Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
Identification, characterization and localization of chagasin, a tight-binding cysteine protease inhibitor in Trypanosoma cruzi.
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Chagasin is a heat-stable, 12 kDa cruzipain-binding protein isolated from T. cruzi epimastigote lysates and characterised kinetically as a tight-binding, reversible inhibitor of papain-like cysteine proteases.
"This protein was subsequently characterized as a tight-binding and reversible inhibitor of papain-like cysteine proteases."
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Immunogold electron microscopy localised chagasin to the flagellar pocket and cytoplasmic vesicles of trypomastigotes and to the cell surface of amastigotes.
"Gold-labeled antibodies localized chagasin to the flagellar pocket and cytoplasmic vesicles of trypomastigotes and to the cell surface of amastigotes."
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Binding assays on living parasites with FITC-labelled probes independently placed both chagasin and cruzipain at the amastigote cell surface.
"Binding assays performed by probing living parasites with fluorescein (FITC)-cruzipain or FITC-chagasin revealed the presence of both inhibitor and protease at the cell surface of amastigotes."
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Chagasin expression is developmentally regulated and inversely correlated with that of cruzipain.
"Immunoblotting indicated that the expression of chagasin is developmentally regulated and inversely correlated with that of cruzipain."
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The authors propose that the intersection of the chagasin and cruzipain trafficking pathways is the regulatory checkpoint, which frames the localisation annotations as mechanism rather than incidental.
"The intersection of chagasin and cruzipain trafficking pathways may represent a checkpoint for downstream regulation of proteolysis in trypanosomatid protozoa."
Chagasin, the endogenous cysteine-protease inhibitor of Trypanosoma cruzi, modulates parasite differentiation and invasion of mammalian cells.
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Ultrastructural immunolocalisation placed chagasin and cruzipain together in the Golgi complex and in reservosomes, with free chagasin in unidentified small intracellular vesicles.
"Ultrastructural studies showed that chagasin and cruzipain both localize to the Golgi complex and reservosomes (lysosome-like organelles), whereas free chagasin was found in small intracellular vesicles"
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Inhibitory chagasin-cruzipain complexes were recovered directly from living epimastigotes using the differential detergent sensitivity of preformed complexes.
"we obtained direct evidence that chagasin-cruzipain complexes are indeed formed in epimastigotes"
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Chagasin overexpression raised cysteine peptidase inhibitory activity fourfold, slowed metacyclogenesis and conferred resistance to a synthetic peptidase inhibitor.
"Chagasin transfectants (fourfold increase in CP inhibitory activity) displayed low rates of differentiation (metacyclogenesis) and exhibited increased resistance to a synthetic CP inhibitor."
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The transfectant phenotypes were accompanied by a drastic loss of soluble cruzipain activity and increased secretion of cruzipain-chagasin complexes, which is the direct in vivo evidence that chagasin negatively regulates endogenous peptidase activity.
"These phenotypic changes were accompanied by a drastic reduction of soluble cruzipain activity and by upregulated secretion of cruzipain-chagasin molecular complexes."
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Reduced infectivity of chagasin overexpressers was traced to lower membrane-associated peptidase activity and rescued by wild-type membranes in an E-64-sensitive manner, locating the effect on the parasite's own enzyme.
"The deficiency of chagasin overexpressers is caused by lower activity of membrane-associated CPs, because membranes recovered from wild-type trypomastigotes restored infectivity and this effect was nullified by the CP inhibitor E-64."
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The authors state that the effects on differentiation and invasion are indirect, mediated through the endogenous peptidase, which is why no process term for metacyclogenesis or invasion is proposed in this review.
"our studies suggest that chagasin regulates the endogenous activity of CP, thus indirectly modulating proteolytic functions that are essential for parasite differentiation and invasion of mammalian cells."
The structure of chagasin in complex with a cysteine protease clarifies the binding mode and evolution of an inhibitor family.
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The 2.2 A crystal structure of chagasin in complex with a cysteine protease (falcipain-2 of Plasmodium falciparum, UniProtKB:Q9NBD4) is the structural basis of the IntAct protein-binding row.
"We report the 2.2 A crystal structure of the signature member of the I42 family, chagasin, in complex with a cysteine protease."
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Chagasin has an immunoglobulin-like fold with homology to human CD8alpha and is structurally unrelated to the cystatins, although the two converge on a similar mode of protease engagement.
"Chagasin has a unique variant of the immunoglobulin fold with homology to human CD8alpha."
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Chagasin engages its target protease in a manner reminiscent of the cystatins, which is the informative function the bare protein-binding row should be replaced by.
"Interactions of chagasin with a target protease are reminiscent of the cystatin family inhibitors."
Crystal structure of the parasite protease inhibitor chagasin in complex with a host target cysteine protease.
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The chagasin-human cathepsin L complex was solved at 1.75 A, establishing that chagasin inhibits a host cysteine protease with 39 pM affinity.
"We have crystallized and solved the structure of chagasin in complex with the host cysteine protease, cathepsin L, at 1.75 A resolution."
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Three loops (L2, L4, L6) form an inhibitory wedge whose contacts account for the high-affinity binding.
"An inhibitory wedge composed of three loops (L2, L4, and L6) forms a number of contacts responsible for high-affinity binding (K(i), 39 pM) to the enzyme."
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The central loop L2 inserts directly into the catalytic centre, which is the mechanism underlying the cysteine-type endopeptidase inhibitor activity.
"All three loops interact with the catalytic groove, with the central loop L2 inserted directly into the catalytic center."
Displacement of the occluding loop by the parasite protein, chagasin, results in efficient inhibition of human cathepsin B.
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Chagasin inhibits human cathepsin B with sub-nanomolar affinity by displacing the occluding loop that normally restricts access to its active site.
"Kinetic analysis revealed that substitution of His-110e, which anchors the loop in occluding position, results in 3-fold increased chagasin affinity (Ki for H110A cathepsin B, 0.35 nm) due to an improved association rate"
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All three chagasin loops span the entire active-site cleft of cathepsin B, confirming the shared wedge mechanism across targets.
"demonstrating that the occluding loop is displaced to allow chagasin binding with its three loops, L4, L2, and L6, spanning the entire active site cleft"
Crystal structure of the parasite inhibitor chagasin in complex with papain allows identification of structural requirements for broad reactivity and specificity determinants for target proteases.
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Chagasin inactivates papain with near-diffusion-limited kinetics to form a very tight, reversible complex, quantifying the tight-binding and reversible characterisation.
"Kinetic studies revealed that inactivation of papain by chagasin is very fast (k(on) = 1.5 x 10(6) M(-1) x s(-1)), and results in the formation of a very tight, reversible complex (K(i) = 36 pM)"
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Papain is structurally close to the cruzipain catalytic domain, so this complex is a reliable model of the physiological chagasin-cruzipain interaction.
"As papain exhibits high structural similarity to the catalytic domain of the T. cruzi enzyme cruzipain, the present chagasin-papain complex provides a reliable model of chagasin-cruzipain interactions."
The role of conserved residues of chagasin in the inhibition of cysteine peptidases.
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Chagasin is a single 110-residue chain that inactivates papain-like enzymes by forming 1:1 tight-binding complexes, with no sequence similarity to other inhibitor groups.
"It is a single 110-residue polypeptide chain with no sequence similarity to other known groups of CP inhibitors and potently inactivates papain-like enzymes through the formation of 1:1 tight–binding complexes."
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Genetic evidence shows that chagasin regulates endogenous cruzipain activity, in that overexpressing lines lose about 80% of active cysteine peptidase and are impaired in differentiation and infectivity.
"T. cruzi lines that overexpress chagasin have an 80% decrease in active CPs, are less capable to differentiate and are less infective to mammalian cells in vitro"
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Mutational dissection separates the parasite and host targets, since W93A costs 100-fold affinity for human cathepsin L while T31 substitutions cost 10-100-fold for cruzipain, and the difference is traced to contacts with cathepsin L helix 8 that do not occur with cruzipain.
"These differential effects reflect the occurrence of direct interactions between chagasin and helix 8 of cathepsin L, interactions that do not occur with cruzipain."
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Wild-type chagasin inhibits papain with a dissociation constant of about 5 pM.
"WT Ki = 5 ± 0.7 pM"
Natural cysteine protease inhibitors in protozoa: Fifteen years of the chagasin family.
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Chagasin is an approximately 11 kDa, highly thermostable, tight-binding inhibitor of papain, cysteine cathepsins and endogenous parasite cysteine proteases, and is the signature member of inhibitor family I42.
"Chagasin was identified in the pathogenic protozoa Trypanosoma cruzi as an approximately 11 kDa protein that is a tight-binding and highly thermostable inhibitor of papain, cysteine cathepsins and endogenous parasite cysteine proteases."
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The inhibitory mechanism differs from that of the cystatins, in that loop 2 enters the active-site cleft and contacts the catalytic residues while loops 4 and 6 embrace the enzyme from the sides.
"Differently from cystatins, the loop 2 of chagasin enters the active-site cleft, making direct contact with the catalytic residues, while loops 4 and 6 embrace the enzyme from the sides."
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Biological roles of chagasin-family inhibitors have been studied in few species, mostly by transgenesis, and the conclusions vary, which bounds how far any process annotation can be taken.
"Studies on their biological roles were carried out in a few species, mainly using transgenic protozoa, and the conclusions vary."
UniProtKB/Swiss-Prot flat-file record for Trypanosoma cruzi chagasin (Q966X9)
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The curated function is cysteine protease inhibition, specifically inhibition of cruzipain, with experimental evidence from the discovery paper.
"Inhibits cysteine protease cruzipain"
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Chagasin interacts with cruzipain, curated with experimental evidence.
"SUBUNIT: Interacts with cruzipain."
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The curated subcellular locations are flagellar pocket, cytoplasmic vesicle and cell surface, each with experimental ECO:0000269 evidence from PubMed:11719560, which is what the three GO_REF:0000044 IEA rows restate.
"SUBCELLULAR LOCATION: Flagellar pocket {ECO:0000269|PubMed:11719560}."
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Cytoplasmic vesicle and cell surface are curated from the same experimental reference.
"Cytoplasmic vesicle {ECO:0000269|PubMed:11719560}. Cell surface"
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The BC, DE and FG loops are curated as a tripartite wedge that blocks the substrate-binding site of target cysteine proteases, the structural basis of the inhibitor activity.
"DOMAIN: The BC, DE and FG loops form a tripartite wedge that blocks the"
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The entry is classified in the protease inhibitor I42 family, supporting family membership as a basis for the inhibitor function.
"SIMILARITY: Belongs to the protease inhibitor I42 family."
Curation notes for Trypanosoma cruzi chagasin (Q966X9)
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Deep research was unavailable for this review and the GONUTS wiki is offline, so the CACAO rows were judged from the cached publications, UniProt and PubMed/PMC lookups.
"**Deep research was unavailable for this review.**"
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All four CACAO rows on this gene are sound as written, making chagasin a negative control for the CACAO failure taxonomy; the one row needing correction is an IntAct import, not a CACAO row.
"All four CACAO rows on this gene are **sound as written**."
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The review keeps parasite-directed and host-directed inhibition apart, annotating a process term only for the demonstrated regulation of the parasite's own cruzipain.
"This gene's annotation hinges on keeping two claims apart."