, levels of the high molecular weight complex are significantly reduced in response to a systemic increase of insulin. The ratio of the two complexes is restored upon normalization of glucose levels. Structurally, we show that oligomer formation of Acrp30 critically depends on disulfide bond formation mediated by Cys-39. Mutation of Cys-39 results in trimers that are subject to proteolytic cleavage in the collagenous domain. Surprisingly, Acrp30(C39S) or wild-type Acrp30 treated with dithiothreitol are significantly more bioactive than the higher order oligomeric forms of the protein with respect to reduction of serum glucose levels. Furthermore, treatment of primary hepatocytes with trimeric and higher order forms of Acrp30 confirms that the increased bioactivity seen in vivo is reflected in an augmented potency to reduce glucose output in the presence of gluconeogenic stimuli. Combined, these results shed new light on the regulation of this complex protein and suggest a new model for in vivo activation of the protein, implicating a serum reductase activity.

Redox control on the cell surface: implications for HIV-1 entry

Proteins that work outside cells nearly always contain disulfide bonds. The prevailing view is that these bonds have been added during evolution to enhance protein stability. Recent evidence suggests that disulfide bonds can also control protein function. Certain secreted proteins contain one or more disulfide bonds that can control function by breaking and reforming in a controlled way. This review focuses on disulfide exchange events on the cell surface, with a particular reference to two proteins involved in HIV-1 infection. The primary HIV-1 receptor on immune cells, CD4, and the viral envelope glycoprotein, gp120, play a central role in HIV-1 entry. Redox change in a disulfide bond or bonds in one or both of these proteins appears to be important for HIV-1 entry.

Phosphorylation of pyrimidine L-deoxynucleoside analog diphosphates. Kinetics of phosphorylation and dephosphorylation of nucleoside analog diphosphates and triphosphates by 3-phosphoglycerate kinase

Anticancer and antiviral D- and L-nucleoside analogs are phosphorylated stepwise in the cells to the pharmacologically active triphosphate metabolites. We recently reported that in the last step, L-deoxynucleoside analog diphosphates are phosphorylated by 3-phosphoglycerate kinase (PGK). To explain the preference of PGK for L- over D-deoxynucleoside analog diphosphates, the kinetics of their phosphorylation were compared with the dephosphorylation of the respective triphosphates using recombinant human PGK. The results attributed favorable phosphorylation of L-deoxynucleoside analog diphosphates by PGK to differences in k(cat), which were consequences of varied orientations of the sugar and diphosphates in the catalytic site of PGK. The amino acids involved in the catalytic reaction of PGK (including Glu(344), Lys(220), and Asn(337)) were therefore mutated. The impact of mutations on the phosphorylation of L- and D-deoxynucleoside analog diphosphates was different from those on dephosphorylation of the respective triphosphates. This suggested that the interactions of the nucleoside analogs with amino acids during the transition state are different in the phosphorylation and dephosphorylation reactions. Thus, reversible action of the enzyme may not involve the same configuration of the active site. Furthermore, the amino acid determinants of the action of PGK for L-deoxynucleotides were not the same as for the D-deoxynucleotides. This study also suggests the potential impact of nucleoside analog diphosphates and triphosphates on the multiple cellular functions of PGK, which may contribute to the action of the analogs.