The 2.2-A crystal structure of human pro-granzyme K reveals a rigid zymogen with unusual features

Granzyme serine proteases in inflammation and rheumatic diseases

Granzymes (granule-secreted enzymes) are a family of serine proteases that have been viewed as redundant cytotoxic enzymes since their discovery more than 30 years ago. Predominantly produced by cytotoxic lymphocytes and natural killer cells, granzymes are delivered into the cytoplasm of target cells through immunological synapses in cooperation with the pore-forming protein perforin. After internalization, granzymes can initiate cell death through the cleavage of intracellular substrates. However, evidence now also demonstrates the existence of non-cytotoxic, pro-inflammatory, intracellular and extracellular functions that are granzyme specific. Under pathological conditions, granzymes can be produced and secreted extracellularly by immune cells as well as by non-immune cells. Depending on the granzyme, accumulation in the extracellular milieu might contribute to inflammation, tissue injury, impaired wound healing, barrier dysfunction, osteoclastogenesis and/or autoantigen generation.

Granzyme K drives a newly-intentified pathway of complement activation

Granzymes are a family of serine proteases mainly expressed by CD8 + T cells, natural killer cells, and innate-like lymphocytes 1,2 . Although their major role is thought to be the induction of cell death in virally infected and tumor cells, accumulating evidence suggests some granzymes can regulate inflammation by acting on extracellular substrates 2 . Recently, we found that the majority of tissue CD8 + T cells in rheumatoid arthritis (RA) synovium, inflammatory bowel disease and other inflamed organs express granzyme K (GZMK) 3 , a tryptase-like protease with poorly defined function. Here, we show that GZMK can activate the complement cascade by cleaving C2 and C4. The nascent C4b and C2a fragments form a C3 convertase that cleaves C3, allowing further assembly of a C5 convertase that cleaves C5. The resulting convertases trigger every major event in the complement cascade, generating the anaphylatoxins C3a and C5a, the opsonins C4b and C3b, and the membrane attack complex. In RA synovium, GZMK is enriched in areas with abundant complement activation, and fibroblasts are the major producers of complement C2, C3, and C4 that serve as targets for GZMK-mediated complement activation. Our findings describe a previously unidentified pathway of complement activation that is entirely driven by lymphocyte-derived GZMK and proceeds independently of the classical, lectin, or alternative pathways. Given the widespread abundance of GZMK -expressing T cells in tissues in chronic inflammatory diseases and infection, GZMK-mediated complement activation is likely to be an important contributor to tissue inflammation in multiple disease contexts.

Intracellular and Extracellular Roles of Granzyme K

Granzymes are a family of serine proteases stored in granules inside cytotoxic cells of the immune system. Granzyme K (GrK) has been only limitedly characterized and knowledge on its molecular functions is emerging. Traditionally GrK is described as a granule-secreted, pro-apoptotic serine protease. However, accumulating evidence is redefining the functions of GrK by the discovery of novel intracellular (e.g. cytotoxicity, inhibition of viral replication) and extracellular roles (e.g. endothelial activation and modulation of a pro-inflammatory immune cytokine response). Moreover, elevated GrK levels are associated with disease, including viral and bacterial infections, airway inflammation and thermal injury. This review aims to summarize and discuss the current knowledge of i) intracellular and extracellular GrK activity, ii) cytotoxic and non-cytotoxic GrK functioning, iii) the role of GrK in disease, and iv) GrK as a potential therapeutic target.

Role of microRNA-145 in protection against myocardial ischemia/reperfusion injury in mice by regulating expression of GZMK with the treatment of sevoflurane

This study aims to investigate the role of microRNA-145 (miR-145) in protection against myocardial ischemia/reperfusion (I/R) injury in mice by regulating expression of granzyme K (GZMK) with the treatment of sevoflurane. The mice model of myocardial I/R injury was established by left coronary artery ligation. The expression of miR-145 and GZMK in myocardial tissues of mice was detected by Reverse transcription quantitative polymerase chain reaction and western blot analysis. The changes of the cardiac function and hemodynamics, pathological changes of myocardial tissues, the ultrastructure of cardiomyocytes, myocardial infarction area, and cardiomyocyte apoptosis were observed. The expression of the apoptosis-related protein cleaved-caspase-3, Bax, and Bcl-2 was detected by western blot analysis. The levels of malondialdehyde, myeloperoxidase, superoxide dismutase in myocardial tissues were detected by spectrophotometric colorimetry. The levels of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α in the serum of mice were detected by the enzyme-linked immunosorbent assay. The level of oxidative stress and the expression of inflammatory factors increased in mice with myocardial I/R injury. Sevoflurane postconditioning could reduce myocardial I/R injury in mice. Sevoflurane postconditioning may protect myocardial I/R injury through miR-145-regulation of GZMK in mice. Inhibition of miR-145 expression could reduce the protective effect of sevoflurane posttreatment on myocardial I/R injury in mice. Low expression of GZMK could attenuate the inhibitory effect of miR-145 on myocardial I/R injury after sevoflurane treatment in mice. Our study suggests that sevoflurane postconditioning may protect against myocardial I/R injury by upregulating miR-145 expression and downregulating GZMK expression.

Identification and annotation of bovine granzyme genes reveals a novel granzyme encoded within the trypsin-like locus

Granzymes are a family of serine proteases found in the lytic granules of cytotoxic T lymphocytes and natural killer (NK) cells, which are involved in killing of susceptible target cells. Most information on granzymes and their enzymatic specificities derive from studies in humans and mice. Although granzymes shared by both species show a high level of conservation, the complement of granzyme genes differs between the species. The aim of this study was to identify granzyme genes expressed in cattle, determine their genomic locations and analyse their sequences to predict likely functional specificities. Orthologues of the five granzyme genes found in humans (A, B, H, K and M) were identified, as well a novel gene designated granzyme O, most closely related to granzyme A. An orthologue of granzyme O was found in pigs and a non-function version was detected in the human genome. Use of specific PCRs demonstrated that all of these genes, including granzyme O, are expressed in activated subsets of bovine lymphocytes, with particularly high levels in CD8 T cells. Consistent with findings in humans and mice, the granzyme-encoding genes were located on three distinct genomic loci, which correspond to different proteolytic enzymatic activities, namely trypsin-like, chymotrypsin-like and metase-like. Analysis of amino acid sequences indicated that the granzyme proteins have broadly similar enzymatic specificities to their human and murine counterparts but indicated that granzyme B has a different secondary specificity. These findings provide the basis for further work to examine their role in the cytotoxic activity of bovine CD8 T cells.

Protein conformational plasticity and complex ligand-binding kinetics explored by atomistic simulations and Markov models

Understanding the structural mechanisms of protein–ligand binding and their dependence on protein sequence and conformation is of fundamental importance for biomedical research. Here we investigate the interplay of conformational change and ligand-binding kinetics for the serine protease Trypsin and its competitive inhibitor Benzamidine with an extensive set of 150 μs molecular dynamics simulation data, analysed using a Markov state model. Seven metastable conformations with different binding pocket structures are found that interconvert at timescales of tens of microseconds. These conformations differ in their substrate-binding affinities and binding/dissociation rates. For each metastable state, corresponding solved structures of Trypsin mutants or similar serine proteases are contained in the protein data bank. Thus, our wild-type simulations explore a space of conformations that can be individually stabilized by adding ligands or making suitable changes in protein sequence. These findings provide direct evidence of conformational plasticity in receptors.

Conformational plasticity influences several aspects of protein function. Here the authors combine extensive MD simulations with Markov state models—using trypsin as model—to reveal new mechanistic details of how conformational plasticity influence ligand-receptors interactions.

Live cell evaluation of granzyme delivery and death receptor signaling in tumor cells targeted by human natural killer cells

Growing interest in natural killer (NK) cell-based therapy for treating human cancer has made it imperative to develop new tools to measure early events in cell death. We recently demonstrated that protease-cleavable luciferase biosensors detect granzyme B and pro-apoptotic caspase activation within minutes of target cell recognition by murine cytotoxic lymphocytes. Here we report successful adaptation of the biosensor technology to assess perforin-dependent and -independent induction of death pathways in tumor cells recognized by human NK cell lines and primary cells. Cell-cell signaling via both Fc receptors and NK-activating receptors led to measurable luciferase signal within 15 minutes. In addition to the previously described aspartase-cleavable biosensors, we report development of granzyme A and granzyme K biosensors, for which no other functional reporters are available. The strength of signaling for granzyme biosensors was dependent on perforin expression in IL-2-activated NK effectors. Perforin-independent induction of apoptotic caspases was mediated by death receptor ligation and was detectable after 45 minutes of conjugation. Evidence of both FasL and TRAIL-mediated signaling was seen after engagement of Jurkat cells by perforin-deficient human cytotoxic lymphocytes. Although K562 cells have been reported to be insensitive to TRAIL, robust activation of pro-apoptotic caspases by NK cell-derived TRAIL was detectable in K562 cells. These studies highlight the sensitivity of protease-cleaved luciferase biosensors to measure previously undetectable events in live cells in real time. Further development of caspase and granzyme biosensors will allow interrogation of additional features of granzyme activity in live cells including localization, timing, and specificity.

Proteolytic clipping of histone tails: the emerging role of histone proteases in regulation of various biological processes

Chromatin is a dynamic DNA scaffold structure that responds to a variety of external and internal stimuli to regulate the fundamental biological processes. Majority of the cases chromatin dynamicity is exhibited through chemical modifications and physical changes between DNA and histones. These modifications are reversible and complex signaling pathways involving chromatin-modifying enzymes regulate the fluidity of chromatin. Fluidity of chromatin can also be impacted through irreversible change, proteolytic processing of histones which is a poorly understood phenomenon. In recent studies, histone proteolysis has been implicated as a regulatory process involved in the permanent removal of epigenetic marks from histones. Activities responsible for clipping of histone tails and their significance in various biological processes have been observed in several organisms. Here, we have reviewed the properties of some of the known histone proteases, analyzed their significance in biological processes and have provided future directions.

Substrate specificities of the granzyme tryptases A and K

The physiological roles of the granzymes A and K have been debated, especially concerning their involvement in cytotoxic and inflammatory processes. By performing N-terminal COFRADIC assisted N-terminomics on the homologous human granzymes A and K, we here provide detailed data on their substrate repertoires, their specificities, and differences in efficiency by which they cleave their substrates, all of which may aid in elucidating their key substrates. In addition, the so far uncharacterized mouse granzyme K was profiled alongside its human orthologue. While the global primary specificity profiles of these granzymes appear quite similar as they revealed only subtle differences and pointed to substrate occupancies in the P1, P1', and P2' position as the main determinants for substrate recognition, differential analyses unveiled distinguishing substrate subsite features, some of which were confirmed by the more selective cleavage of specifically designed probes.

New selective peptidyl di(chlorophenyl) phosphonate esters for visualizing and blocking neutrophil proteinase 3 in human diseases

The function of neutrophil protease 3 (PR3) is poorly understood despite of its role in autoimmune vasculitides and its possible involvement in cell apoptosis. This makes it different from its structural homologue neutrophil elastase (HNE). Endogenous inhibitors of human neutrophil serine proteases preferentially inhibit HNE and to a lesser extent, PR3. We constructed a single-residue mutant PR3 (I217R) to investigate the S4 subsite preferences of PR3 and HNE and used the best peptide substrate sequences to develop selective phosphonate inhibitors with the structure Ac-peptidyl(P)(O-C6H4-4-Cl)2. The combination of a prolyl residue at P4 and an aspartyl residue at P2 was totally selective for PR3. We then synthesized N-terminally biotinylated peptidyl phosphonates to identify the PR3 in complex biological samples. These inhibitors resisted proteolytic degradation and rapidly inactivated PR3 in biological fluids such as inflammatory lung secretions and the urine of patients with bladder cancer. One of these inhibitors revealed intracellular PR3 in permeabilized neutrophils and on the surface of activated cells. They hardly inhibited PR3 bound to the surface of stimulated neutrophils despite their low molecular mass, suggesting that the conformation and reactivity of membrane-bound PR3 is altered. This finding is relevant for autoantibody binding and the subsequent activation of neutrophils in granulomatosis with polyangiitis (formerly Wegener disease). These are the first inhibitors that can be used as probes to monitor, detect, and control PR3 activity in a variety of inflammatory diseases.

Granzyme K synergistically potentiates LPS-induced cytokine responses in human monocytes

Granzymes are serine proteases released by cytotoxic lymphocytes to induce apoptosis in virus-infected cells and tumor cells. Evidence is emerging that granzymes also play a role in controlling inflammation. Granzyme serum levels are elevated in patients with autoimmune diseases and infections, including sepsis. However, the function of extracellular granzymes in inflammation largely remains unknown. Here, we show that granzyme K (GrK) binds to Gram-negative bacteria and their cell-wall component lipopolysaccharide (LPS). GrK synergistically enhances LPS-induced cytokine release in vitro from primary human monocytes and in vivo in a mouse model of LPS challenge. Intriguingly, these extracellular effects are independent of GrK catalytic activity. GrK disaggregates LPS from micelles and augments LPS-CD14 complex formation, thereby likely boosting monocyte activation by LPS. We conclude that extracellular GrK is an unexpected direct modulator of LPS-TLR4 signaling during the antimicrobial innate immune response.

Allosteric inactivation of a trypsin-like serine protease by an antibody binding to the 37- and 70-loops

Serine protease catalytic activity is in many cases regulated by conformational changes initiated by binding of physiological modulators to exosites located distantly from the active site. Inhibitory monoclonal antibodies binding to such exosites are potential therapeutics and offer opportunities for elucidating fundamental allosteric mechanisms. The monoclonal antibody mU1 has previously been shown to be able to inhibit the function of murine urokinase-type plasminogen activator in vivo. We have now mapped the epitope of mU1 to the catalytic domain's 37- and 70-loops, situated about 20 Å from the S1 specificity pocket of the active site. Our data suggest that binding of mU1 destabilizes the catalytic domain and results in conformational transition into a state, in which the N-terminal amino group of Ile16 is less efficiently stabilizing the oxyanion hole and in which the active site has a reduced affinity for substrates and inhibitors. Furthermore, we found evidence for functional interactions between residues in uPA's C-terminal catalytic domain and its N-terminal A-chain, as deletion of the A-chain facilitates the mU1-induced conformational distortion. The inactive, distorted state is by several criteria similar to the E* conformation described for other serine proteases. Hence, agents targeting serine protease conformation through binding to exosites in the 37- and 70-loops represent a new class of potential therapeutics.

Conformational selection in trypsin-like proteases

For over four decades, two competing mechanisms of ligand recognition--conformational selection and induced-fit--have dominated our interpretation of protein allostery. Defining the mechanism broadens our understanding of the system and impacts our ability to design effective drugs and new therapeutics. Recent kinetics studies demonstrate that trypsin-like proteases exist in equilibrium between two forms: one fully accessible to substrate (E) and the other with the active site occluded (E*). Analysis of the structural database confirms existence of the E* and E forms and vouches for the allosteric nature of the trypsin fold. Allostery in terms of conformational selection establishes an important paradigm in the protease field and enables protein engineers to expand the repertoire of proteases as therapeutics.

Crystal structures of prethrombin-2 reveal alternative conformations under identical solution conditions and the mechanism of zymogen activation

Prethrombin-2 is the immediate zymogen precursor of the clotting enzyme thrombin, which is generated upon cleavage at R15 and separation of the A chain and catalytic B chain. The X-ray structure of prethrombin-2 determined in the free form at 1.9 Å resolution shows the 215-217 segment collapsed into the active site and occluding 49% of the volume available for substrate binding. Remarkably, some of the crystals harvested from the same crystallization well, under identical solution conditions, diffract to 2.2 Å resolution in the same space group but produce a structure in which the 215-217 segment moves >5 Å and occludes 24% of the volume available for substrate binding. The two alternative conformations of prethrombin-2 have the side chain of W215 relocating >9 Å within the active site and are relevant to the allosteric E*-E equilibrium of the mature enzyme. Another unanticipated feature of prethrombin-2 bears on the mechanism of prothrombin activation. R15 is found buried within the protein in ionic interactions with E14e, D14l, and E18, thereby making its exposure to solvent necessary for proteolytic attack and conversion to thrombin. On the basis of this structural observation, we constructed the E14eA/D14lA/E18A triple mutant to reduce the level of electrostatic coupling with R15 and promote zymogen activation. The mutation causes prethrombin-2 to spontaneously convert to thrombin, without the need for the snake venom ecarin or the physiological prothrombinase complex.

Allostery in trypsin-like proteases suggests new therapeutic strategies

Trypsin-like proteases (TLPs) are a large family of enzymes responsible for digestion, blood coagulation, fibrinolysis, development, fertilization, apoptosis and immunity. A current paradigm posits that the irreversible transition from an inactive zymogen to the active protease form enables productive interaction with substrate and catalysis. Analysis of the entire structural database reveals two distinct conformations of the active site: one fully accessible to substrate (E) and the other occluded by the collapse of a specific segment (E*). The allosteric E*-E equilibrium provides a reversible mechanism for activity and regulation in addition to the irreversible zymogen to protease conversion and points to new therapeutic strategies aimed at inhibiting or activating the enzyme. In this review, we discuss relevant examples, with emphasis on the rational engineering of anticoagulant thrombin mutants.

Neutrophil elastase, proteinase 3, and cathepsin G as therapeutic targets in human diseases

Polymorphonuclear neutrophils are the first cells recruited to inflammatory sites and form the earliest line of defense against invading microorganisms. Neutrophil elastase, proteinase 3, and cathepsin G are three hematopoietic serine proteases stored in large quantities in neutrophil cytoplasmic azurophilic granules. They act in combination with reactive oxygen species to help degrade engulfed microorganisms inside phagolysosomes. These proteases are also externalized in an active form during neutrophil activation at inflammatory sites, thus contributing to the regulation of inflammatory and immune responses. As multifunctional proteases, they also play a regulatory role in noninfectious inflammatory diseases. Mutations in the ELA2/ELANE gene, encoding neutrophil elastase, are the cause of human congenital neutropenia. Neutrophil membrane-bound proteinase 3 serves as an autoantigen in Wegener granulomatosis, a systemic autoimmune vasculitis. All three proteases are affected by mutations of the gene (CTSC) encoding dipeptidyl peptidase I, a protease required for activation of their proform before storage in cytoplasmic granules. Mutations of CTSC cause Papillon-Lefèvre syndrome. Because of their roles in host defense and disease, elastase, proteinase 3, and cathepsin G are of interest as potential therapeutic targets. In this review, we describe the physicochemical functions of these proteases, toward a goal of better delineating their role in human diseases and identifying new therapeutic strategies based on the modulation of their bioavailability and activity. We also describe how nonhuman primate experimental models could assist with testing the efficacy of proposed therapeutic strategies.

Crystal structure of prethrombin-1

Prothrombin is the zymogen precursor of the clotting enzyme thrombin, which is generated by two sequential cleavages at R271 and R320 by the prothrombinase complex. The structure of prothrombin is currently unknown. Prethrombin-1 differs from prothrombin for the absence of 155 residues in the N-terminal domain and is composed of a single polypeptide chain containing fragment 2 (residues 156-271), A chain (residues 272-320), and B chain (residues 321-579). The X-ray crystal structure of prethrombin-1 solved at 2.2-Å resolution shows an overall conformation significantly different (rmsd = 3.6 Å) from that of its active form meizothrombin desF1 carrying a cleavage at R320. Fragment 2 is rotated around the y axis by 29° and makes only few contacts with the B chain. In the B chain, the oxyanion hole is disrupted due to absence of the I16-D194 ion pair and the Na(+) binding site and adjacent primary specificity pocket are highly perturbed. A remarkable feature of the structure is that the autolysis loop assumes a helical conformation enabling W148 and W215, located 17 Å apart in meizothrombin desF1, to come within 3.3 Å of each other and completely occlude access to the active site. These findings suggest that the zymogen form of thrombin possesses conformational plasticity comparable to that of the mature enzyme and have significant implications for the mechanism of prothrombin activation and the zymogen → protease conversion in trypsin-like proteases.

Evidence of the E*-E equilibrium from rapid kinetics of Na+ binding to activated protein C and factor Xa

Na(+) binding to thrombin enhances the procoagulant and prothrombotic functions of the enzyme and obeys a mechanism that produces two kinetic phases: one fast (in the microsecond time scale) due to Na(+) binding to the low activity form E to produce the high activity form E:Na(+) and another considerably slower (in the millisecond time scale) that reflects a pre-equilibrium between E and the inactive form E*. In this study, we demonstrate that this mechanism also exists in other Na(+)-activated clotting proteases like factor Xa and activated protein C. These findings, along with recent structural data, suggest that the E*-E equilibrium is a general feature of the trypsin fold.

Orphan granzymes find a home

Cytotoxic lymphocytes are armed with granules that are released in the granule-exocytosis pathway to kill tumor cells and virus-infected cells. Cytotoxic granules contain the pore-forming protein perforin and a family of structurally homologues serine proteases called granzymes. While perforin facilitates the entry of granzymes into a target cell, the latter initiate distinct apoptotic routes. Granzymes are also implicated in extracellular functions such as extracellular matrix degradation, immune regulation, and inflammation. The family of human granzymes consists of five members, of which granzyme A and B have been studied most extensively. Recently, elucidation of the specific characteristics of the other three human granzymes H, K, and M, also referred to as orphan granzymes, have started. In this review, we summarize and discuss what is currently known about the biology of the human orphan granzymes.

Structures of human proteinase 3 and neutrophil elastase--so similar yet so different

Proteinase 3 and neutrophil elastase are serine proteinases of the polymorphonuclear neutrophils, which are considered to have both similar localization and ligand specificity because of their high sequence similarity. However, recent studies indicate that they might have different and yet complementary physiologic roles. Specifically, proteinase 3 has intracellular specific protein substrates resulting in its involvement in the regulation of intracellular functions such as proliferation or apoptosis. It behaves as a peripheral membrane protein and its membrane expression is a risk factor in chronic inflammatory diseases. Moreover, in contrast to human neutrophil elastase, proteinase 3 is the preferred target antigen in Wegener's granulomatosis, a particular type of vasculitis. We review the structural basis for the different ligand specificities and membrane binding mechanisms of both enzymes, as well as the putative anti-neutrophil cytoplasm autoantibody epitopes on human neutrophil elastase 3. We also address the differences existing between murine and human enzymes, and their consequences with respect to the development of animal models for the study of human proteinase 3-related pathologies. By integrating the functional and the structural data, we assemble many pieces of a complicated puzzle to provide a new perspective on the structure-function relationship of human proteinase 3 and its interaction with membrane, partner proteins or cleavable substrates. Hence, precise and meticulous structural studies are essential tools for the rational design of specific proteinase 3 substrates or competitive ligands that modulate its activities.

Recombinant human progranzyme 3 expressed in Escherichia coli for analysis of its activation mechanism

Gr3 is reported to play an important role in defense against viral infection. Although it is known that Gr3 is synthesized as a proenzyme and activated in the cytotoxic granules of NK cells and CTL, the activation mechanism is not clearly understood. In an attempt to analyze the activation mechanism of human Gr3, a recombinant pro-Gr3 was expressed in the periplasm of E. coli and purified to homogeneity. On SDS-PAGE the recombinant pro-Gr3 showed a slightly higher molecular weight than the enzymatically active Gr3, because the former possesses a small propeptide at its N-terminal. The recombinant pro-Gr3 was enzymatically inactive. It could be activated by treatment with cathepsin C, which concomitantly decreased the molecular weight to that of active Gr3. The proteolytic reaction of cathepsin C did not continue after one dipeptide had been removed, indicating that the recombinant pro-Gr3 had the native conformation without any refolding process. The recombinant pro-Gr3 would be a valuable tool for analyzing the activation mechanism and exploring other activating enzymes besides cathepsin C.

Granzyme A activates another way to die

Granzyme A (GzmA) is the most abundant serine protease in killer cell cytotoxic granules. GzmA activates a novel programed cell death pathway that begins in the mitochondrion, where cleavage of NDUFS3 in electron transport complex I disrupts mitochondrial metabolism and generates reactive oxygen species (ROS). ROS drives the endoplasmic reticulum-associated SET complex into the nucleus, where it activates single-stranded DNA damage. GzmA also targets other important nuclear proteins for degradation, including histones, the lamins that maintain the nuclear envelope, and several key DNA damage repair proteins (Ku70, PARP-1). Cells that are resistant to the caspases or GzmB by overexpressing bcl-2 family anti-apoptotic proteins or caspase or GzmB protease inhibitors are sensitive to GzmA. By activating multiple cell death pathways, killer cells provide better protection against a variety of intracellular pathogens and tumors. GzmA also has proinflammatory activity; it activates pro-interleukin-1beta and may also have other proinflammatory effects that remain to be elucidated.

Granzyme M: characterization with sites of post-translational modification and specific sites of interaction with substrates and inhibitors

Granzymes kill cells in a variety of ways. They induce mitochondrial dysfunction through caspase dependent and caspase-independent pathways and destroy DNA and the integrity of the nucleus. For gaining a better understanding of the molecular function of granzyme M and its NK cell specificity, structural characterization of this enzyme by molecular modeling as well as its detailed comparison with other granzymes is presented in this study. The study includes mode of action of granzyme M using cationic binding sites, substrate specificity, post-translational structural modification and its functional relationship and interaction of the enzyme with inhibitor in an attempt to explore how the activity of human granzyme M is controlled under physiological conditions. It is concluded from the present study that the post-translational modification, including Oglycosylation of serine, phosphorylation of serine and threonine and myristoylation of glycine, play an important role in the interaction of enzyme with the cell surface membrane and regulate protein trafficking and stability. Phosphorylated serine and threonine also plays a role in tumor elimination, viral clearance and tissue repair. In Gzm M there are cationic sites, cs1 and cs2 that may participate in binding of Gzm M to the cell surface, thereby promoting its uptake and eventual release into the cytoplasm. Gzm M shows apoptotic activity both by caspase dependent and independent pathways. Modeling of inhibitors bound to the granzyme active site shows that the dimer also contributes to substrate specificity in a unique manner by extending the active-site cleft.

Structural basis for proteolytic specificity of the human apoptosis-inducing granzyme M

Granzyme M (GzmM), a unique serine protease constitutively expressed in NK cells, is important for granule-mediated cytolysis and can induce rapid caspase-dependent apoptosis of tumor cells. However, few substrates of GzmM have been reported to date, and the mechanism by which this enzyme recognizes and hydrolyzes substrates is unknown. To provide structural insights into the proteolytic specificity of human GzmM (hGzmM), crystal structures of wild-type hGzmM, the inactive D86N-GzmM mutant with bound peptide substrate, and the complexes with a catalytic product and with a tetrapeptide chloromethylketone inhibitor were solved to 1.96 A, 2.30 A, 2.17 A and 2.70 A, respectively. Structure-based mutagenesis revealed that the N terminus and catalytic triad of hGzmM are most essential for proteolytic function. In particular, D86N-GzmM was found to be an ideal inactive enzyme for functional studies. Structural comparisons indicated a large conformational change of the L3 loop upon substrate binding, and suggest this loop mediates the substrate specificity of hGzmM. Based on the complex structure of GzmM with its catalytic product, a tetrapeptide chloromethylketone inhibitor was designed and found to specifically block the catalytic activity of hGzmM.

Granzyme F induces a novel death pathway characterized by Bid-independent cytochrome c release without caspase activation

Granzyme F (GzmF) belongs to a unique group of granzymes in mice. Murine GzmF is highly expressed in NK3.1 cells and in lymphokine-activated killer (LAK) cells. However, the manner in which GzmF works in granule-mediated cytolysis is unknown. In this study, we first demonstrated that GzmF causes a novel cell death pathway. The death is characterized by an externalization of phosphatidylserine, by nuclear condensation, mitochondrial damage, cytochrome c (cyt c) release, caspase inactivation and single-stranded DNA nicking. GzmF-induced chromatin was incompletely condensed and segmented at the nuclear periphery. Cellular organelles were damaged and the cytoplasm showed an extensive vacuolization that is reminiscent of necroptosis. GzmF can cause rapid mitochondrial swelling, depolarization and reactive oxygen species accumulation. GzmF-induced death does not involve caspase activation, Bid cleavage or activation of DNA nickase NM23H1. GzmF-silenced LAK cells showed reduced cytotoxicity against caspase-inhibited target tumor cells. Moreover, cyt c release is independent of Bid or Bax/Bak. We further showed that GzmF impairs mitochondrial electron transport to abolish ATP generation. ATP decline may contribute to a failure of apoptosome formation, leading to caspase inactivation.

Serine proteases

Over one third of all known proteolytic enzymes are serine proteases. Among these, the trypsins underwent the most predominant genetic expansion yielding the enzymes responsible for digestion, blood coagulation, fibrinolysis, development, fertilization, apoptosis, and immunity. The success of this expansion resides in a highly efficient fold that couples catalysis and regulatory interactions. Added complexity comes from the recent observation of a significant conformational plasticity of the trypsin fold. A new paradigm emerges where two forms of the protease, E* and E, are in allosteric equilibrium and determine biological activity and specificity.

Mechanism of the anticoagulant activity of thrombin mutant W215A/E217A

The thrombin mutant W215A/E217A (WE) is a potent anticoagulant both in vitro and in vivo. Previous x-ray structural studies have shown that WE assumes a partially collapsed conformation that is similar to the inactive E* form, which explains its drastically reduced activity toward substrate. Whether this collapsed conformation is genuine, rather than the result of crystal packing or the mutation introduced in the critical 215-217 beta-strand, and whether binding of thrombomodulin to exosite I can allosterically shift the E* form to the active E form to restore activity toward protein C are issues of considerable mechanistic importance to improve the design of an anticoagulant thrombin mutant for therapeutic applications. Here we present four crystal structures of WE in the human and murine forms that confirm the collapsed conformation reported previously under different experimental conditions and crystal packing. We also present structures of human and murine WE bound to exosite I with a fragment of the platelet receptor PAR1, which is unable to shift WE to the E form. These structural findings, along with kinetic and calorimetry data, indicate that WE is strongly stabilized in the E* form and explain why binding of ligands to exosite I has only a modest effect on the E*-E equilibrium for this mutant. The E* --> E transition requires the combined binding of thrombomodulin and protein C and restores activity of the mutant WE in the anticoagulant pathway.

Stabilization of the E* form turns thrombin into an anticoagulant

Previous studies have shown that deletion of nine residues in the autolysis loop of thrombin produces a mutant with an anticoagulant propensity of potential clinical relevance, but the molecular origin of the effect has remained unresolved. The x-ray crystal structure of this mutant solved in the free form at 1.55 A resolution reveals an inactive conformation that is practically identical (root mean square deviation of 0.154 A) to the recently identified E* form. The side chain of Trp(215) collapses into the active site by shifting > 10 A from its position in the active E form, and the oxyanion hole is disrupted by a flip of the Glu(192)-Gly(193) peptide bond. This finding confirms the existence of the inactive form E* in essentially the same incarnation as first identified in the structure of the thrombin mutant D102N. In addition, it demonstrates that the anticoagulant profile often caused by a mutation of the thrombin scaffold finds its likely molecular origin in the stabilization of the inactive E* form that is selectively shifted to the active E form upon thrombomodulin and protein C binding.

Structure of human prostasin, a target for the regulation of hypertension

Prostasin (also called channel activating protease-1 (CAP1)) is an extracellular serine protease implicated in the modulation of fluid and electrolyte regulation via proteolysis of the epithelial sodium channel. Several disease states, particularly hypertension, can be affected by modulation of epithelial sodium channel activity. Thus, understanding the biochemical function of prostasin and developing specific agents to inhibit its activity could have a significant impact on a widespread disease. We report the expression of the prostasin proenzyme in Escherichia coli as insoluble inclusion bodies, refolding and activating via proteolytic removal of the N-terminal propeptide. The refolded and activated enzyme was shown to be pure and monomeric, with kinetic characteristics very similar to prostasin expressed from eukaryotic systems. Active prostasin was crystallized, and the structure was determined to 1.45 A resolution. These apoprotein crystals were soaked with nafamostat, allowing the structure of the inhibited acyl-enzyme intermediate structure to be determined to 2.0 A resolution. Comparison of the inhibited and apoprotein forms of prostasin suggest a mechanism of regulation through stabilization of a loop which interferes with substrate recognition.

Granzyme B delivery via perforin is restricted by size, but not by heparan sulfate-dependent endocytosis

How granzymes gain entry into the cytosol of target cells during killer cell attack has been the subject of several studies in the past, but the effective delivery mechanism during target cell encounter has not been clarified. Here we show that granzyme B (GzmB) mutants lacking binding to negatively charged, essentially heparan-sulfate-containing membrane receptors are poorly endocytosed yet are delivered to the cytosol with efficacy similar to that of WT GzmB. In a cell-based system GzmB-deficient natural killer cells provided perforin (pfn) by natural polarized secretion and synergized with externally added GzmB. Whereas receptor (heparan sulfate)-dependent endocytosis was dispensable, delivery of larger cargo like that of GzmB fusion proteins and GzmB-antibody complexes was restricted by their size. Our data support the model in which granzymes are primarily translocated through repairable membrane pores of finite size and not by the disruption of endocytosed vesicles. We conclude that structurally related translocators, i.e., perforin and cholesterol-dependent cytolysins, deliver deathly cargo across host cell membranes in a similar manner.

Death by a thousand cuts: granzyme pathways of programmed cell death

The granzymes are cell death-inducing enzymes, stored in the cytotoxic granules of cytotoxic T lymphocytes and natural killer cells, that are released during granule exocytosis when a specific virus-infected or transformed target cell is marked for elimination. Recent work suggests that this homologous family of serine esterases can activate at least three distinct pathways of cell death. This redundancy likely evolved to provide protection against pathogens and tumors with diverse strategies for evading cell death. This review discusses what is known about granzyme-mediated pathways of cell death as well as recent studies that implicate granzymes in immune regulation and extracellular proteolytic functions in inflammation.

Granzyme H destroys the function of critical adenoviral proteins required for viral DNA replication and granzyme B inhibition

Granzymes are key components of the immune response that play important roles in eliminating host cells infected by intracellular pathogens. Several granzymes are potent inducers of cell death. However, whether granzymes use additional mechanisms to exert their antipathogen activity remains elusive. Here, we show that in adenovirus-infected cells in which granzyme B (gzmB) and downstream apoptosis pathways are inhibited, granzyme H (gzmH), an orphan granzyme without known function, directly cleaves the adenovirus DNA-binding protein (DBP), a viral component absolutely required for viral DNA replication. We directly addressed the functional consequences of the cleavage of the DBP by gzmH through the generation of a virus that encodes a gzmH-resistant DBP. This virus demonstrated that gzmH directly induces an important decay in viral DNA replication. Interestingly, gzmH also cleaves the adenovirus 100K assembly protein, a major inhibitor of gzmB, and relieves gzmB inhibition. These results provide the first evidence that granzymes can mediate antiviral activity through direct cleavage of viral substrates, and further suggest that different granzymes have synergistic functions to outflank viral defenses that block host antiviral activities.

Granzyme K cleaves the nucleosome assembly protein SET to induce single-stranded DNA nicks of target cells

Although granzymes (Gzms) A- and B-induced cell death pathways have been defined, little is known about how other orphan Gzms function in CTL-mediated cytotoxicity. GzmK and A are tryptases among all the Gzms of humans and they are closely linked on the same chromosome. In this study, we showed that GzmK can be efficiently delivered into target cells with a cationic lipid protein transfection reagent Pro-Ject. We found human GzmK triggers rapid cell death independently of caspase activation. The features of death are characterized by rapid externalization of phosphatidylserine, nuclear morphological changes and single-stranded DNA nicks. GzmK hydrolyzes the nucleosome assembly protein SET in its recombinant and native forms or in intact cells. Cleavage of SET by GzmK abrogates its nucleosome assembly activity. After GzmK loading, SET and DNase NM23H1 rapidly translocate into the nucleus and SET is cleaved, where the nuclease activity of NM23H1 is activated to nick chromosomal DNA.

A Pulmonary Perspective on GASPIDs: Granule-Associated Serine Peptidases of Immune Defense

Airways are protected from pathogens by forces allied with innate and adaptive immunity. Recent investigations establish critical defensive roles for leukocyte and mast cell serine-class peptidases garrisoned in membrane-bound organelles-here termed Granule-Associated Serine Peptidases of Immune Defense, or GASPIDs. Some better characterized GASPIDs include neutrophil elastase and cathepsin G (which defend against bacteria), proteinase-3 (targeted by antineutrophil antibodies in Wegener's vasculitis), mast cell beta-tryptase and chymase (which promote allergic inflammation), granzymes A and B (which launch apoptosis pathways in infected host cells), and factor D (which activates complement's alternative pathway). GASPIDs can defend against pathogens but can harm host cells in the process, and therefore become targets for pharmaceutical inhibition. They vary widely in specificity, yet are phylogenetically similar. Mammalian speciation supported a remarkable flowering of these enzymes as they co-evolved with specialized immune cells, including mast cells, basophils, eosinophils, cytolytic T-cells, natural killer cells, neutrophils, macrophages and dendritic cells. Many GASPIDs continue to evolve rapidly, providing some of the most conspicuous examples of divergent protein evolution. Consequently, students of GASPIDs are rewarded not only with insights into their roles in lung immune defense but also with clues to the origins of cellular specialization in vertebrate immunity.

Functional and structural characterization of Spl proteases from Staphylococcus aureus

Staphylococcus aureus is the major cause of nosocomial infections world-wide, with increasing prevalence of community-acquired diseases. The recent dramatic increase in multi-antibiotic resistance, including resistance to the last-resort drug, vancomycin, together with the lack of an effective vaccine highlight the need for better understanding of S.aureus pathogenicity. Comparative analysis of available bacterial genomes allows for the identification of previously uncharacterized S.aureus genes with potential roles in pathogenicity. A good example is a cluster of six serine protease-like (spl) genes encompassed in one operon, which encode for putative proteases with similarity to staphylococcal glutamylendopeptidase (V8 protease). Here, we describe an efficient expression system for the production of recombinant SplB and SplC proteases in Escherichia coli, together with structural and functional characterization of the purified enzymes. A unique mechanism of cytoplasm protection against activity of misdirected SplB was uncovered. Apparently, the co-translated signal peptide maintains protease latency until it is cleaved by the signal peptidase during protein secretion. Furthermore, the crystal structure of the SplC protease revealed a fold resembling that of the V8 protease and epidermolytic toxins. Arrangement of the active site cleft and substrate-binding pocket of SplC explains the mechanism of enzyme latency and suggests that some Spl proteases possess restricted substrate specificity similar to that of the V8 protease and epidermolytic toxins.

Molecular characterization and expression of a granzyme of an ectothermic vertebrate with chymase-like activity expressed in the cytotoxic cells of Nile tilapia (Oreochromis niloticus)

We have identified the gene coding for a novel serine protease with close similarities to mammalian granzymes from nonspecific cytotoxic cells of a teleost fish Oreochromis niloticus. The genomic organization of tilapia granzyme-1 (TLGR-1) has the signature five-exon-four-intron structure shared by all granzymes and similar hematopoietic Ser proteases. Molecular modeling studies suggested a granzyme-like structure for this protein with four disulfide linkages and two additional Cys residues. The expression of this gene is found to be restricted to cytotoxic cell populations with a low level of constitutive expression when compared to similar granzymes in other teleost species. High levels of transcriptional activation of TLGR-1 with different stimuli suggested that this gene is highly induced during immune reactions. Triplet residues around the active site Ser of TLGR, which determines the primary substrate specificity of granzymes, differ significantly from that of other granzymes. Recombinant TLGR-1 was expressed in the mature and proenzyme forms using pPICZ-alpha vector in the Pichia pastoris expression system. Recombinant TLGR-1 was used to determine the primary substrate specificity of this protease using various synthetic thiobenzyl ester substrates. In vitro enzyme kinetics assays suggested a preference for residues with bulky side chains at the P1 site, indicating a chymase-like activity for this protease. These results indicate the presence of novel granzymes in cytotoxic cells from ectothermic vertebrates.

Expression of enzymatically active human granzyme 3 in Escherichia coli for analysis of its substrate specificity

Human granzyme 3 (Gr3) is a serine protease contained in the granules of natural killer cells and cytotoxic T lymphocytes. To elucidate the biochemical and physiological characteristics of Gr3, we attempted to prepare an enzymatically active recombinant human Gr3 without refolding and proteolytic activation. An expression vector was constructed, in which the pre-/pro-peptide coding sequence of Gr3 was replaced with the bacterial pelB leader sequence. The resultant expression product was a fully active protease in the periplasmic fraction of Escherichia coli and was purified to homogeneity. The purified enzyme effectively hydrolyzed Z-Lys-SBzl, a conventionally used substrate of Gr3. In addition, it also hydrolyzed the peptide substrate library FRETS-25Xaa series, required basic amino acid residues, Arg or Lys, at the P1 position, and most efficiently hydrolyzed the carboxylic side of Phe-Tyr-Arg downward arrow (P3-P2-P1) sequence of the 475 tripeptide combinations.

X-ray structures of free and leupeptin-complexed human alphaI-tryptase mutants: indication for an alpha-->beta-tryptase transition

Tryptases alpha and beta are trypsin-like serine proteinases expressed in large amounts by mast cells. Beta-tryptase is a tetramer that has enzymatic activity, but requires heparin binding to maintain functional and structural stability, whereas alpha-tryptase has little, if any, enzymatic activity but is a stable tetramer in the absence of heparin. As shown previously, these differences can be mainly attributed to the different conformations of the 214-220 segment. Interestingly, the replacement of Asp216 by Gly, which is present in beta-tryptase, results in enzymatically active but less stable alpha-tryptase mutants. We have solved the crystal structures of both the single (D216G) and the double (K192Q/D216G) mutant forms of recombinant human alphaI-tryptase in complex with the peptide inhibitor leupeptin, as well as the structure of the non-inhibited single mutant. The inhibited mutants exhibited an open functional substrate binding site, while in the absence of an inhibitor, the open (beta-tryptase-like) and the closed (alpha-tryptase-like) conformations were present simultaneously. This shows that both forms are in a two-state equilibrium, which is influenced by the residues in the vicinity of the active site and by inhibitor/substrate binding. Novel insights regarding the observed stability differences as well as a potential proteolytic activity of wild-type alpha-tryptase, which may possess a cryptic active site, are discussed.

The N-terminal tetrapeptide of neutrophil proteinase 3 causes S-phase arrest in granulopoietic progenitors

OBJECTIVE

Secreted enzymatically inactive proforms of hematopoietic serine proteases proteinase 3 (PR3), azurocidin, and granzymes A, B, H, K, and M are able to reduce the fraction of granulopoietic progenitors (CFU-GM) in S-phase, whereas human leukocyte elastase (HLE) and cathepsin G lack this ability. The objective of the present study was to map the specific sequence(s) of PR3 and other hematopoietic serine proteases responsible for the downmodulation of S-phase.

METHODS

Synthetic peptides corresponding to N-terminal sequences of PR3, purified recombinant PR3, and HLE, as well as hybrid proteins constructed by interchanging the N-terminal regions of PR3 and HLE, thus creating PR3/HLE and HLE/PR3, respectively, were tested for their ability to reduce the fraction of human marrow CFU-GM killed by cytosine arabinoside. In addition, we measured the effect of synthetic peptides on bromodeoxyuridine (BrdU) incorporation in common myeloid progenitors (CMP) and granulocyte/macrophage progenitors (GMP) isolated by cell sorting.

RESULTS

The common N-terminal motif of PR3 and other serine proteases (i.e., IVGG or IIGG) downmodulate the S-phase of CFU-GM at 40 to 80 nM concentration. Tetrapeptide IVGG, but not IVGR, significantly reduces BrdU incorporation in GMP within the CD34+ population. When the N-terminal of HLE is presented by the HLE/PR3 hybrid protein it is fully active.

CONCLUSION

These findings demonstrate that the downmodulatory effect on CFU-GM in S-phase is an S-phase arrest mediated by the first four N-terminal amino acids of PR3, and also suggest that this activity is dependent on the configuration of the proform providing the correct presentation of this N-terminal motif.

A fourier fingerprint-based method for protein surface representation

A crucial enabling technology for structural genomics is the development of algorithms that can predict the putative function of novel protein structures: the proposed functions can subsequently be experimentally tested by functional studies. Testable assignments of function can be made if it is possible to attribute a putative, or indeed probable, function on the basis of the shapes of the binding sites on the surface of a protein structure. However the comparison of the surfaces of 3D protein structures is a computationally demanding task. Here we present four surface representations that can be used locally to describe the global shape of specifically bounded local region models. The most successful of these representations is obtained by a Fourier analysis of the distribution of surface curvature on concentric spheres around a surface point and summarizes a 24 A diameter spherically clipped region of protein surface by a fingerprint of 18 Fourier amplitude values. Searching experiments using these fingerprints on a set of 366 proteins demonstrate that this provides an effective and an efficient technique for the matching of protein surfaces.

Differential expression of human granzymes A, B, and K in natural killer cells and during CD8+ T cell differentiation in peripheral blood

NK cells and cytotoxic T lymphocytes can induce apoptosis in virus-infected and transformed target cells via the granule exocytosis pathway. The key components of the cytolytic granules are perforin and several serine esterases, termed granzymes. While the cellular distribution of human granzymes A (GrA) and B (GrB) has been well characterized much less is known about the expression pattern of human granzyme K (GrK). In this study GrA, GrB, and GrK expression was analyzed in human peripheral blood lymphocytes using flow cytometry. There was a distinct population of GrK expressing CD8+ T cells with a CD27+/CD28+/CCR5high/CCR7-/perforin-/low/IFN-gamma+ memory-like phenotype, while all CD56bright NK cells were also positive for GrK. In addition, GrK was also expressed in subpopulations of CD56+ T cells, CD4+ T cells, and TCRgammadelta+ T cells. In contrast, GrB was primarily expressed in CD56dim NK cells and differentiated memory CD8+ T cells with the CD27-/low/CD28-/low/CCR5-/low/CCR7-/CD11b+/perforinhigh phenotype. Only few CD8+ T cells expressed both GrB and GrK. GrA was found to be co-expressed in all GrB- and GrK-expressing T cells. Our findings suggest that granzyme expression during the differentiation process of memory CD8+ T cells might be as follows: GrA+/GrB-/GrK+ --> GrA+/GrB+/GrK+ --> GrA+/GrB+/GrK-.

Toxicological effects of acrylamide on rat testicular gene expression profile

Toxicological effects of acrylamide on differential gene expression profile of rat testis were evaluated. Acrylamide induced morphological sperm defects, and decreased sperm concentration in cauda epididymis. Serum testosterone level and Leydig cell viability were also decreased dose-dependently, which resulted in decreased spermatogenesis. Acrylamide-induced histopathological lesions, such as formation of multinucleated giant cells and vacuolation, and numerous apoptotic cells were observed in seminiferous tubules. cDNA microarray analysis revealed that genes related to testicular-functions, apoptosis, cellular redox, cell growth, cell cycle, and nucleic acid-binding were up/down-regulated in testes isolated from acrylamide-treated group (60 mg/kg/day). Acrylamide toxicity appears to increase Leydig cell death and perturb gene expression levels, contributing to sperm defects and various abnormal histopathological lesions including apoptosis in rat testis.

Bioinformatics of granzymes: sequence comparison and structural studies on granzyme family by homology modeling

Cytotoxic lymphocytes (CTLs), the key players of cell mediated immunity, induce apoptosis by engaging death receptors or through exocytosis of cytolytic granules containing granzyme (proteases) and pore-forming protein (perforin). The crystal structure of granzyme B from human (B(h)) and rat (B(r)), as well as that of pro-granzyme K (K(h)) has been reported recently. In the present communication, we describe the homology modeling of granzyme family (in particular Gzm A(h), M(h), B(m), and C(m) from human and mouse) based on the crystal structural coordinates of trypsin, granzyme K (K(h)), and granzyme B (B(h)). These models have been used for establishing phylogenetic relationship as well as identifying characteristic features for designing specific inhibitors. The paper also highlights key residues at the S1, S2, and S2(') binding subsites in all granzyme, which may be involved in the structure-function relationship of this enzyme family. The predicted 3D homology models show a conserved two similar domain structure, i.e., an N-terminal domain and a C-terminal domain comprising predominantly of beta-sheet structure with a little alpha-helical content. Micro-heterogeneities have been observed in the vicinity of the active site in all granzymes as compared to granzyme B(h). For example, in granzyme M(h), valine is present at the S1 subsite instead of arginine. Similarly differences at S2 (Leu-->Phe), S3 (Ser-->Gly), and S4 (Arg-->Asn) subsites are quite apparent and appear to hold the potential for selective designing of inhibitors for possible therapeutic applications. Furthermore, analysis of the electrostatic surface potential on the shape of granzyme-inhibitor binding groove reveals clear differences at the reactive site. Additionally the different posttranslational modification sites such as phosphorylation (e.g., in granzyme M Thr101, Ser109), myristoylation (Gly22, 117, and 131), and glycosylation (Ser160) have been identified, as very little is known about the functional significance of these modifications in the granzyme family. Thus, glycosylation at Ser160 in granzyme M may influence the net charge of the enzyme, resulting in altered substrate binding as compared to granzyme B. Also this modification may influence the rate of complexation and binding affinity with proteoglycans. These studies are expected to contribute towards the basic understanding of functional associations of the granzymes with other molecules and their possible role in apoptosis.

Crystal structure of the apoptosis-inducing human granzyme A dimer

Granzyme A (GzmA) belongs to a family of trypsin-like serine proteases localized in cytoplasmic granules of activated lymphocytes and natural killer (NK) cells. In contrast to the related granzyme B (GzmB), GzmA forms a stable disulfide-linked homodimer and triggers target-cell death in a caspase-independent way. Limited proteolysis of a high-molecular-mass complex containing SET (also named putative HLA-associated protein II or PHAPII), PHAPI (pp32, leucine-rich acidic nuclear protein) and HMG2 by GzmA liberates NM23-H1, a Mg2+-dependent DNase that causes single-stranded breaks in nuclear DNA. By analyzing the dimeric GzmA structure at a resolution of 2.5 A, we determined the substrate-binding constraints and selective advantages of the two domains arranged as a unique functional tandem. The active sites of the two subunits point in opposite directions and the nearby noncatalytic surfaces can function as exosites, presenting substrates to the active site region of the adjacent partner in a manner analogous to staphylokinase or streptokinase, which present plasminogen to the cofactor-plasmin and cofactor-plasminogen complexes.