Molecular characterization of a puromycin-insensitive leucyl-specific aminopeptidase, PILS-AP

Molecular pathways for antigenic peptide generation by ER aminopeptidase 1

Endoplasmic Reticulum aminopeptidase 1 (ERAP1) is an intracellular enzyme that can generate or destroy potential peptide ligands for MHC class I molecules. ERAP1 activity influences the cell-surface immunopeptidome and epitope immunodominance patterns but in complex and poorly understood manners. Two main distinct pathways have been proposed to account for ERAP1's effects on the nature and quantity of MHCI-bound peptides: i) ERAP1 trims peptides in solution, generating the correct length for binding to MHCI or overtrimming peptides so that they are too short to bind, and ii) ERAP1 trims peptides while they are partially bound onto MHCI in manner that leaves the peptide amino terminus accessible. For both pathways, once an appropriate length peptide is generated it could bind conventionally to MHCI, competing with further trimming by ERAP1. The two pathways, although not necessarily mutually exclusive, provide distinct vantage points for understanding of the rules behind the generation of the immunopeptidome. Resolution of the mechanistic details of ERAP1-mediated antigenic peptide generation can have important consequences for pharmacological efforts to regulate the immunopeptidome for therapeutic applications, and for understanding association of ERAP1 alleles with susceptibility to autoimmune disease and cancer. We review current evidence in support of these two pathways and discuss their relative importance and potential complementarity.

Complex antigen presentation pathway for an HLA-A*0201-restricted epitope from Chikungunya 6K protein

BACKGROUND

The adaptive cytotoxic T lymphocyte (CTL)-mediated immune response is critical for clearance of many viral infections. These CTL recognize naturally processed short viral antigenic peptides bound to human leukocyte antigen (HLA) class I molecules on the surface of infected cells. This specific recognition allows the killing of virus-infected cells. The T cell immune T cell response to Chikungunya virus (CHIKV), a mosquito-borne Alphavirus of the Togaviridae family responsible for severe musculoskeletal disorders, has not been fully defined; nonetheless, the importance of HLA class I-restricted immune response in this virus has been hypothesized.

METHODOLOGY/PRINCIPAL FINDINGS

By infection of HLA-A*0201-transgenic mice with a recombinant vaccinia virus that encodes the CHIKV structural polyprotein (rVACV-CHIKV), we identified the first human T cell epitopes from CHIKV. These three novel 6K transmembrane protein-derived epitopes are presented by the common HLA class I molecule, HLA-A*0201. One of these epitopes is processed and presented via a complex pathway that involves proteases from different subcellular locations. Specific chemical inhibitors blocked these events in rVACV-CHIKV-infected cells.

CONCLUSIONS/SIGNIFICANCE

Our data have implications not only for the identification of novel Alphavirus and Togaviridae antiviral CTL responses, but also for analyzing presentation of antigen from viruses of different families and orders that use host proteinases to generate their mature envelope proteins.

Complex Consisting of β-Glucan and Antigenic Peptides with Cleavage Site for Glutathione and Aminopeptidases Induces Potent Cytotoxic T Lymphocytes

The efficient induction of antigen-specific immune responses requires not only promotion of the uptake of antigens and adjuvant molecules into antigen-presenting cells but also control of their intracellular behavior. We previously demonstrated that the β-glucan schizophyllan (SPG) can form complexes with CpG oligonucleotides with attached dA₄₀ (CpG-dA/SPG), which can accumulate in macrophages in the draining inguinal lymph nodes and induce strong immune responses. In this study, we prepared various conjugates composed of antigenic peptide (OVA_257-264) and dA₄₀ and made complexes with SPG. The conjugates with a disulfide bond between OVA_257-264 and dA₄₀ were easily cleaved by glutathione. The resultant peptides with a hydrophobic amino acid at the C-terminal end was recognized by puromycin-insensitive leucine aminopeptidase (PILS-AP), which trims antigenic peptide precursors and prepares peptides of eight or nine amino acids in length, which is the optimal length for binding to major histocompatibility complex (MHC)-I. The conjugate exposed to such enzymes induced a high antigen presentation level. The antigen presentation level was almost the same before and after the complexation with SPG. Immunization with a mixture of dA-OVA_257-264/SPG and CpG-dA/SPG induced high antigen-specific cytotoxic T-lymphocyte activity at a much lower peptide dose than in previous studies. These results can be strongly ascribed to not only the cell-specific delivery by SPG but also the control of the intracellular behavior by the introduction of cleavage sites. Therefore, peptide-dA/SPG complexes could be used as potent vaccine antigens for the treatment of cancers and infectious diseases.

Transcription factors LRF and BCL11A independently repress expression of fetal hemoglobin

Genes encoding human β-type globin undergo a developmental switch from embryonic to fetal to adult-type expression. Mutations in the adult form cause inherited hemoglobinopathies or globin disorders, including sickle cell disease and thalassemia. Some experimental results have suggested that these diseases could be treated by induction of fetal-type hemoglobin (HbF). However, the mechanisms that repress HbF in adults remain unclear. We found that the LRF/ZBTB7A transcription factor occupies fetal γ-globin genes and maintains the nucleosome density necessary for γ-globin gene silencing in adults, and that LRF confers its repressive activity through a NuRD repressor complex independent of the fetal globin repressor BCL11A. Our study may provide additional opportunities for therapeutic targeting in the treatment of hemoglobinopathies.

Autoimmune disease-associated variants of extracellular endoplasmic reticulum aminopeptidase 1 induce altered innate immune responses by human immune cells

Endoplasmic reticulum aminopeptidase 1 (ERAP1) gene polymorphisms have been linked to several autoimmune diseases; however, the molecular mechanisms underlying these associations are not well understood. Recently, we demonstrated that ERAP1 regulates key aspects of the innate immune response. Previous studies show ERAP1 to be endoplasmic reticulum-localized and secreted during inflammation. Herein, we investigate the possible roles that ERAP1 polymorphic variants may have in modulating the innate immune responses of human peripheral blood mononuclear cells (hPBMCs) using two experimental methods: extracellular exposure of hPBMCs to ERAP1 variants and adenovirus (Ad)-based ERAP1 expression. We found that exposure of hPBMCs to ERAP1 variant proteins as well as ERAP1 overexpression by Ad5 vectors increased inflammatory cytokine and chemokine production, and enhanced immune cell activation. Investigating the molecular mechanisms behind these responses revealed that ERAP1 is able to activate innate immunity via multiple pathways, including the NLRP3 (NOD-like receptor, pyrin domain-containing 3) inflammasome. Importantly, these responses varied if autoimmune disease-associated variants of ERAP1 were examined in the assay systems. Unexpectedly, blocking ERAP1 cellular internalization augmented IL-1β production. To our knowledge, this is the first report identifying ERAP1 as being involved in modulating innate responses of human immune cells, a finding that may explain why ERAP1 has been genetically associated with several autoimmune diseases.

Purpurin inhibits adipocyte-derived leucine aminopeptidase and angiogenesis in a zebrafish model

Adipocyte-derived leucine aminopeptidase (A-LAP) is a novel member of the M1 family of zinc metallopeptidases, which has been reported to play a crucial role in angiogenesis. In the present study, we conducted a target-based screening of natural products and synthetic chemical libraries using the purified enzyme to search novel inhibitors of A-LAP. Amongst several hits isolated, a natural product purpurin was identified as one of the most potent inhibitors of A-LAP from the screening. In vitro enzymatic analyses demonstrated that purpurin inhibited A-LAP activity in a non-competitive manner with a Ki value of 20 M. In addition, purpurin showed a strong selectivity toward A-LAP versus another member of M1 family of zinc metallopeptidase, aminopeptidase N (APN). In angiogenesis assays, purpurin inhibited the vascular endothelial growth factor (VEGF)-induced invasion and tube formation of human umbilical vein endothelial cells (HUVEC). Moreover, purpurin inhibited in vivo angiogenesis in zebrafish embryo without toxicity. These data demonstrate that purpurin is a novel specific inhibitor of A-LAP and could be developed as a new anti-angiogenic agent.

TLR-mediated secretion of endoplasmic reticulum aminopeptidase 1 from macrophages

Macrophages play an important role in host defense under several immunological, inflammatory, and/or infectious conditions. In our previous work, we demonstrated that endoplasmic reticulum aminopeptidase 1 (ERAP1) was secreted from macrophages in response to LPS and IFN-γ, and it enhanced their phagocytic activity. In this study, we analyzed the mechanism of LPS/IFN-γ-induced ERAP1 secretion. LPS/IFN-γ-induced secretion of the enzyme from the murine macrophage cell line RAW264.7 was suppressed by polymyxin B. Several agonists of TLRs, such as Pam3CSK4, FSL-1, and ODN1826, induced its secretion. In contrast, neutralizing Abs to IFN-β and TNF-α receptor type 1 suppressed its secretion. Using murine peritoneal macrophages derived from TNF-α and type 1 IFNR knockout mice, we confirmed the involvement of these two cytokines in ERAP1 secretion. In addition, secretion of ERAP1 from both RAW264.7 cells and murine peritoneal macrophages was induced by A23187 and thapsigargin and inhibited by BAPTA-AM and the calmodulin inhibitor W7. These results suggest that LPS/IFN-γ-induced secretion of ERAP1 is mediated by TLRs via induction of intermediate cytokines such as IFN-β and TNF-α, which in turn lead to enhanced cytosolic Ca(2+) levels and calmodulin activation.

Multiple classes of immune-related proteases associated with the cell death response in pepper plants

Proteases regulate a large number of biological processes in plants, such as metabolism, physiology, growth, and defense. In this study, we carried out virus-induced gene silencing assays with pepper cDNA clones to elucidate the biological roles of protease superfamilies. A total of 153 representative protease genes from pepper cDNA were selected and cloned into a Tobacco rattle virus-ligation independent cloning vector in a loss-of-function study. Silencing of 61 proteases resulted in altered phenotypes, such as the inhibition of shoot growth, abnormal leaf shape, leaf color change, and lethality. Furthermore, the silencing experiments revealed that multiple proteases play a role in cell death and immune response against avirulent and virulent pathogens. Among these 153 proteases, 34 modulated the hypersensitive cell death response caused by infection with an avirulent pathogen, and 16 proteases affected disease symptom development caused by a virulent pathogen. Specifically, we provide experimental evidence for the roles of multiple protease genes in plant development and immune defense following pathogen infection. With these results, we created a broad sketch of each protease function. This information will provide basic information for further understanding the roles of the protease superfamily in plant growth, development, and defense.

Role of metalloproteases in vaccinia virus epitope processing for transporter associated with antigen processing (TAP)-independent human leukocyte antigen (HLA)-B7 class I antigen presentation

The transporter associated with antigen processing (TAP) translocates the viral proteolytic peptides generated by the proteasome and other proteases in the cytosol to the endoplasmic reticulum lumen. There, they complex with nascent human leukocyte antigen (HLA) class I molecules, which are subsequently recognized by the CD8(+) lymphocyte cellular response. However, individuals with nonfunctional TAP complexes or tumor or infected cells with blocked TAP molecules are able to present HLA class I ligands generated by TAP-independent processing pathways. Herein, using a TAP-independent polyclonal vaccinia virus-polyspecific CD8(+) T cell line, two conserved vaccinia-derived TAP-independent HLA-B*0702 epitopes were identified. The presentation of these epitopes in normal cells occurs via complex antigen-processing pathways involving the proteasome and/or different subsets of metalloproteinases (amino-, carboxy-, and endoproteases), which were blocked in infected cells with specific chemical inhibitors. These data support the hypothesis that the abundant cellular proteolytic systems contribute to the supply of peptides recognized by the antiviral cellular immune response, thereby facilitating immunosurveillance. These data may explain why TAP-deficient individuals live normal life spans without any increased susceptibility to viral infections.

Structural insights into the molecular ruler mechanism of the endoplasmic reticulum aminopeptidase ERAP1

Endoplasmic reticulum aminopeptidase 1 (ERAP1) is an essential component of the immune system, because it trims peptide precursors and generates the N--restricted epitopes. To examine ERAP1's unique properties of length- and sequence-dependent processing of antigen precursors, we report a 2.3 Å resolution complex structure of the ERAP1 regulatory domain. Our study reveals a binding conformation of ERAP1 to the carboxyl terminus of a peptide, and thus provides direct evidence for the molecular ruler mechanism.

Aminopeptidase activity profile in cultured human osteoblasts

Aminopeptidases (APs) are enzymes involved in a wide variety of biological processes and present in a variety of different cell populations. The authors studied these enzymes in primary cultured human osteoblasts in order to establish an activity profile and thereby contribute to knowledge of bone tissue. The authors used 13 different substrates (N-terminal amino acids) and a fluorimetric assay to examine AP activity associated with the membranes of cultured osteoblasts. The authors demonstrated activity > 10 pmol/min/10(4) cells when glycine, alanine, leucine, arginine, phenylalanine, methionine, and lysine were used as substrates. The activity was markedly lower (<1.6 pmol/min/10(4) cells) when the other N-terminal amino acids were used. Puromycin and bestatin inhibited AP activity, though not completely, when we used AlaNA or LeuNA as substrates. Further studies are warranted to determine the role of these enzymes in bone tissue physiology.

Secretion of endoplasmic reticulum aminopeptidase 1 is involved in the activation of macrophages induced by lipopolysaccharide and interferon-gamma

Endoplasmic reticulum aminopeptidase 1 (ERAP1) is a multifunctional enzyme with an important role in processing antigenic peptides presented to class I major histocompatibility complex in the endoplasmic reticulum. In this study, we found that endoplasmic reticulum-retained ERAP1 was secreted from macrophages in response to activation by treatment with lipopolysaccharide (LPS) and interferon (IFN)-γ and enhanced their phagocytic activity. Enhancement of the phagocytic activity of murine macrophage RAW264.7 cells induced by LPS/IFN-γ was inhibited by a potent aminopeptidase inhibitor, amastatin. The addition of recombinant wild-type but not inactive mutant ERAP1 to culture medium enhanced phagocytosis. These results suggest that enhancement of phagocytic activity is at least in part mediated by secreted ERAP1 through the generation of active peptides processed by the enzyme. Our data reveal ERAP1-mediated activation of macrophages for the first time and will provide new insights into the role of this enzyme in innate immunity.

Aminopeptidases and Angiogenesis

A number of proteases, including matrix metalloproteinases and plasminogen activators, have been shown to be involved in angiogenesis. In addition, recent reports suggest that aminopeptidases also play roles in angiogenesis. These peptidases regulate the N-terminal modification of proteins and peptides required in processes such as maturation, activation, or degradation, and thereby they are related to a variety of physiological and pathological processes. At least three aminopeptidases are reported to be involved in angiogenesis, namely, type 2 methionine aminopeptidase, aminopeptidase N, and adipocyte-derived leucine aminopeptidase/puromycin-insensitive leucyl-specific aminopeptidase. This review will focus on the possible role of these aminopeptidases in angiogenesis.

Glutamine-181 is crucial in the enzymatic activity and substrate specificity of human endoplasmic-reticulum aminopeptidase-1

ERAP-1 (endoplasmic-reticulum aminopeptidase-1) is a multifunctional enzyme with roles in the regulation of blood pressure, angiogenesis and the presentation of antigens to MHC class I molecules. Whereas the enzyme shows restricted specificity toward synthetic substrates, its substrate specificity toward natural peptides is rather broad. Because of the pathophysiological significance of ERAP-1, it is important to elucidate the molecular basis of its enzymatic action. In the present study we used site-directed mutagenesis to identify residues affecting the substrate specificity of human ERAP-1 and identified Gln(181) as important for enzymatic activity and substrate specificity. Replacement of Gln(181) by aspartic acid resulted in a significant change in substrate specificity, with Q181D ERAP-1 showing a preference for basic amino acids. In addition, Q181D ERAP-1 cleaved natural peptides possessing a basic amino acid at the N-terminal end more efficiently than did the wild-type enzyme, whereas its cleavage of peptides with a non-basic amino acid was significantly reduced. Another mutant enzyme, Q181E, also revealed some preference for peptides with a basic N-terminal amino acid, although it had little hydrolytic activity toward the synthetic peptides tested. Other mutant enzymes, including Q181N and Q181A ERAP-1s, revealed little enzymatic activity toward synthetic or peptide substrates. These results indicate that Gln(181) is critical for the enzymatic activity and substrate specificity of ERAP-1.

The internal sequence of the peptide-substrate determines its N-terminus trimming by ERAP1

Background

Endoplasmic reticulum aminopeptidase 1 (ERAP1) trims N-terminally extended antigenic peptide precursors down to mature antigenic peptides for presentation by major histocompatibility complex (MHC) class I molecules. ERAP1 has unique properties for an aminopeptidase being able to trim peptides in vitro based on their length and the nature of their C-termini.

Methodology/Principal Findings

In an effort to better understand the molecular mechanism that ERAP1 uses to trim peptides, we systematically analyzed the enzyme's substrate preferences using collections of peptide substrates. We discovered strong internal sequence preferences of peptide N-terminus trimming by ERAP1. Preferences were only found for positively charged or hydrophobic residues resulting to trimming rate changes by up to 100 fold for single residue substitutions and more than 40,000 fold for multiple residue substitutions for peptides with identical N-termini. Molecular modelling of ERAP1 revealed a large internal cavity that carries a strong negative electrostatic potential and is large enough to accommodate peptides adjacent to the enzyme's active site. This model can readily account for the strong preference for positively charged side chains.

Conclusions/Significance

To our knowledge no other aminopeptidase has been described to have such strong preferences for internal residues so distal to the N-terminus. Overall, our findings indicate that the internal sequence of the peptide can affect its trimming by ERAP1 as much as the peptide's length and C-terminus. We therefore propose that ERAP1 recognizes the full length of its peptide-substrate and not just the N- and C- termini. It is possible that ERAP1 trimming preferences influence the rate of generation and the composition of antigenic peptides in vivo.

Aminopeptidase O contains a functional nucleolar localization signal and is implicated in vascular biology

We have identified a gene trap integration into Aminopeptidase O, the gene encoding a member of the M1 family of metalloproteases. Using the betagal reporter of the gene trap vector, we have revealed that at least some ApO isoforms are expressed predominantly in embryonic and adult blood vessels leading us to propose that ApO plays a role in vascular cell biology. The protein produced from an engineered Gfp-ApO fusion cDNA localises to the nucleolus in transfected COS7 cells. We confirm that indeed the APO protein contains a functional nucleolar localisation domain by demonstrating that GFP-APO fusion proteins that lack the predicted nucleolar localisation signal are retained in the cytoplasm. We report the existence of multiple alternatively spliced Apo isoforms that differ with respect to the presence of exons encoding important functional domains. Alternative splicing predictably produces protein products with or without the catalytic domain and/or a nucleolar localisation signal and therefore likely represents an important mechanism in regulating the biological activity of APO that has been reported to cleave one of the peptides of the renin angiotensin pathway.

The significance of brain aminopeptidases in the regulation of the actions of angiotensin peptides in the brain

From the outset, the concept of a brain renin-angiotensin system (RAS) has been controversial and this controversy continues to this day. In addition to the unresolved questions as to the means by which, and location(s) where brain Ang II is synthesized, and the uncertainties regarding the functionality of the different subtypes of Ang II receptors in the brain, a new controversy has arisen with respect to the identity of the angiotensin peptide(s) that activate brain AT(1) receptors. While it has been known for some time that Ang III can activate Ang II receptors with equivalent or near-equivalent efficacy to Ang II, it has been proposed that in the brain, only Ang III is active. This proposal, which we have named "The Angiotensin III Hypothesis" states that Ang II must be converted to Ang III in order to activate brain AT(1) receptors. This review examines several aspects of the controversies regarding the brain RAS with a special focus on brain aminopeptidases, studies that either support or refute The Angiotensin III Hypothesis, and the implications of The Angiotensin III Hypothesis for the activity of the brain RAS. It also addresses the need for further research that can test The Angiotensin III Hypothesis and definitively identify the angiotensin peptide(s) that activate brain AT(1) receptor-mediated effects.

Enzymatic pathways of the brain renin-angiotensin system: unsolved problems and continuing challenges

The brain renin-angiotensin system continues to be enigmatic more than 40 years after the brain was first recognized to be a site of action of angiotensin II. This review focuses on the enzymatic pathways for the formation and degradation of the growing number of active angiotensins in the brain. A brief description and nomenclature of the peptidases involved in the processing of angiotensin peptides in the brain is given. Of primary interest is the array of enzymes that degrade radiolabeled angiotensins in receptor binding assays. This poses major challenges to studies of brain angiotensin receptors and it is debatable whether an accurate determination of brain angiotensin receptor binding kinetics has yet been made. The quandary facing the investigator of brain angiotensin receptors is the need to protect the radioligand from metabolic alteration while maintaining the characteristics of the receptors in situ. It is the tenet of this review that we have yet to fully understand the binding characteristics of brain angiotensin receptors and the extent of their distribution in the brain because of our inability to fully protect the angiotensins from metabolic alteration until equilibrium binding conditions can be attained.

Endoplasmic reticulum aminopeptidase 1 (ERAP1) trims MHC class I-presented peptides in vivo and plays an important role in immunodominance

CD8(+) T cells respond to short peptides bound to MHC class I molecules. Although most antigenic proteins contain many sequences that could bind to MHC class I, few of these peptides actually stimulate CD8(+) T cell responses. Moreover, the T cell responses that are generated often follow a very reproducible hierarchy to different peptides for reasons that are poorly understood. We find that the loss of a single enzyme, endoplasmic reticulum aminopeptidase 1 (ERAP1), in the antigen-processing pathway results in a marked shift in the hierarchy of immunodominance in viral infections, even when the responding T cells have the same T cell receptor repertoire. In mice, ERAP1 is the major enzyme that trims precursor peptides in the endoplasmic reticulum and, in this process, can generate or destroy antigenic peptides. Consequently, when ERAP1 is lost, the immune response to some viral peptides is reduced, to others increased, and to yet others unchanged. Therefore, many epitopes must be initially generated as precursors that are normally trimmed by ERAP1 before binding to MHC class I, whereas others are normally degraded by ERAP1 to lengths that are too short to bind to MHC class I. Moreover, peptide trimming and the resulting abundance of peptide-MHC complexes are dominant factors in establishing immunodominance.

Expression of endoplasmic reticulum aminopeptidases in EBV-B cell lines from healthy donors and in leukemia/lymphoma, carcinoma, and melanoma cell lines

Peptide trimming in the endoplasmic reticulum (ER), the final step required for the generation of most HLA class I-binding peptides, implicates the concerted action of two aminopeptidases, ERAP1 and ERAP2. Because defects in the expression of these peptidases could lead to aberrant surface HLA class I expression in tumor cells, we quantitatively assayed 14 EBV-B cell lines and 35 human tumor cell lines of various lineages for: 1) expression and enzymatic activities of ERAP1 and ERAP2; 2) ER peptide-trimming activity in microsomes; 3) expression of HLA class I H chains and TAP1; and 4) surface HLA class I expression. ERAP1 and ERAP2 expression was detectable in all of the EBV-B and tumor cell lines, but in the latter it was extremely variable, sometimes barely detectable, and not coordinated. The expression of the two aminopeptidases corresponded well to the respective enzymatic activities in most cell lines. A peptide-trimming assay in microsomes revealed additional enzymatic activities, presumably contributed by other unidentified aminopeptidases sharing substrate specificity with ERAP2. Interestingly, surface HLA class I expression showed significant correlation with ERAP1 activity, but not with the activity of either ERAP2 or other unidentified aminopeptidases. Transfection with ERAP1 or ERAP2 of two tumor cell lines selected for simultaneous low expression of the two aminopeptidases resulted in the expected, moderate increases of class I surface expression. Thus, low and/or imbalanced expression of ERAP1 and probably ERAP2 may cause improper Ag processing and favor tumor escape from the immune surveillance.

Reduced activity of the hypertension-associated Lys528Arg mutant of human adipocyte-derived leucine aminopeptidase (A-LAP)/ER-aminopeptidase-1

The adipocyte-derived leucine aminopeptidase (A-LAP)/ER aminopeptidase-1 is a multi-functional enzyme belonging to the M1 family of aminopeptidases. It was reported that the polymorphism Lys528Arg in the human A-LAP gene is associated with essential hypertension. In this study, the role of Lys528 in the enzymatic activity of human A-LAP was examined by site-directed mutagenesis. Among non-synonymous polymorphisms tested, only Lys528Arg reduced enzymatic activity. The replacement of Lys528 with various amino acids including Ala, Met, His and Arg caused a significant decrease in the enzymatic activity. Molecular modeling of the enzyme suggested that Lys528 is located near the entrance of the substrate pocket. These results suggest that Lys528 is important for maximal activity of A-LAP by maintaining the appropriate structure of the substrate pocket of the enzyme. The reduced enzymatic activity of A-LAP may cause high blood pressure and the observed association between the polymorphism and hypertension.

Extracellular TNFR1 release requires the calcium-dependent formation of a nucleobindin 2-ARTS-1 complex

Extracellular tumor necrosis factor (TNF) receptors function as TNF-binding proteins that modulate TNF activity. In human vascular endothelial cells (HUVEC), extracellular TNFR1 (type I TNF receptor, TNFRSF1A) is generated by two mechanisms, proteolytic cleavage of soluble TNFR1 ectodomains and the release of full-length 55-kDa TNFR1 in the membranes of exosome-like vesicles. TNFR1 release from HUVEC is known to involve the association between ARTS-1 (aminopeptidase regulator of TNFR1 shedding), an integral membrane aminopeptidase, and TNFR1. The goal of this study was to identify ARTS-1 binding partners that modulate TNFR1 release to the extracellular space. A yeast two-hybrid screen of a human placenta cDNA library showed that NUCB2 (nucleobindin 2), via its helix-loop-helix domains, binds the ARTS-1 extracellular domain. The association between endogenous ARTS-1 and NUCB2 in HUVEC was demonstrated by co-immunoprecipitation experiments, which showed the formation of a calcium-dependent NUCB2.ARTS-1 complex that associated with a subset of total cellular TNFR1. Confocal microscopy experiments demonstrated that this association involved a distinct population of NUCB2-containing intracytoplasmic vesicles. RNA interference was utilized to specifically knock down NUCB2 and ARTS-1 expression, which demonstrated that both are required for the constitutive release of a full-length 55-kDa TNFR1 within exosome-like vesicles as well as the inducible proteolytic cleavage of soluble TNFR1 ectodomains. We propose that calcium-dependent NUCB2.ARTS-1 complexes, which associate with TNFR1 prior to its commitment to pathways that result in either the constitutive release of TNFR1 exosome-like vesicles or the inducible proteolytic cleavage of TNFR1 ectodomains, play an important role in mediating TNFR1 release to the extracellular compartment.

The aminopeptidase ERAAP shapes the peptide repertoire displayed by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I molecules present thousands of peptides to allow CD8(+) T cells to detect abnormal intracellular proteins. The antigen-processing pathway for generating peptides begins in the cytoplasm, and the MHC molecules are loaded in the endoplasmic reticulum. However, the nature of peptide pool in the endoplasmic reticulum and the proteolytic events that occur in this compartment are unclear. We addressed these issues by generating mice lacking the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP). We found that loss of ERAAP disrupted the generation of naturally processed peptides in the endoplasmic reticulum, decreased the stability of peptide-MHC class I complexes and diminished CD8(+) T cell responses. Thus, trimming of antigenic peptides by ERAAP in the endoplasmic reticulum is essential for the generation of the normal repertoire of processed peptides.

All the peptides that fit: the beginning, the middle, and the end of the MHC class I antigen-processing pathway

The end result of the antigen-processing pathway is the display of peptide-bound major histocompatibility complex I (pMHC I) molecules. The pMHC I molecules are expressed on the cell surface where they can be surveyed by CD8(+) T cells for abnormal proteins. MHC I molecules present a large repertoire of peptides that fit perfectly in their binding grooves and represent the otherwise hidden intracellular contents. Many peptides originate as defective ribosomal products in the cytoplasm. In a stepwise manner, the antigen-processing pathway generates and protects the proteolytic intermediates until they yield the final peptides that can fit the MHC I in the endoplasmic reticulum.

Complexity, contradictions, and conundrums: studying post-proteasomal proteolysis in HLA class I antigen presentation

The vast majority of the peptides produced during protein degradation by the cytosolic proteasome-ubiquitin system are consecutively hydrolyzed to single amino acids by multiple cytosolic peptidases preferring intermediate length or short substrates. The small fraction of peptides surviving the aggressive cytosolic environment can be recruited for presentation by major histocompatibility complex (MHC) class I molecules. However, such peptides may frequently have to be adapted to the strict MHC class I-binding requirements by one or several N-terminal-trimming steps. A recent model proposes that an initial step, in which peptides of 15 or more residues are shortened by cytosolic tripeptidylpeptidase II, is followed by additional trimming by cytosolic or endoplasmic reticulum (ER) aminopeptidases. In humans, at least two ER resident aminopeptidases, ERAP1 and ERAP2, contribute to trimming of human leukocyte antigen class I ligands. These interferon-gamma-regulated metallopeptidases show distinct substrate preferences and may have to act in a concerted fashion to remove some complex or longer N-terminal extensions and to trim the full spectrum of precursor peptides. This task is likely facilitated by the formation of presumably heterodimeric ERAP1-2 complexes. RNA interference experiments suggest that both enzymes are important for normal antigen presentation, but precise determination of the extent and the cellular context of their requirement will be left to future experimentation.

The oxytocinase subfamily of M1 aminopeptidases

The placental leucine aminopeptidase (P-LAP), adipocyte-derived leucine aminopeptidase (A-LAP) and leukocyte-derived aminopeptidase (L-RAP) belong to one distinct group of the M1 family of amimopeptidases, which we term the "Oxytocinase subfamily". They share HEXXH(X)18E Zn-binding and GAMEN motifs essential for the enzymatic activities. Intracellular localization is the characteristic feature of the subfamily members. While P-LAP is translocated from intracellular vesicles to plasma membrane in a stimulus-dependent manner, both A-LAP and L-RAP are retained in the endoplasmic reticulum. They contain sequences necessary for the specific localization in the cell. It is getting evident that the subfamily members play important roles in the maintenance of homeostasis including maintenance of normal pregnancy, memory retention, blood pressure regulation and antigen presentation. In this review, current situation of this newly identified subfamily is summarized.

Concerted peptide trimming by human ERAP1 and ERAP2 aminopeptidase complexes in the endoplasmic reticulum

The generation of many HLA class I peptides entails a final trimming step in the endoplasmic reticulum that, in humans, is accomplished by two 'candidate' aminopeptidases. We show here that one of these, ERAP1, was unable to remove several N-terminal amino acids that were trimmed efficiently by the second enzyme, ERAP2. This trimming of a longer peptide required the concerted action of both ERAP1 and ERAP2, both for in vitro digestion and in vivo for cellular antigen presentation. ERAP1 and ERAP2 localized together in vivo and associated physically in complexes that were most likely heterodimeric. Thus, the human endoplasmic reticulum is equipped with a pair of trimming aminopeptidases that have complementary functions in HLA class I peptide presentation.

N-hydroxy-2-(naphthalene-2-ylsulfanyl)-acetamide, a novel hydroxamic acid-based inhibitor of aminopeptidase N and its anti-angiogenic activity

In the course of our screening, N-hydroxy-2-(naphthalene-2-ylsulfanyl)-acetamide (1), which contains a metal-chelating hydroxamate group, has been identified as a potent inhibitor of aminopeptidase N (APN, EC 3.4.11.2). Compound 1 potently inhibited APN activity with a K(i) value of 3.5 microM. It also inhibited the basic fibroblast growth-factor-induced invasion of bovine aortic endothelial cells at low micromolar concentrations.

Regulation of the human leukocyte-derived arginine aminopeptidase endoplasmic reticulum-aminopeptidase 2 gene by interferon-?

The leukocyte-derived arginine aminopeptidase (L-RAP) is the second aminopeptidase localized in the endoplasmic reticulum (ER) processing antigenic peptides presented to major histocompatibility complex (MHC) class I molecules. In this study, the genomic organization of the gene encoding human L-RAP was determined and the regulatory mechanism of its expression was elucidated. The entire genomic structure of the L-RAP gene is similar to both placental leucine aminopeptidase (P-LAP) and adipocyte-derived leucine aminopeptidase (A-LAP) genes, confirming the close relationship of these three enzymes. Interferon (IFN)-gamma up-regulates the expression of the L-RAP gene. Deletion and site-directed mutagenic analyses of the 5'-flanking region of the L-RAP gene and electrophoretic mobility shift assay indicated that while interferon regulatory factor (IRF)-2 is important in the basal condition, IRF-1 is the primary regulator of IFN-gamma-mediated augmentation of the gene expression. In addition, PU.1, a member of the E26 transformation-specific family of transcription factors, also plays a role in the regulation of gene expression. The maximum expression of the gene was achieved by coexpression of IRF-1 and PU.1 in HEK293 cells and IRF-2 suppressed the IRF-1-mediated enhancement of gene expression, suggesting that IFN-gamma-induced L-RAP gene expression is cooperatively regulated by IRFs and PU.1 transcription factors.

Distribution of adipocyte-derived leucine aminopeptidase (A-LAP)/ER-aminopeptidase (ERAP)-1 in human uterine endometrium

Adipocyte-derived leucine aminopeptidase (A-LAP, endoplasmic reticulum aminopeptidase ERAP1) is specialized to produce peptides presented on the class I major histocompatibility complex (MHC) by trimming epitopes to eight or nine residues, in addition to its enzymatic activity to degrade angiotensin II. Previously we identified placental leucine aminopeptidase (P-LAP), another member of the oxytocinase subfamily of aminopeptidases, in human uterine endometrial epithelial cells. Here we analyzed the distribution of A-LAP in human cyclic endometrium. Western blotting analysis showed that A-LAP was present in the endometrial tissue throughout the menstrual cycle. Immunohistochemical (IHC) analysis of A-LAP showed a similar distribution to that of P-LAP. A-LAP was localized predominantly in the endometrial glands and the luminal surface epithelium. However, the intracellular localization change that accompanied apocrine secretion, which was observed in P-LAP, was not shown in A-LAP. Subcellular localization of A-LAP, demonstrated by immunofluorescence, was ER in the cultured glandular epithelial cells. Our results indicate that A-LAP may fit the endometrial localization as an antigen-presenting ER aminopeptidase. Further understanding of the function(s) of A-LAP in the endometrium appear likely to yield insights into the cyclic changes during the normal endometrial cycle.

Post-proteasomal antigen processing for major histocompatibility complex class I presentation

Peptides presented by major histocompatibility complex class I molecules are derived mainly from cytosolic oligopeptides generated by proteasomes during the degradation of intracellular proteins. Proteasomal cleavages generate the final C terminus of these epitopes. Although proteasomes may produce mature epitopes that are eight to ten residues in length, they more often generate N-extended precursors that are too long to bind to major histocompatibility complex class I molecules. Such precursors are trimmed in the cytosol or in the endoplasmic reticulum by aminopeptidases that generate the N terminus of the presented epitope. Peptidases can also destroy epitopes by trimming peptides to below the size needed for presentation. In the cytosol, endopeptidases, especially thimet oligopeptidase, and aminopeptidases degrade many proteasomal products, thereby limiting the supply of many antigenic peptides. Thus, the extent of antigen presentation depends on the balance between several proteolytic processes that may generate or destroy epitopes.

Transcriptional regulation of angiogenesis-related puromycin-insensitive leucyl-specific aminopeptidase in endothelial cells

Puromycin-insensitive leucyl-specific aminopeptidase (PILSAP) was expressed in endothelial cells (ECs) and played an important role in angiogenesis. Here, we characterized its transcriptional regulation. Mouse PILSAP gene contained 19 exons and located in the chromosome 13C1-C2. We identified two transcripts; one transcribed from exon 1 and the other from exon 2. Mouse ECs expressed dominantly the one from exon 1. The promoter analysis using 5' upstream region of exon 1 revealed that -1868 to -1812 was critical for its transcription in mouse ECs. We identified a motif of the transcription factor PEBP2 in this region, and the deletion or mutation of this motif decreased promoter activity. Protein extracted from mouse ECs bound specifically to this motif. AML1/Runx1/PEBP2alphaB increased PILSAP mRNA in mouse ECs, whereas dominant interfering chimerical PEBP2beta-MYH11 decreased it. These results indicate that the expression of PILSAP in mouse ECs is regulated, at least in part, by PEBP2.

Potential roles of protein oxidation and the immunoproteasome in MHC class I antigen presentation: the 'PrOxI' hypothesis

The major histocompatibility complex (MHC) class I (MHC-I) antigen presentation system is responsible for the cell-surface presentation of self-proteins and intracellular viral proteins. This pathway is important in screening between self, and non-self or infected cells. In this pathway, proteins are partially degraded to peptides in the cytosol and targeted to the cell surface bound to an MHC-I receptor protein. At the cell surface, T cells bypass cells displaying self-peptides but destroy others displaying foreign antigens. Cells contain several isoforms of the proteasome, but it is thought that the immunoproteasome is the major form involved in generating peptides for the MHC-I pathway. How all intracellular proteins are targeted for MHC-I processing is unclear. Oxidative stress is experienced by all cells, and all proteins are exposed to oxidation. We propose that oxidative modification makes proteins susceptible to degradation by the immunoproteasome. This could be called the protein oxidation and immunoproteasome or 'PrOxI' hypothesis of MHC-I antigen processing. Protein oxidation may, thus, be a universal mechanism for peptide generation and presentation in the MHC-I pathway.

Shedding of the type II IL-1 decoy receptor requires a multifunctional aminopeptidase, aminopeptidase regulator of TNF receptor type 1 shedding

Proteolytic cleavage of the extracellular domain of the type II IL-1 decoy receptor (IL-1RII) generates soluble IL-1-binding proteins that prevent excessive bioactivity by binding free IL-1. In this study we report that an aminopeptidase, aminopeptidase regulator of TNFR1 shedding (ARTS-1), is required for IL-1RII shedding. Coimmunoprecipitation experiments demonstrate an association between endogenous membrane-associated ARTS-1 and a 47-kDa IL-1RII, consistent with ectodomain cleavage of the membrane-bound receptor. A direct correlation exists between ARTS-1 protein expression and IL-1RII shedding, as cell lines overexpressing ARTS-1 have increased IL-1RII shedding and decreased membrane-associated IL-1RII. Basal IL-1RII shedding is absent from ARTS-1 knockout cell lines, demonstrating that ARTS-1 is required for constitutive IL-1RII shedding. Similarly, PMA-mediated IL-1RII shedding is almost entirely ARTS-1-dependent. ARTS-1 expression also enhances ionomycin-induced IL-1RII shedding. ARTS-1 did not alter levels of membrane-associated IL-1RI or IL-1R antagonist release from ARTS-1 cell lines, which suggests that the ability of ARTS-1 to promote shedding of IL-1R family members may be specific for IL-1RII. Further, increased IL-1RII shedding by ARTS-1-overexpressing cells attenuates the biological activity of IL-1beta. We conclude that the ability of ARTS-1 to enhance IL-1RII shedding represents a new mechanism by which IL-1-induced cellular events can be modulated. As ARTS-1 also promotes the shedding of the structurally unrelated 55-kDa, type I TNF receptor and the IL-6R, we propose that ARTS-1 may play an important role in regulating innate immune and inflammatory responses by increasing cytokine receptor shedding.

Role of Aminopeptidase in Angiogenesis

Microvessels are composed of endothelial cells and surrounding pericytes. Angiogenesis, a neo-vessel formation from pre-existing microvessels, is a complex phenomenon, which requires following sequential steps: detachment of pre-existing pericytes for vascular destabilization, extracellular matrix turnover, migration, proliferation, tube formation by endothelial cells (ECs), and reattachment of pericytes for vascular stabilization. Aminopeptidases regulate the N-terminal modification of proteins and peptides for maturation, activation or degradation, and thereby relate to a variety of biological processes. Recently, three aminopeptidases have been reported to be involved in angiogenesis. They include type 2 methionine aminopeptidase, aminopeptidase N, and adipocyte-derived leucine aminopeptidase/puromycin insensitive leucyl-specific aminopeptidase. This review will focus on the possible role of these aminopeptidases in angiogenesis.

Processing of Antigenic Peptides by Aminopeptidases

Antigenic peptides presented to major histocompatibility complex (MHC) class I molecules are generated in the cytosol during degradation of cellular proteins by the ubiquitin-proteasome proteolytic pathway. Proteasome can generate N-extended precursors as well as final epitopes, and then the precursors are processed to mature epitopes by aminopeptidases. Both cytosolic peptidases (i.e. puromycin-sensitive aminopeptidase, bleomycin hydrolase and interferon-gamma-inducible leucine aminopeptidase) and recently identified metallo-aminopeptidase located in the endoplasmic reticulum (i.e. adipocyte-derived leucine aminopeptidase/endoplasmic reticulum aminopeptidase 1 and leukocyte-derived arginine aminopeptidase) can generate final epitopes from precursor peptides. Some of these aminopeptidases are also considered to destroy certain antigenic peptides to limit the antigen presentation. Taken together, it is getting evident that aminopeptidases located in the cytosol and the lumen of endoplasmic reticulum play important roles in the generation of antigenic peptides presented to MHC class I molecules.

Human leukocyte-derived arginine aminopeptidase. The third member of the oxytocinase subfamily of aminopeptidases

In this study we report the cloning and characterization of a novel human aminopeptidase, which we designate leukocyte-derived arginine aminopeptidase (L-RAP). The sequence encodes a 960-amino acid protein with significant homology to placental leucine aminopeptidase and adipocyte-derived leucine aminopeptidase. The predicted L-RAP contains the HEXXH(X)18E zinc-binding motif, which is characteristic of the M1 family of zinc metallopeptidases. Phylogenetic analysis indicates that L-RAP forms a distinct subfamily with placental leucine aminopeptidase and adipocyte-derived leucine aminopeptidase in the M1 family. Immunocytochemical analysis indicates that L-RAP is located in the lumenal side of the endoplasmic reticulum. Among various synthetic substrates tested, L-RAP revealed a preference for arginine, establishing that the enzyme is a novel arginine aminopeptidase with restricted substrate specificity. In addition to natural hormones such as angiotensin III and kallidin, L-RAP cleaved various N-terminal extended precursors to major histocompatibility complex class I-presented antigenic peptides. Like other proteins involved in antigen presentation, L-RAP is induced by interferon-gamma. These results indicate that L-RAP is a novel aminopeptidase that can trim the N-terminal extended precursors to antigenic peptides in the endoplasmic reticulum.

An aminopeptidase, ARTS-1, is required for interleukin-6 receptor shedding

Aminopeptidase regulator of TNFR1 shedding (ARTS-1) binds to the type I tumor necrosis factor receptor (TNFR1) and promotes receptor shedding. Because hydroxamic acid-based metalloprotease inhibitors prevent shedding of both TNFR1 and the interleukin-6 receptor (IL-6Ralpha), we hypothesized that ARTS-1 might also regulate shedding of IL-6Ralpha, a member of the type I cytokine receptor superfamily that is structurally different from TNFR1. Reciprocal co-immunoprecipitation experiments identified that membrane-associated ARTS-1 directly binds to a 55-kDa IL-6Ralpha, a size consistent with soluble IL-6Ralpha generated by ectodomain cleavage of the membrane-bound receptor. Furthermore, ARTS-1 promoted IL-6Ralpha shedding, as demonstrated by a direct correlation between increased membrane-associated ARTS-1 protein, increased IL-6Ralpha shedding, and decreased membrane-associated IL-6Ralpha in cell lines overexpressing ARTS-1. The absence of basal IL-6Ralpha shedding from arts-1 knock-out cells identified that ARTS-1 was required for constitutive IL-6Ralpha shedding. Furthermore, the mechanism of constitutive IL-6Ralpha shedding requires ARTS-1 catalytic activity. Thus, ARTS-1 promotes the shedding of two cytokine receptor superfamilies, the type I cytokine receptor superfamily (IL-6Ralpha) and the TNF receptor superfamily (TNFR1). We propose that ARTS-1 is a multifunctional aminopeptidase that may modulate inflammatory events by promoting IL-6Ralpha and TNFR1 shedding.

An IFN-gamma-induced aminopeptidase in the ER, ERAP1, trims precursors to MHC class I-presented peptides

Precursors to major histocompatibility complex (MHC) class I-presented peptides with extra NH2-terminal residues can be efficiently trimmed to mature epitopes in the endoplasmic reticulum (ER). Here, we purified from liver microsomes a lumenal, soluble aminopeptidase that removes NH2-terminal residues from many antigenic precursors. It was identified as a metallopeptidase named "adipocyte-derived leucine" or "puromycin-insensitive leucine-specific" aminopeptidase. However, because we localized it to the ER, we propose it be renamed ER-aminopeptidase 1 (ERAP1). ERAP1 is inhibited by agents that block precursor trimming in ER vesicles and although it trimmed NH2-extended precursors, it spared presented peptides of 8 amino acid and less. Like other proteins involved in antigen presentation, ERAP1 is induced by interferon-gamma. When overexpressed in vivo, we found that ERAP1 stimulates the processing and presentation of an antigenic precursor in the ER.

The ER aminopeptidase ERAP1 enhances or limits antigen presentation by trimming epitopes to 8-9 residues

Endoplasmic reticulum (ER) aminopeptidase 1 (ERAP1) appears to be specialized to produce peptides presented on class I major histocompatibility complex molecules. We found that purified ERAP1 trimmed peptides that were ten residues or longer, but spared eight-residue peptides. In vivo, ERAP1 enhanced production of an eight-residue ovalbumin epitope from precursors extended on the NH2 terminus that were generated either in the ER or cytosol. Purified ERAP1 also trimmed nearly half the nine-residue peptides tested. By destroying such nine-residue peptides in normal human cells, ERAP1 reduced the overall supply of antigenic peptides. However, after interferon-gamma treatment, which causes proteasomes to produce more NH2-extended antigenic precursors, ERAP1 increased the supply of peptides for MHC class I antigen presentation.

Puromycin insensitive leucyl-specific aminopeptidase (PILSAP) is involved in the activation of endothelial integrins

We previously reported that mouse orthologue of puromycin insensitive leucyl-specific aminopeptidase (mPILSAP) played an important role in angiogenesis by regulating the proliferation and migration of endothelial cells (ECs) (Miyashita et al., 2002. Blood 99:3241-3249). Here, we examined the mechanism as to how mPILSAP regulates the migration of ECs. Cell adhesion through integrins plays a crucial role in cell migration, and ECs use at least type-1 collagen receptor integrin alpha2beta1, fibronectin receptor alpha5beta1, and vitronectin receptors alphavbeta3 and alphavbeta5. mPILSAP antisense oligodeoxynucleotide (AS-ODN) or leucinethiol (LT), a leucyl-aminopeptidase inhibitor, did not affect the attachment but did significantly inhibit the spreading of cells of the murine endothelial cell line MSS31 when they were plated on vitronectin-, fibronectin-, or type-1 collagen, although they did not affect the expression of integrin alpha2, alpha5, alphav, beta1, beta3, and beta5 subunits on the cell surface. AS-ODN and LT also inhibited the tyrosine phosphorylation of FAK when cells were plated on vitronectin, fibronectin, or type-1 collagen. This inhibition of cell spreading and of tyrosine phosphorylation of FAK could be negated by Mg(2+). These results suggest that mPILSAP is involved in the activation of endothelial integrins.

ERAAP customizes peptides for MHC class I molecules in the endoplasmic reticulum

The ability of killer T cells carrying the CD8 antigen to detect tumours or intracellular pathogens requires an extensive display of antigenic peptides by major histocompatibility complex (MHC) class I molecules on the surface of potential target cells. These peptides are derived from almost all intracellular proteins and reveal the presence of foreign pathogens and mutations. How cells produce thousands of distinct peptides cleaved to the precise lengths required for binding different MHC class I molecules remains unknown. The peptides are cleaved from endogenously synthesized proteins by the proteasome in the cytoplasm and then trimmed by an unknown aminopeptidase in the endoplasmic reticulum (ER). Here we identify ERAAP, the aminopeptidase associated with antigen processing in the ER. ERAAP has a broad substrate specificity, and its expression is strongly upregulated by interferon-gamma. Reducing the expression of ERAAP through RNA interference prevents the trimming of peptides for MHC class I molecules in the ER and greatly reduces the expression of MHC class I molecules on the cell surface. Thus, ERAAP is the missing link between the products of cytosolic processing and the final peptides presented by MHC class I molecules on the cell surface.

Characterisation of aminopeptidase activity in scab mites (Psoroptes spp.)

Soluble and membrane-bound aminopeptidase activities were demonstrated in extracts of P. cuniculi (Delafond). Leucine aminopeptidase (LAP) activity in the soluble fraction of P. cuniculi extracts displayed substrate preference for amino acid derivatives with terminal leucine and methionine over those with acidic, basic or heterocyclic groups. P. cuniculi LAP was inhibited by leucinethiol (IC(50) = 1.4 +/- 0.4 nM), bestatin (IC(50) = 3.9 +/- 1.7 microM), Arphamenine A (IC(50) = 0.37 +/- 0.03 mM) the chelating agent 1,10-phenanthroline (IC(50) = 2.3 +/- 0.5 mM), Zn(2+), Cu(2+) Ni(2+), and Co(2+), and activated by Mn(2+) and Mg(2+). The LAP activity was visualised as a single major band after electrophoresis on native gels and eluted from a size exclusion column as a single major peak representing a molecular mass range of 85-116 kDa. Degenerate oligonucleotide primers were used to amplify short fragments of genomic DNA containing nucleotide sequence coding for the cation-binding motifs of the co-catalytic Zn(2+) binding domains of dizinc leucine aminopeptidases in both P. cuniculi and P.ovis (Hering). The major soluble aminopeptidase from these mites therefore displays most of the characteristics associated with typical cytosolic leucine aminopeptidases belonging to the M17 family of metalloproteinases.

Identification of ARTS-1 as a novel TNFR1-binding protein that promotes TNFR1 ectodomain shedding

Proteolytic cleavage of TNF receptor 1 (TNFR1) generates soluble receptors that regulate TNF bioactivity. We hypothesized that the mechanism of TNFR1 shedding might involve interactions with regulatory ectoproteins. Using a yeast two-hybrid approach, we identified ARTS-1 (aminopeptidase regulator of TNFR1 shedding) as a type II integral membrane protein that binds to the TNFR1 extracellular domain. In vivo binding of membrane-associated ARTS-1 to TNFR1 was confirmed by coimmunoprecipitation experiments using human pulmonary epithelial and umbilical vein endothelial cells. A direct relationship exists between membrane-associated ARTS-1 protein levels and concordant changes in TNFR1 shedding. Cells overexpressing ARTS-1 demonstrated increased TNFR1 shedding and decreased membrane-associated TNFR1, while cells expressing antisense ARTS-1 mRNA demonstrated decreased membrane-associated ARTS-1, decreased TNFR1 shedding, and increased membrane-associated TNFR1. ARTS-1 neither bound to TNFR2 nor altered its shedding, suggesting specificity for TNFR1. Although a recombinant ARTS-1 protein demonstrated selective aminopeptidase activity toward nonpolar amino acids, multiple lines of negative evidence suggest that ARTS-1 does not possess TNFR1 sheddase activity. These data indicate that ARTS-1 is a multifunctional ectoprotein capable of binding to and promoting TNFR1 shedding. We propose that formation of a TNFR1-ARTS-1 molecular complex represents a novel mechanism by which TNFR1 shedding is regulated.