Deep research was unavailable for this review. No deep-research API keys are
configured in this container, so just deep-research was not run and no
-deep-research-<provider>.md file exists for this gene. The literature below was
assembled directly from the cached publications in publications/, PubMed/PMC
lookups, and the curated UniProtKB record cha-uniprot.txt.
The GONUTS wiki was unavailable. https://gowiki.tamu.edu/wiki/index.php/PMID:11719560
returns only "This site is temporarily offline while we perform upgrades", matching
what projects/CACAO.md records for 2026-10-29, so the student's own evidence page
for the four CACAO rows could not be consulted. Fortunately the cached abstract of
PMID:11719560 states each annotated claim explicitly (see below), so no CACAO row on
this gene had to be judged blind.
Chagasin is a 110-residue, heat-stable protein of T. cruzi and the signature member
of MEROPS inhibitor family I42 (DR MEROPS; I42.001). It is a tight-binding,
reversible, 1:1 inhibitor of papain-family (clan CA, family C1) cysteine peptidases
PMID:18201565. It has no sequence or
fold relationship to the cystatins: it adopts an immunoglobulin-like fold with homology
to human CD8alpha PMID:17502099, and presents three loops (L2/BC, L4/DE, L6/FG)
on one face as a tripartite inhibitory wedge
PMID:17561110.
Loop L2 inserts directly into the catalytic cleft and contacts the catalytic residues
PMID:17561110. UniProt records the same three
motifs as FT MOTIF BC (29-34), DE (59-68) and FG (91-100).
Affinities are picomolar and broad across the C1 family: papain K(i) = 5 pM
PMID:18201565, human cathepsin L K(i) = 39 pM
PMID:17561110, human cathepsin B
K(i) sub-nanomolar PMID:18515357, and papain
association essentially diffusion-limited
PMID:19143838.
This gene's annotation hinges on keeping two claims apart.
Chagasin regulates the parasite's own cruzipain in vivo. This is the
established biological role. Chagasin and cruzipain co-localise in the secretory
and endocytic pathway, chagasin-cruzipain complexes are recovered from living
epimastigotes, and raising chagasin levels genetically collapses parasite cysteine
peptidase activity PMID:15713748;
PMID:15713748; PMID:15713748. The consequences are
parasite-intrinsic: reduced metacyclogenesis and reduced infectivity, both traced
back to loss of parasite peptidase activity rather than to anything done to the host
PMID:15713748; PMID:15713748; PMID:18201565. Note the
direction: the authors are explicit that the modulation of differentiation and
invasion is indirect, via the endogenous peptidase PMID:15713748.
Chagasin inhibits host cathepsins in vitro. Chagasin binds and inhibits human
cathepsin L and cathepsin B, and the complexes have been crystallised
PMID:17561110;
PMID:18515357. It even inhibits the
Plasmodium protease falcipain-2 (UniProtKB:Q9NBD4), which is the basis of the
IntAct IPI row, and plant papain. But these are cross-reactivities of a broad-spectrum
C1 inhibitor measured with purified enzymes. No published work shows chagasin
inhibiting a host protease inside an infected host cell, and the family review is
candid that the biological roles vary by species and rest mostly on transgenic work
PMID:26546840. The host-protease
affinity is also not merely incidental: chagasin makes contacts with cathepsin L
helix 8 that it does not make with cruzipain PMID:18201565, so the two targets are
discriminable and a host-directed role remains a live hypothesis rather than a
finding.
Consequence for annotation: the molecular function GO:0004869 cysteine-type
endopeptidase inhibitor activity is target-agnostic and is supported by both lines.
A biological process term is only warranted for claim 1 — hence GO:0010951 negative
regulation of endopeptidase activity proposed here against the parasite's own
peptidases. No symbiont-mediated / interspecies term (the GO:0052[0-9]* branch, e.g.
modulation of host protease activity) is proposed, because claim 2 has never been
demonstrated in an infected cell. Likewise no involved_in term for metacyclogenesis or
host-cell invasion: the authors themselves call that modulation indirect, and chagasin
performs no step of either process — it only sets the level of the peptidase that does.
Immunogold EM in PMID:11719560 gives three locations directly, in the two life stages
examined PMID:11719560, and the cell-surface result is independently confirmed on living cells
by FITC-probe binding PMID:11719560. "Flagellar pocket" is
an exact synonym of GO:0020016 ciliary pocket (verified in QuickGO), so the CACAO
term choice is a direct match rather than a stretch. UniProt curates the same three
locations from the same paper with ECO:0000269|PubMed:11719560, which is what the
three GO_REF:0000044 IEA rows restate.
PMID:15713748 adds two compartments that are absent from GOA: the Golgi complex and the
reservosome (GO:0106123, a term created specifically for the Schizotrypanum
organelle). Both are proposed as NEW rows here.
All four CACAO rows on this gene are sound as written. The paper is about chagasin,
it is the paper that named the protein, every annotated claim is stated in the abstract
with the assay named, the evidence code IDA fits (immunogold EM; enzyme-inhibition
kinetics on the purified protein), and each term sits at the granularity the assay
resolved. None of the eight failure shapes catalogued in projects/CACAO.md applies:
cell surface has two independent assaysECO:0000269.cytoplasmic vesicle looks like a parent-level term, butThe one thing the students did not do is the part-of/located_in distinction (part_of
cell surface is odd for a non-complex), but qualifiers are inert for review purposes
per repository policy, and the underlying claim is right.
This makes chagasin a useful negative control for the CACAO project: a case where
community curation from a single classic paper produced four annotations that need no
correction. The one row that does need work (GO:0005515 protein binding) is not a
CACAO row — it is an IntAct IPI import.
GO:0005515 with UniProtKB:Q9NBD4 (verified in UniProt as "Cysteine protease
falcipain-2", Plasmodium falciparum) comes from PMID:17502099, the chagasin-falcipain-2
crystal structure. The interaction is real but the term carries no functional
information, and the paper supports the informative replacement directly: UniProt cites
this very paper with ECO:0000269 for chagasin's inhibitor function. Per the
protein-binding policy this is a MODIFY to GO:0004869, not a REMOVE and not an
ACCEPT. Worth noting the partner is a Plasmodium enzyme used as a crystallographic
surrogate — T. cruzi and P. falciparum share no host cell, so this is not a
physiological interaction and should not be read as evidence of a host- or
co-infection-directed role.