A giant protease with a twist: the TPP II complex from Drosophila studied by electron microscopy

Early-onset Evans syndrome, immunodeficiency, and premature immunosenescence associated with tripeptidyl-peptidase II deficiency

Autoimmune cytopenia is a frequent manifestation of primary immunodeficiencies. Two siblings presented with Evans syndrome, viral infections, and progressive leukopenia. DNA available from one patient showed a homozygous frameshift mutation in tripeptidyl peptidase II (TPP2) abolishing protein expression. TPP2 is a serine exopeptidase involved in extralysosomal peptide degradation. Its deficiency in mice activates cell death programs and premature senescence. Similar to cells from naïve, uninfected TPP2-deficient mice, patient cells showed increased major histocompatibility complex I expression and most CD8(+) T-cells had a senescent CCR7-CD127(-)CD28(-)CD57(+) phenotype with poor proliferative responses and enhanced staurosporine-induced apoptosis. T-cells showed increased expression of the effector molecules perforin and interferon-γ with high expression of the transcription factor T-bet. Age-associated B-cells with a CD21(-) CD11c(+) phenotype expressing T-bet were increased in humans and mice, combined with antinuclear antibodies. Moreover, markers of senescence were also present in human and murine TPP2-deficient fibroblasts. Telomere lengths were normal in patient fibroblasts and granulocytes, and low normal in lymphocytes, which were compatible with activation of stress-induced rather than replicative senescence programs. TPP2 deficiency is the first primary immunodeficiency linking premature immunosenescence to severe autoimmunity. Determination of senescent lymphocytes should be part of the diagnostic evaluation of children with refractory multilineage cytopenias.

The structure of human tripeptidyl peptidase II as determined by a hybrid approach

Tripeptidyl-peptidase II (TPPII) is a high molecular mass (∼5 MDa) serine protease, which is thought to act downstream of the 26S proteasome, cleaving peptides released by the latter. Here, the structure of human TPPII (HsTPPII) has been determined to subnanometer resolution by cryoelectron microscopy and single-particle analysis. The complex is built from two strands forming a quasihelical structure harboring a complex system of inner cavities. HsTPPII particles exhibit some polymorphism resulting in complexes consisting of nine or of eight dimers per strand. To obtain deeper insights into the architecture and function of HsTPPII, we have created a pseudoatomic structure of the HsTPPII spindle using a comparative model of HsTPPII dimers and molecular dynamics flexible fitting. Analyses of the resulting hybrid structure of the HsTPPII holocomplex provide new insights into the mechanism of maturation and activation.

Molecular ruler of tripeptidylpeptidase II: mechanistic principle of exopeptidase selectivity

The structure of tripeptidylpeptidase II (TPPII) has shown that it belongs to the group of exopeptidases which use a double-Glu motif to convey aminopeptidase activity. TPPII has been implicated in vital biological processes. At least one of these, antigen processing, requires the involvement of its endopeptidase activity. In order to understand the extent and molecular basis of this unusual functional promiscuity we have performed a systematic kinetic analysis of wild type Drosophila melanogaster TPPII and five point mutants of the double-Glu-motif (E312/E343) involving natural substrates. Unlike the known double-Glu motives of other exopeptidases, the double-Glu motif of TPPII is distinctly asymmetrical: E312 is the crucial determinant of the aminotripeptidolytic ruler mechanism. It both blocks the active-site cleft at substrate position P4 and forms a salt bridge with the N-terminus of the substrate. In contrast, E343 forms a much weaker salt bridge than E312 and it does not have a blocking role. An endopeptidase substrate can bind at relatively high affinity if the length of the substrate permits binding to several S' sites. However, the lacking alignment of the substrate by the double-Glu motif causes the endopeptidolytic K(cat)/K(M) of TPPII to be very low.

Structure and function of tripeptidyl peptidase II, a giant cytosolic protease

Tripeptidyl peptidase II is the largest known eukaryotic peptidase. It has been described as a multi-purpose peptidase, which, in addition to its house-keeping function in intracellular protein degradation, plays a role in several vital cellular processes such as antigen processing, apoptosis, or cell division, and is involved in diseases like muscle wasting, obesity, and in cancer. Biochemical studies and bioinformatics have identified TPPII as a subtilase, but its structure is very unusual: it forms a large homooligomeric complex (6 MDa) with a spindle-like shape. Recently, the high-resolution structure of TPPII homodimers (300 kDa) was solved and a hybrid structure of the holocomplex built of 20 dimers was obtained by docking it into the EM-density. Here, we summarize our current knowledge about TPPII with a focus on structural aspects. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

The Enigma of Tripeptidyl-Peptidase II: Dual Roles in Housekeeping and Stress

The tripeptidyl-peptidase II complex consists of repeated 138 kDa subunits, assembled into two twisted strands that form a high molecular weight complex (>5 MDa). TPPII, like many other cytosolic peptidases, plays a role in the ubiquitin-proteasome pathway downstream of the proteasome as well as in the production and destruction of MHC class I antigens and degradation of neuropeptides. Tripeptidyl-peptidase II activity is increased in cells with an increased demand for protein degradation, but whether degradation of cytosolic peptides is the only cell biological role for TPPII has remained unclear. Recent data indicated that TPPII translocates into the nucleus to control DNA damage responses in malignant cells, supporting that cytosolic “housekeeping peptidases” may have additional roles in cell biology, besides their contribution to protein turnover. Overall, TPPII has an emerging importance in several cancer-related fields, such as metabolism, cell death control, and control of genome integrity; roles that are not understood in detail. The present paper reviews the cell biology of TPPII and discusses distinct roles for TPPII in the nucleus and cytosol.

Role of tripeptidyl peptidase II in MHC class I antigen processing - the end of controversies?

Peptide ligands presented by MHC class I molecules are generated in a cascade of proteolytic events starting with the proteasome in the cytosol and frequently terminating with trimming aminopeptidases in the endoplasmic reticulum. Several cytosolic proteases can carry out intermediate proteolytic steps between these start and endpoints. Among these, tripeptidyl peptidase II (TPP II), an exceptionally large homo-oligomeric protease, has been proposed to be involved in the generation of many or most MHC class I ligands by cleaving long precursor peptides. In this issue of the European Journal of Immunology, the effect of pharmacological or genetic TPP II inhibition on peptide loading of HLA-B27 and other HLA class I molecules is examined, and no evidence for a role of TPP II in this process is detected. Although further studies using more efficient inhibitors and focusing on HLA class I alleles such as HLA-A3 are warranted, these results, together with other recently published data, suggest that the role of TPP II in MHC class I processing may be much more limited than previously appreciated.

Activation of cellular death programs associated with immunosenescence-like phenotype in TPPII knockout mice

The giant cytosolic protease tripeptidyl peptidase II (TPPII) has been implicated in the regulation of proliferation and survival of malignant cells, particularly lymphoma cells. To address its functions in normal cellular and systemic physiology we have generated TPPII-deficient mice. TPPII deficiency activates cell type-specific death programs, including proliferative apoptosis in several T lineage subsets and premature cellular senescence in fibroblasts and CD8(+) T cells. This coincides with up-regulation of p53 and dysregulation of NF-kappaB. Prominent degenerative alterations at the organismic level were a decreased lifespan and symptoms characteristic of immunohematopoietic senescence. These symptoms include accelerated thymic involution, lymphopenia, impaired proliferative T cell responses, extramedullary hematopoiesis, and inflammation. Thus, TPPII is important for maintaining normal cellular and systemic physiology, which may be relevant for potential therapeutic applications of TPPII inhibitors.

Analysis of direct and cross-presentation of antigens in TPPII knockout mice

Tripeptidyl peptidase II (TPPII) is an oligopeptidase forming giant complexes in the cytosol that have high exo-, but also, endoproteolytic activity. Immunohistochemically, the complexes appear as distinct foci in the cytosol. In part controversial biochemical and functional studies have suggested that TPPII contributes, on the one hand, positively to Ag processing by generating epitope carboxyl termini or by trimming epitope precursors, and, on the other, negatively by destroying potentially antigenic peptides. To clarify which of these roles is predominant, we generated and analyzed TPPII-deficient mice. Cell surface levels of MHC class I peptide complexes tended to be increased on most cell types of these mice. Although presentation of three individual epitopes derived from lymphocytic choriomeningitis virus was not elevated on TPPII-/- cells, that of the immunodominant OVA epitope SIINFEKL was significantly enhanced. Consistent with this, degradation of a synthetic peptide corresponding to the OVA epitope and of another corresponding to a precursor thereof, both being proteasomally generated OVA fragments, was delayed in TPPII-deficient cytosolic extracts. In addition, dendritic cell cross-presentation of phagocytosed OVA and of OVA internalized as an immune complex was increased to about the same level as direct presentation of the Ag. The data suggest a moderate, predominantly destructive role of TPPII in class I Ag processing, in line with our finding that TPPII is not induced by IFN-gamma, which up-regulates numerous, predominantly constructive components of the Ag processing and presentation machinery.

A tale of two giant proteases

The 26S proteasome and tripeptidyl peptidase II (TPPII) are two exceptionally large eukaryotic protein complexes involved in intracellular proteolysis, where they exert their function sequentially: the proteasome, a multisubunit complex of 2.5 MDa, acts at the downstream end of the ubiquitin pathway and degrades ubiquitinylated proteins into small oligopeptides. Such oligopeptides are substrates for TPPII, a 6-MDa homooligomer, which releases tripeptides from their free N-terminus. Both 26S and TPPII are very fragile complexes refractory to crystallization and in their fully assembled native form have been visualized only by electron microscopy. Here, we will discuss the structural features of the two complexes and their functional implications.

Ultrastructure and morphology of midgut visceral muscle in early pupal Aedes aegypti mosquitoes

These studies focus on the pupal Aedes aegypti midgut muscularis for the first 26 h following larval-pupal transition. The midgut muscularis of Ae. aegypti pupae during this first half of the pupal stadium is a grid of both circularly and longitudinally oriented muscle bands, arranged in a manner resembling that of the larvae. While many muscle bands exhibit signs of degeneration during the time period studied, not all bands degrade, nor is this degradation simultaneous. Band deterioration involves destruction of internal elements while the muscle fiber plasma membrane remains intact. Deterioration of contractile elements may involve proteosome-like structures and associated enzymes. Many features of the larval muscularis including cruciform cells, bifurcating circular bands, and bifurcating longitudinal bands of muscle are retained during the time period investigated. Neuromuscular junctions along some muscle bands are retained through at least 16 h into the pupal stadium. The selective nature of muscle fiber degradation, coupled with the retention of larval features and neural input, may allow for limited functionality of the muscularis during metamorphosis. Evidence of sexual dimorphism in the midgut muscularis of male and female Ae. aegypti pupae was not observed during the time period studied.

Tripeptidyl-peptidase II Controls DNA Damage Responses and In vivo γ-Irradiation Resistance of Tumors

Cellular responses to gamma-irradiation exposure are controlled by phosphatidylinositol 3-kinase-related kinases (PIKK) in the nucleus, and in addition, cytosolic PIKKs may have a role in such responses. Here, we show that the expression of tripeptidyl-peptidase II (TPPII), a high molecular weight cytosolic peptidase, required PIKK signaling and that TPPII was rapidly translocated into the nucleus of gamma-irradiated cells. These events were dependent on mammalian target of rapamycin, a cytosolic/mitochondrial PIKK that is activated by gamma-irradiation. Lymphoma cells with inhibited expression of TPPII failed to efficiently stabilize p53 and had reduced ability to arrest proliferation in response to gamma-irradiation. We observed that TPPII contains a BRCA COOH-terminal-like motif, contained within sequences of several proteins involved in DNA damage signaling pathways, and this motif was important for nuclear translocation of TPPII and stabilization of p53. Novel tripeptide-based inhibitors of TPPII caused complete in vivo tumor regression in mice in response to relatively low doses of gamma-irradiation (3-4 Gy/wk). This was observed with established mouse and human tumors of diverse tissue backgrounds, with no tumor regrowth after cancellation of treatment. These TPPII inhibitors had minor effects on tumor growth as single agent and had low cellular toxicity. Our data indicated that TPPII connects signaling by cytosolic/mitochondrial and nuclear PIKK-dependent pathways and that TPPII can be targeted for inhibition of tumor therapy resistance.

Puromycin-sensitive aminopeptidase is the major peptidase responsible for digesting polyglutamine sequences released by proteasomes during protein degradation

Long stretches of glutamine (Q) residues are found in many cellular proteins. Expansion of these polyglutamine (polyQ) sequences is the underlying cause of several neurodegenerative diseases (e.g. Huntington's disease). Eukaryotic proteasomes have been found to digest polyQ sequences in proteins very slowly, or not at all, and to release such potentially toxic sequences for degradation by other peptidases. To identify these key peptidases, we investigated the degradation in cell extracts of model Q-rich fluorescent substrates and peptides containing 10-30 Q's. Their degradation at neutral pH was due to a single aminopeptidase, the puromycin-sensitive aminopeptidase (PSA, cytosol alanyl aminopeptidase). No other known cytosolic aminopeptidase or endopeptidase was found to digest these polyQ peptides. Although tripeptidyl peptidase II (TPPII) exhibited limited activity, studies with specific inhibitors, pure enzymes and extracts of cells treated with siRNA for TPPII or PSA showed PSA to be the rate-limiting activity against polyQ peptides up to 30 residues long. (PSA digests such Q sequences, shorter ones and typical (non-repeating) peptides at similar rates.) Thus, PSA, which is induced in neurons expressing mutant huntingtin, appears critical in preventing the accumulation of polyQ peptides in normal cells, and its activity may influence susceptibility to polyQ diseases.

Size Matters for the Tripeptidylpeptidase II Complex from Drosophila

Tripeptidylpeptidase II (TPP II) is an exopeptidase of the subtilisin type of serine proteases, a key component of the protein degradation cascade in many eukaryotes, which cleaves tripeptides from the N terminus of proteasome-released products. The Drosophila TPP II is a large homooligomeric complex (approximately 6 MDa) that is organized in a unique repetitive structure with two strands each composed of ten stacked homodimers; two strands intertwine to form a spindle-shaped structure. We report a novel procedure of preparing an active, structurally homogeneous TPP II holo-complex overexpressed in Escherichia coli. Assembly studies revealed that the specific activity of TPP II increases with oligomer size, which in turn is strongly concentration-dependent. At a TPP II concentration such as prevailing in Drosophila, equilibration of size and activity proceeds on a time scale of hours and leads to spindle formation at a TPP II concentration of > or =0.03 mg/ml. Before equilibrium is reached, activation lags behind assembly, suggesting that activation occurs in a two-step process consisting of (i) assembly and (ii) a subsequent conformational change leading to a switch from basal to full activity. We propose a model consistent with the hyperbolic increase of activity with oligomer size. Spindle formation by strand pairing causes both significant thermodynamic and kinetic stabilization. The strands inherently heterogeneous in length are thus locked into a discrete oligomeric state. Our data indicate that the unique spindle form of the holo-complex represents an assembly motif stabilizing a highly active state.

Tripeptidyl Peptidase II Is the Major Peptidase Needed to Trim Long Antigenic Precursors, but Is Not Required for Most MHC Class I Antigen Presentation

Recent reports concluded that tripeptidyl peptidase (TPPII) is essential for MHC class I Ag presentation and that the proteasome in vivo mainly releases peptides 16 residues or longer that require processing by TPPII. However, we find that eliminating TPPII from human cells using small interfering RNA did not decrease the overall supply of peptides to MHC class I molecules and reduced only modestly the presentation of SIINFEKL from OVA, while treatment with proteasome inhibitors reduced these processes dramatically. Purified TPPII digests peptides from 6 to 30 residues long at similar rates, but eliminating TPPII in cells reduced the processing of long antigenic precursors (14-17 residues) more than short ones (9-12 residues). Therefore, TPPII appears to be the major peptidase capable of processing proteasome products longer than 14 residues. However, proteasomes in vivo (like purified proteasomes) release relatively few such peptides, and these peptides processed by TPPII require further trimming in the endoplasmic reticulum (ER) by ER aminopeptidase 1 for presentation. Taken together, these observations demonstrate that TPPII plays a specialized role in Ag processing and one that is not essential for the generation of most presented peptides. Moreover, these findings reveal that three sequential proteolytic steps (by proteasomes, TPPII, and then ER aminopepsidase 1) are required for the generation of a subset of epitopes.

Particle picking by segmentation: a comparative study with SPIDER-based manual particle picking

Boxing hundreds of thousands of particles in low-dose electron micrographs is one of the major bottle-necks in advancing toward achieving atomic resolution reconstructions of biological macromolecules. We have shown that a combination of pre-processing operations and segmentation can be used as an effective, automatic tool for identifying and boxing single-particle images. This paper provides a brief description of how this method has been applied to a large data set of micrographs of ice-embedded ribosomes, including a comparative analysis of the efficiency of the method. Some results on processing micrographs of tripeptidyl peptidase II particles are also shown. In both cases, we have achieved our goal of selecting at least 80% of the particles that an expert would select with less than 10% false positives.

Tripeptidyl-peptidase II: A multi-purpose peptidase

Tripeptidyl-peptidase II is a high-molecular weight peptidase with a widespread distribution in eukaryotic cells. The enzyme sequentially removes tripeptides from a free N-terminus of longer peptides and also displays a low endopeptidase activity. A role for tripeptidyl-peptidase II in the formation of peptides for antigen presentation has recently become evident, and the enzyme also appears to be important for the degradation of some specific substrates, e.g. the neuropeptide cholecystokinin. However, it is likely that the main biological function of tripeptidyl-peptidase II is to participate in a general intracellular protein turnover. This peptidase may act on oligopeptides generated by the proteasome, or other endopeptidases, and the tripeptides formed would subsequently be good substrates for other exopeptidases. The fact that tripeptidyl-peptidase II activity is increased in sepsis-induced muscle wasting, a situation of enhanced protein turnover, corroborates this biological role.

Molecular machines for protein degradation

One of the most precisely regulated processes in living cells is intracellular protein degradation. The main component of the degradation machinery is the 20S proteasome present in both eukaryotes and prokaryotes. In addition, there exist other proteasome-related protein-degradation machineries, like HslVU in eubacteria. Peptides generated by proteasomes and related systems can be used by the cell, for example, for antigen presentation. However, most of the peptides must be degraded to single amino acids, which are further used in cell metabolism and for the synthesis of new proteins. Tricorn protease and its interacting factors are working downstream of the proteasome and process the peptides into amino acids. Here, we summarise the current state of knowledge about protein-degradation systems, focusing in particular on the proteasome, HslVU, Tricorn protease and its interacting factors and DegP. The structural information about these protein complexes opens new possibilities for identifying, characterising and elucidating the mode of action of natural and synthetic inhibitors, which affects their function. Some of these compounds may find therapeutic applications in contemporary medicine.

Molecular architecture and assembly mechanism of Drosophila tripeptidyl peptidase II

In eukaryotes, tripeptidyl peptidase II (TPPII) is a crucial component of the proteolytic cascade acting downstream of the 26S proteasome in the ubiquitin-proteasome pathway. It is an amino peptidase belonging to the subtilase family removing tripeptides from the free N terminus of oligopeptides. The 150-kDa subunits of Drosophila TPPII assemble into a giant proteolytic complex of 6 MDa with a remarkable architecture consisting of two segmented and twisted strands that form a spindle-shaped structure. A refined 3D model has been obtained by cryoelectron microscopy, which reveals details of the molecular architecture and, in conjunction with biochemical data, provides insight into the assembly mechanism. The building blocks of this complex are apparently dimers, within which the 150-kDa monomers are oriented head to head. Stacking of these dimers leads to the formation of twisted single strands, two of which comprise the fully assembled spindle. This spindle also forms when TPPII is heterologously expressed in Escherichia coli, demonstrating that no scaffolding protein is required for complex formation and length determination. Reciprocal interactions of the N-terminal part of subunits from neighboring strands are probably involved in the formation of the native quaternary structure, lending the TPPII spindle a stability higher than that of single strands.

Tripeptidyl peptidase II. An oligomeric protease complex from Arabidopsis

The breakdown of most nuclear and cytoplasmic proteins involves their partial cleavage by the 26S proteasome followed by further disassembly to free amino acids by the combined action of endo- and exopeptidases. In animals, one important intermediate exopeptidase is tripeptidyl peptidase (TPP)II, which digests peptide products of the 26S proteasome and other endopeptidases into tripeptides. Here, we describe the purification and characterization of TPPII from Arabidopsis (Arabidopsis thaliana). Like its animal counterparts, Arabidopsis TPPII exists as a soluble, approximately 5- to 9-MD complex. Two related species of 153 and 142 kD are present in the purified preparations that are derived from a single TPP2 gene. Sequencing by Edman degradation of the intact polypeptides and mass spectrometry of proteolytic fragments demonstrated that the 142-kD form mainly differs from the 153-kD form by a truncation at the C-terminal end. This serine protease is a member of the subtilisin superfamily and is sensitive to the inhibitors alanine-alanine-phenylalanine-chloromethylketone and butabindide, which are diagnostic for the TPPII subfamily. The Arabidopsis TPP2 gene is widely expressed in many tissue types with related genes evident in other plant genomes. Whereas the 26S proteasome is essential, TPPII appears not as important for plant physiology. An Arabidopsis T-DNA mutant defective in TPP2 expression displays no phenotypic abnormalities and is not hypersensitive to either amino acid analogs or the 26S proteasome inhibitor MG132. As a consequence, plants likely contain other intermediate exopeptidases that assist in amino acid recycling.

The cyanide degrading nitrilase from Pseudomonas stutzeri AK61 is a two-fold symmetric, 14-subunit spiral

The quaternary structure of the cyanide dihydratase from Pseudomonas stutzeri AK61 was determined by negative stain electron microscopy and three-dimensional reconstruction using the single particle technique. The structure is a spiral comprising 14 subunits with 2-fold symmetry. Interactions across the groove cause a decrease in the radius of the spiral at the ends and the resulting steric hindrance prevents the addition of further subunits. Similarity to two members of the nitrilase superfamily, the Nit domain of NitFhit and N-carbamyl-D-amino acid amidohydrolase, enabled the construction of a partial atomic model that could be unambiguously fitted to the stain envelope. The model suggests that interactions involving two significant insertions in the sequence relative to these structures leads to the left-handed spiral assembly.

Purification of the Arabidopsis 26 S Proteasome

The 26 S proteasome is a multisubunit protease complex responsible for degrading a wide range of intracellular proteins in eukaryotes, especially those modified with polyubiquitin chains. It is composed of a self-compartmentalized core protease (CP) that houses the peptidase active sites appended on either or both ends by a regulatory particle (RP) that identifies appropriate substrates and translocates them into the lumen of the CP for breakdown. Here, we describe the molecular and biochemical properties of the 26 S proteasome from the plant Arabidopsis thaliana. Like the CP and the ATPase ring of the RP, the RP non-ATPase subunits are often encoded by two transcriptionally active genes with some pairs displaying sufficient sequence divergence to suggest functional differences. Most RPN subunits could functionally replace their yeast counterparts, implying that they have retained their positions and activities within the complex. A method was developed to purify the 26 S proteasome intact from whole Arabidopsis seedlings. These preparations are biochemically indistinguishable from those from yeast and mammals, including the need for ATP to maintain integrity and a strong sensitivity to the inhibitors MG115, MG132, lactacystin, and epoxomicin. Mass spectrometric analysis of the complex detected the presence of almost all CP and RP subunits. In many cases, both products of paralogous genes were detected, demonstrating that each isoform assembles into the mature particle. As with the yeast and animal 26 S proteasomes, attenuation of individual RP genes induces a coordinated up-regulation of many of the other 26 S proteasome genes, suggesting that plants contain a negative feedback mechanism to regulate the 26 S proteasome levels. The incorporation of paralogous subunits into the Arabidopsis holoprotease raises the intriguing possibility that plants synthesize multiple 26 S proteasome types with unique properties and/or target specificities.

ACE2 X-ray structures reveal a large hinge-bending motion important for inhibitor binding and catalysis

The angiotensin-converting enzyme (ACE)-related carboxypeptidase, ACE2, is a type I integral membrane protein of 805 amino acids that contains one HEXXH + E zinc-binding consensus sequence. ACE2 has been implicated in the regulation of heart function and also as a functional receptor for the coronavirus that causes the severe acute respiratory syndrome (SARS). To gain further insights into this enzyme, the first crystal structures of the native and inhibitor-bound forms of the ACE2 extracellular domains were solved to 2.2- and 3.0-Å resolution, respectively. Comparison of these structures revealed a large inhibitor-dependent hinge-bending movement of one catalytic subdomain relative to the other (∼16°) that brings important residues into position for catalysis. The potent inhibitor MLN-4760 ((S,S)-2-{1-carboxy-2-[3-(3,5-dichlorobenzyl)-3H-imidazol4-yl]-ethylamino}-4-methylpentanoic acid) makes key binding interactions within the active site and offers insights regarding the action of residues involved in catalysis and substrate specificity. A few active site residue substitutions in ACE2 relative to ACE appear to eliminate the S₂′ substrate-binding subsite and account for the observed reactivity change from the peptidyl dipeptidase activity of ACE to the carboxypeptidase activity of ACE2.

From words to literature in structural proteomics

Technical advances on several frontiers have expanded the applicability of existing methods in structural biology and helped close the resolution gaps between them. As a result, we are now poised to integrate structural information gathered at multiple levels of the biological hierarchy - from atoms to cells - into a common framework. The goal is a comprehensive description of the multitude of interactions between molecular entities, which in turn is a prerequisite for the discovery of general structural principles that underlie all cellular processes.