Spy (Spheroplast protein Y) is an ATP-independent periplasmic chaperone in E. coli that functions primarily as a holdase, preventing protein aggregation under stress conditions (tannins, butanol, ethanol). It forms a thin, cradle-shaped homodimer with a novel alpha-helical fold. Spy is massively upregulated (up to 25-50% of periplasmic protein) under envelope stress via the Bae and Cpx two-component systems. Remarkably, Spy can also allow substrate proteins to fold while remaining bound to its surface, making it unusual among ATP-independent chaperones. It binds unfolded, intermediate, and native conformations of its substrates with comparable micromolar affinities.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0030288
outer membrane-bounded periplasmic space
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: IBA annotation for periplasmic localization of Spy, inferred from phylogenetic analysis. Spy is well established as a periplasmic protein based on extensive experimental evidence from multiple studies (PMID:9068658, PMID:21317898, PMID:9694902).
Reason: Spy was originally identified as a periplasmic protein (PMID:9068658) and its periplasmic localization has been confirmed repeatedly. The IBA annotation is consistent with all available experimental evidence and the UniProt record. The IDA annotation (below) provides direct experimental support.
Supporting Evidence:
PMID:9068658
It encodes a precursor of a so far unknown 139-residue, rather basic periplasmic protein.
PMID:21317898
Spy overexpression leads to the accumulation of an otherwise highly unstable protein instead suggested that Spy might function as a chaperone that facilitates protein folding in the bacterial periplasm.
|
|
GO:0051082
unfolded protein binding
|
IBA
GO_REF:0000033 |
MODIFY |
Summary: IBA annotation for unfolded protein binding, inferred by phylogeny. Spy is indeed one of the best-characterized unfolded protein binders, demonstrated by ITC, stopped-flow fluorescence, and crystallographic studies (PMID:26619265, PMID:27239796). However, GO:0051082 is proposed for obsoletion and Spy is primarily a holdase chaperone that prevents aggregation in the periplasm.
Reason: Spy is primarily a holdase -- an ATP-independent chaperone that binds unfolded/misfolded proteins to prevent aggregation in situ. It does not escort proteins between compartments, so GO:0140309 (unfolded protein carrier activity) does not apply. The ideal replacement is a proposed "holdase chaperone activity" NTR. Until that NTR exists, GO:0051082 should be retained. Spy also has some foldase-like activity (PMID:26619265), but its primary mechanism is holdase. GO:0044183 (protein folding chaperone, already annotated separately) captures the foldase aspect.
Proposed replacements:
unfolded protein binding (retain until holdase NTR created)
Supporting Evidence:
PMID:21317898
In vitro studies demonstrate that the Spy protein is an effective ATP-independent chaperone that suppresses protein aggregation and aids protein refolding.
PMID:26619265
It has previously been thought that only ATP dependent "foldase" chaperones can actively facilitate protein folding. The ATP independent "holdase" chaperones were thought to play a more passive role, holding onto aggregation-sensitive folding intermediates
|
|
GO:0042597
periplasmic space
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: IEA annotation for periplasmic space based on InterPro and UniProt subcellular location mappings. This is a broader term than GO:0030288 (outer membrane-bounded periplasmic space), which is also annotated. Both are correct.
Reason: The IEA mapping to periplasmic space is consistent with all experimental evidence. While GO:0030288 is more specific, the broader GO:0042597 is not incorrect for an IEA annotation. Spy is unambiguously a periplasmic protein (PMID:9068658, PMID:21317898, PMID:9694902).
Supporting Evidence:
PMID:9068658
It encodes a precursor of a so far unknown 139-residue, rather basic periplasmic protein.
|
|
GO:0005515
protein binding
|
IPI
PMID:26619265 Substrate protein folds while it is bound to the ATP-indepen... |
REMOVE |
Summary: IPI annotation for protein binding based on physical interaction evidence from IntAct (Spy interacts with immunity protein Imm). The PMID:26619265 study used ITC and stopped-flow fluorescence to demonstrate direct binding of Spy to Im7 in multiple conformational states (Kd values of 3.5-20.5 uM).
Reason: GO:0005515 (protein binding) is uninformative for a protein whose core function is binding unfolded proteins as a chaperone. The chaperone-substrate interaction is already captured by GO:0051082 (unfolded protein binding) and GO:0044183 (protein folding chaperone). Per curation guidelines, protein binding should be avoided in favor of more specific MF terms.
Supporting Evidence:
PMID:26619265
We found that Spy binds to all three variants of Im7, with affinities of 10.4 ΞM, 3.5 ΞM, and 20.5 ΞM for Im7-L18A L19A L37A (Im7U), Im7-L53A I54A (Im7I), and Im7-WT (Im7N), respectively
|
|
GO:0006457
protein folding
|
IDA
PMID:21317898 Genetic selection designed to stabilize proteins uncovers a ... |
ACCEPT |
Summary: IDA annotation for involvement in protein folding, based on the discovery paper showing that Spy suppresses protein aggregation and aids protein refolding in vitro. Spy increased refolding yield of chemically and thermally unfolded substrates in the absence of ATP or cofactors (PMID:21317898). Subsequent work showed that substrates can actually fold while bound to Spy's surface (PMID:26619265).
Reason: Spy is directly involved in protein folding in the periplasm. The original discovery paper demonstrated that Spy suppresses aggregation of multiple substrates (MDH, aldolase, GAPDH) and significantly increases refolding yield (PMID:21317898). The follow-up study (PMID:26619265) provided the remarkable finding that Im7 folds while bound to Spy. This BP annotation accurately captures Spy's role.
Supporting Evidence:
PMID:21317898
To test if Spy can support protein folding, even though it is localized to the ATP-devoid environment of the bacterial periplasm, we analyzed its influence on the refolding yield of chemically and thermally unfolded proteins. We found that Spy significantly increased the refolding yield of a number of substrates
PMID:26619265
Spy then allows Im7 to fully fold into its native state while it remains bound to the surface of the chaperone.
|
|
GO:0044183
protein folding chaperone
|
IDA
PMID:26619265 Substrate protein folds while it is bound to the ATP-indepen... |
ACCEPT |
Summary: IDA annotation for protein folding chaperone activity, based on the landmark study demonstrating that Im7 substrate folds while bound to Spy (PMID:26619265). Global kinetic fitting showed that the only model consistent with the data requires Im7 to fold completely (U -> I -> N) while remaining associated with Spy. This is unusual for an ATP-independent chaperone.
Reason: Spy is demonstrated to function as a protein folding chaperone, as the substrate Im7 folds through all intermediates while continuously bound to Spy's surface (PMID:26619265). Although Spy is primarily considered a holdase (preventing aggregation), it clearly also promotes folding without ATP. GO:0044183 is appropriate for this experimentally demonstrated activity. The structural basis for this was further elucidated by READ crystallography (PMID:27239796).
Supporting Evidence:
PMID:26619265
A good fit was only achieved when we globally fit the data to the kinetic mechanism that allows both folding steps 4 and 5, i.e., complete folding of Im7 while bound to Spy, making this the simplest kinetic mechanism that can explain all of the experimental data
PMID:27239796
The ensemble shows that Spy-associated Im7 samples conformations ranging from unfolded to partially folded to native-like states and reveals how a substrate can explore its folding landscape while being bound to a chaperone.
|
|
GO:0051082
unfolded protein binding
|
IDA
PMID:26619265 Substrate protein folds while it is bound to the ATP-indepen... |
MODIFY |
Summary: IDA annotation for unfolded protein binding based on ITC and stopped-flow kinetics showing Spy binds unfolded, intermediate, and native Im7 with micromolar affinities (PMID:26619265). Spy bound Im7U with Kd of 10.4 uM and association rate constant of 1.3 x 10^7 M-1 s-1, indicating rapid capture of unfolded substrate.
Reason: The experimental evidence robustly supports Spy binding to unfolded proteins. However, GO:0051082 is proposed for obsoletion. Spy is primarily a holdase -- it prevents aggregation in situ in the periplasm, not a carrier that escorts proteins between compartments. GO:0140309 (unfolded protein carrier activity) does NOT fit because Spy acts in situ. The ideal replacement is a proposed "holdase chaperone activity" NTR. Until the NTR exists, retain GO:0051082.
Proposed replacements:
unfolded protein binding (retain until holdase NTR created)
Supporting Evidence:
PMID:26619265
We found that Spy binds to all three variants of Im7, with affinities of 10.4 ΞM, 3.5 ΞM, and 20.5 ΞM for Im7-L18A L19A L37A (Im7U), Im7-L53A I54A (Im7I), and Im7-WT (Im7N), respectively
PMID:26619265
At this concentration, association between Spy and Im7U would be very rapid (occurring with a half-time of 26 Ξs)
|
|
GO:0051082
unfolded protein binding
|
IDA
PMID:27239796 Visualizing chaperone-assisted protein folding. |
MODIFY |
Summary: IDA annotation for unfolded protein binding based on crystallographic visualization of Spy-Im7 complex using the READ (Residual Electron and Anomalous Density) technique (PMID:27239796). The structural ensemble captured Im7 in multiple conformations (unfolded, partially folded, native-like) bound within Spy's cradle-shaped concave surface.
Reason: Same rationale as the other GO:0051082 annotations. The READ structural study provides direct visualization of an unfolded substrate bound to Spy, confirming unfolded protein binding. However, GO:0051082 is proposed for obsoletion and Spy is an in-situ holdase, not a carrier. Retain GO:0051082 until the holdase NTR is created.
Proposed replacements:
unfolded protein binding (retain until holdase NTR created)
Supporting Evidence:
PMID:27239796
The ensemble shows that Spy-associated Im7 samples conformations ranging from unfolded to partially folded to native-like states and reveals how a substrate can explore its folding landscape while being bound to a chaperone.
PMID:27239796
we observed that Im76-45 takes on several different conformations while bound. We found these conformations to be highly heterogeneous and to include unfolded, partially folded, and native-like states
|
|
GO:0042803
protein homodimerization activity
|
IDA
PMID:20799348 The crystal structure Escherichia coli Spy. |
ACCEPT |
Summary: IDA annotation for protein homodimerization based on the crystal structure of Spy (PMID:20799348), which revealed an antiparallel homodimer with a curved oval shape. The dimer interface buries approximately 1850 A^2 per monomer, indicating a stable dimeric assembly.
Reason: The crystal structure unambiguously shows Spy as a homodimer (PMID:20799348), confirmed independently by a second crystal structure (PMID:21317898), size exclusion chromatography, and analytical ultracentrifugation. Homodimerization is functionally essential for forming the cradle-shaped substrate binding surface. This is a core structural feature of Spy.
Supporting Evidence:
PMID:20799348
which reveals a long kinked hairpin-like structure of four Îą-helices that form an antiparallel dimer
PMID:21317898
The crystal structure shows that Spy molecules associate into tightly bound dimers
|
|
GO:0030288
outer membrane-bounded periplasmic space
|
IDA
PMID:9068658 A new periplasmic protein of Escherichia coli which is synth... |
ACCEPT |
Summary: IDA annotation for periplasmic localization based on the original discovery of Spy (PMID:9068658). Spy was identified as a new periplasmic protein produced abundantly in spheroplasts but not in intact cells. The protein contains a signal peptide (residues 1-23) directing it to the periplasm.
Reason: The original identification of Spy as a periplasmic protein (PMID:9068658) is the foundational localization evidence. The signal peptide cleaves at position 23 to generate the mature periplasmic form. This has been confirmed by multiple subsequent studies (PMID:9694902, PMID:21317898). UniProt also annotates Spy to the periplasm with multiple experimental evidence codes.
Supporting Evidence:
PMID:9068658
It encodes a precursor of a so far unknown 139-residue, rather basic periplasmic protein. It was not detectable immunologically in intact cells but was produced abundantly in spheroplasts.
|
|
GO:0042803
protein homodimerization activity
|
IDA
PMID:21317898 Genetic selection designed to stabilize proteins uncovers a ... |
ACCEPT |
Summary: IDA annotation for protein homodimerization from the Spy chaperone discovery paper (PMID:21317898). The crystal structure, size exclusion chromatography, and analytical ultracentrifugation all confirmed that Spy is a homodimer with extensive monomer-monomer contacts burying ~1850 A^2 per monomer.
Reason: Independent confirmation of Spy homodimerization using multiple biophysical methods (crystallography, SEC, AUC) in the landmark chaperone discovery paper. This is a duplicate of the PMID:20799348 annotation (both correct) and reflects that two independent groups solved the Spy dimer structure. Homodimerization is essential for the cradle architecture that enables chaperone function.
Supporting Evidence:
PMID:21317898
The crystal structure shows that Spy molecules associate into tightly bound dimers
PMID:21317898
The contacts between the two monomers are extensive, burying a surface of âž1850 Ã
2 per monomer upon dimerization and suggesting high dimeric stability
|
Exported on March 22, 2026 at 02:54 AM
Organism: Escherichia coli
Sequence:
MRKLTALFVASTLALGAANLAHAADTTTAAPADAKPMMHHKGKFGPHQDMMFKDLNLTDAQKQQIREIMKGQRDQMKRPPLEERRAMHDIIASDTFDKVKAEAQIAKMEEQRKANMLAHMETQNKIYNILTPEQKKQFNANFEKRLTERPAAKGKMPATAE
I begin with the domain architecture. The polypeptide is fully encompassed by IPR052211 (Cpx two-component system auxiliary protein family, residues 1â161), establishing that the entire sequence is dedicated to supporting a membrane-proximal stress-signaling pathway rather than catalysis. Nested within this, residues 6â148 carry IPR012899 (LTXXQ motif family protein), a conserved structural motif class frequently associated with periplasmic or membrane-associated adaptors that stabilize stress-signaling assemblies. The absence of enzymatic signatures and the dominance of an auxiliary-system family signature together argue that this protein functions through binding and assembly rather than chemistry, steering me toward a binding-centric molecular role.
From this architecture, the molecular function resolves as protein binding (GO:0005515). Auxiliary modules in two-component systems typically tune signal flow by forming transient complexes with the cognate histidine kinase and downstream effectors. The LTXXQ motif family further supports a scaffold/adaptor role that organizes periplasmic or membrane-proximal assemblies. This binding-centric architecture causes regulatory control over pathway activity rather than direct substrate turnover.
Translating binding into process, auxiliary components in envelope-stress pathways modulate signal transduction cascades that govern proteostasis and stress adaptation. The Cpx system in particular monitors envelope and periplasmic stress; an auxiliary factor that stabilizes or targets the pathway naturally participates in signal relay and attenuation. Thus, the most parsimonious biological process is signal transduction (GO:0007165), where this protein shapes pathway throughput and timing rather than executing terminal reactions.
Cellular location follows from the system context and motif class. Cpx signaling operates at the envelope, and LTXXQ-bearing families are often peripherally associated with or resident in membrane-proximal compartments. In Escherichia coli, this places the protein at the cell envelope where sensory and adaptor modules concentrate. Therefore, I infer residence at the cell envelope (GO:0042657), consistent with an auxiliary assembly factor that needs to dock near membrane-embedded kinases and periplasmic substrates.
Mechanistically, I hypothesize that the LTXXQ-bearing core forms a stable adaptor that recruits or stabilizes the membrane-associated histidine kinase and its periplasmic partners, thereby biasing the pathway toward efficient envelope-stress surveillance. By binding the kinase and periplasmic targets, the protein likely enhances signal fidelity and spatial targeting. Expected partners include the cognate membrane histidine kinase and envelope-stress effectors; together they form a membrane-proximal signaling hub that routes information from the cell envelope to downstream transcriptional responses.
An auxiliary component of a membrane-proximal stress-signaling pathway in Escherichia coli that operates at the cell envelope. It lacks catalytic motifs and instead functions through binding and assembly, stabilizing and tuning a two-component signaling module that monitors envelope and periplasmic stress. By organizing transient complexes near the membrane, it enhances signal relay and targeting within the stress-response network.
Auxiliary component of the Cpx stress protein system.
IPR052211, family) â residues 1-161IPR012899, family) â residues 6-148Molecular Function: molecular_function (GO:0003674), protein folding chaperone (GO:0044183), binding (GO:0005488), protein binding (GO:0005515), unfolded protein binding (GO:0051082)
Biological Process: biological_process (GO:0008150), response to stimulus (GO:0050896), cellular process (GO:0009987), cellular response to stimulus (GO:0051716), response to abiotic stimulus (GO:0009628), response to stress (GO:0006950), response to chemical (GO:0042221), response to hypoxia (GO:0001666), cellular response to chemical stimulus (GO:0070887), cellular response to stress (GO:0033554), response to oxygen levels (GO:0070482), cellular response to hypoxia (GO:0071456), cellular response to oxygen levels (GO:0071453), response to decreased oxygen levels (GO:0036293), cellular response to decreased oxygen levels (GO:0036294)
Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), intracellular anatomical structure (GO:0005622), organelle (GO:0043226), membrane (GO:0016020), envelope (GO:0031975), cytoplasm (GO:0005737), membrane-enclosed lumen (GO:0031974), organelle membrane (GO:0031090), organelle envelope (GO:0031967), intracellular organelle (GO:0043229), outer membrane (GO:0019867), plastid (GO:0009536), non-membrane-bounded organelle (GO:0043228), vacuole (GO:0005773), mitochondrion (GO:0005739), organelle lumen (GO:0043233), membrane-bounded organelle (GO:0043227), mitochondrial envelope (GO:0005740), intracellular membrane-bounded organelle (GO:0043231), bounding membrane of organelle (GO:0098588), intracellular non-membrane-bounded organelle (GO:0043232), plastid envelope (GO:0009526), plastid membrane (GO:0042170), organelle outer membrane (GO:0031968), intracellular organelle lumen (GO:0070013), mitochondrial membrane (GO:0031966), mitochondrial outer membrane (GO:0005741), ribosome (GO:0005840)
Generated by BioReason
Exported on March 22, 2026 at 02:54 AM
Organism: Escherichia coli
Sequence:
MRKLTALFVASTLALGAANLAHAADTTTAAPADAKPMMHHKGKFGPHQDMMFKDLNLTDAQKQQIREIMKGQRDQMKRPPLEERRAMHDIIASDTFDKVKAEAQIAKMEEQRKANMLAHMETQNKIYNILTPEQKKQFNANFEKRLTERPAAKGKMPATAE
I begin with the domain architecture. The polypeptide is fully encompassed by IPR052211 (Cpx two-component system auxiliary protein family, residues 1â161), establishing that the entire sequence is dedicated to supporting a membrane-proximal stress-signaling pathway rather than catalysis. Nested within this, residues 6â148 carry IPR012899 (LTXXQ motif family protein), a conserved structural motif class frequently associated with periplasmic or membrane-associated adaptors that stabilize stress-signaling assemblies. The absence of enzymatic signatures and the dominance of an auxiliary-system family signature together argue that this protein functions through binding and assembly rather than chemistry, steering me toward a binding-centric molecular role.
From this architecture, the molecular function resolves as protein binding (GO:0005515). Auxiliary modules in two-component systems typically tune signal flow by forming transient complexes with the cognate histidine kinase and downstream effectors. The LTXXQ motif family further supports a scaffold/adaptor role that organizes periplasmic or membrane-proximal assemblies. This binding-centric architecture causes regulatory control over pathway activity rather than direct substrate turnover.
Translating binding into process, auxiliary components in envelope-stress pathways modulate signal transduction cascades that govern proteostasis and stress adaptation. The Cpx system in particular monitors envelope and periplasmic stress; an auxiliary factor that stabilizes or targets the pathway naturally participates in signal relay and attenuation. Thus, the most parsimonious biological process is signal transduction (GO:0007165), where this protein shapes pathway throughput and timing rather than executing terminal reactions.
Cellular location follows from the system context and motif class. Cpx signaling operates at the envelope, and LTXXQ-bearing families are often peripherally associated with or resident in membrane-proximal compartments. In Escherichia coli, this places the protein at the cell envelope where sensory and adaptor modules concentrate. Therefore, I infer residence at the cell envelope (GO:0042657), consistent with an auxiliary assembly factor that needs to dock near membrane-embedded kinases and periplasmic substrates.
Mechanistically, I hypothesize that the LTXXQ-bearing core forms a stable adaptor that recruits or stabilizes the membrane-associated histidine kinase and its periplasmic partners, thereby biasing the pathway toward efficient envelope-stress surveillance. By binding the kinase and periplasmic targets, the protein likely enhances signal fidelity and spatial targeting. Expected partners include the cognate membrane histidine kinase and envelope-stress effectors; together they form a membrane-proximal signaling hub that routes information from the cell envelope to downstream transcriptional responses.
An auxiliary component of a membrane-proximal stress-signaling pathway in Escherichia coli that operates at the cell envelope. It lacks catalytic motifs and instead functions through binding and assembly, stabilizing and tuning a two-component signaling module that monitors envelope and periplasmic stress. By organizing transient complexes near the membrane, it enhances signal relay and targeting within the stress-response network.
Auxiliary component of the Cpx stress protein system.
IPR052211, family) â residues 1-161IPR012899, family) â residues 6-148Molecular Function: molecular_function (GO:0003674), protein folding chaperone (GO:0044183), binding (GO:0005488), protein binding (GO:0005515), unfolded protein binding (GO:0051082)
Biological Process: biological_process (GO:0008150), response to stimulus (GO:0050896), cellular process (GO:0009987), cellular response to stimulus (GO:0051716), response to abiotic stimulus (GO:0009628), response to stress (GO:0006950), response to chemical (GO:0042221), response to hypoxia (GO:0001666), cellular response to chemical stimulus (GO:0070887), cellular response to stress (GO:0033554), response to oxygen levels (GO:0070482), cellular response to hypoxia (GO:0071456), cellular response to oxygen levels (GO:0071453), response to decreased oxygen levels (GO:0036293), cellular response to decreased oxygen levels (GO:0036294)
Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), intracellular anatomical structure (GO:0005622), organelle (GO:0043226), membrane (GO:0016020), envelope (GO:0031975), cytoplasm (GO:0005737), membrane-enclosed lumen (GO:0031974), organelle membrane (GO:0031090), organelle envelope (GO:0031967), intracellular organelle (GO:0043229), outer membrane (GO:0019867), plastid (GO:0009536), non-membrane-bounded organelle (GO:0043228), vacuole (GO:0005773), mitochondrion (GO:0005739), organelle lumen (GO:0043233), membrane-bounded organelle (GO:0043227), mitochondrial envelope (GO:0005740), intracellular membrane-bounded organelle (GO:0043231), bounding membrane of organelle (GO:0098588), intracellular non-membrane-bounded organelle (GO:0043232), plastid envelope (GO:0009526), plastid membrane (GO:0042170), organelle outer membrane (GO:0031968), intracellular organelle lumen (GO:0070013), mitochondrial membrane (GO:0031966), mitochondrial outer membrane (GO:0005741), ribosome (GO:0005840)
Generated by BioReason
Source: Spy-deep-research-bioreason-rl.md
The BioReason functional summary describes Spy as:
An auxiliary component of a membrane-proximal stress-signaling pathway in Escherichia coli that operates at the cell envelope. It lacks catalytic motifs and instead functions through binding and assembly, stabilizing and tuning a two-component signaling module that monitors envelope and periplasmic stress. By organizing transient complexes near the membrane, it enhances signal relay and targeting within the stress-response network.
This summary is fundamentally wrong about Spy's function. Spy (Spheroplast protein Y) is an ATP-independent periplasmic chaperone, not a signaling pathway component. The curated review establishes that Spy:
- Functions primarily as a holdase, preventing protein aggregation under stress conditions
- Forms a thin, cradle-shaped homodimer with a novel alpha-helical fold
- Is massively upregulated under envelope stress via the Bae and Cpx two-component systems
- Remarkably allows substrate proteins to fold while remaining bound to its surface (PMID:26619265)
BioReason misidentified Spy as a Cpx pathway auxiliary protein (IPR052211), treating it as analogous to CpxP. The InterPro domains listed are "Cpx two-component system auxiliary protein" (IPR052211) and "LTXXQ motif family protein" (IPR012899). While Spy is regulated by the Cpx system, it is not a signaling component of that system -- it is an effector/chaperone induced by the system.
The GO term predictions include many inappropriate terms: mitochondrial outer membrane, plastid envelope, ribosome, thylakoid -- all nonsensical for a bacterial periplasmic chaperone. The predictions also include protein folding chaperone (GO:0044183) and unfolded protein binding (GO:0051082), which are correct but contradict the narrative.
Comparison with interpro2go:
Spy has no GO_REF:0000002 annotations in the curated review. BioReason's classification of Spy under IPR052211 (Cpx auxiliary protein) appears to be the root cause of the misidentification. If the InterPro family assignment is incorrect or overly broad, BioReason would propagate that error. The curated review identifies Spy's core function as protein folding chaperone (GO:0044183) with periplasmic localization -- a very different picture from what BioReason produces.
The trace builds its entire functional model on IPR052211 (Cpx auxiliary protein) and IPR012899 (LTXXQ motif), leading it to describe Spy as a signaling adaptor rather than a chaperone. This is a case where incorrect or misleading InterPro family assignment leads to a fundamentally wrong functional prediction. The mention of "response to hypoxia" and "cellular response to decreased oxygen levels" in the GO predictions is baffling for Spy.
id: P77754
gene_symbol: Spy
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:83333
label: Escherichia coli (strain K12)
description: Spy (Spheroplast protein Y) is an ATP-independent periplasmic chaperone
in E. coli that functions primarily as a holdase, preventing protein aggregation
under stress conditions (tannins, butanol, ethanol). It forms a thin, cradle-shaped
homodimer with a novel alpha-helical fold. Spy is massively upregulated (up to 25-50%
of periplasmic protein) under envelope stress via the Bae and Cpx two-component
systems. Remarkably, Spy can also allow substrate proteins to fold while remaining
bound to its surface, making it unusual among ATP-independent chaperones. It binds
unfolded, intermediate, and native conformations of its substrates with comparable
micromolar affinities.
existing_annotations:
- term:
id: GO:0030288
label: outer membrane-bounded periplasmic space
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: IBA annotation for periplasmic localization of Spy, inferred from phylogenetic
analysis. Spy is well established as a periplasmic protein based on extensive
experimental evidence from multiple studies (PMID:9068658, PMID:21317898, PMID:9694902).
action: ACCEPT
reason: Spy was originally identified as a periplasmic protein (PMID:9068658)
and its periplasmic localization has been confirmed repeatedly. The IBA annotation
is consistent with all available experimental evidence and the UniProt record.
The IDA annotation (below) provides direct experimental support.
supported_by:
- reference_id: PMID:9068658
supporting_text: It encodes a precursor of a so far unknown 139-residue, rather
basic periplasmic protein.
- reference_id: PMID:21317898
supporting_text: Spy overexpression leads to the accumulation of an otherwise
highly unstable protein instead suggested that Spy might function as a chaperone
that facilitates protein folding in the bacterial periplasm.
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: IBA annotation for unfolded protein binding, inferred by phylogeny. Spy
is indeed one of the best-characterized unfolded protein binders, demonstrated
by ITC, stopped-flow fluorescence, and crystallographic studies (PMID:26619265,
PMID:27239796). However, GO:0051082 is proposed for obsoletion and Spy is primarily
a holdase chaperone that prevents aggregation in the periplasm.
action: MODIFY
reason: Spy is primarily a holdase -- an ATP-independent chaperone that binds
unfolded/misfolded proteins to prevent aggregation in situ. It does not escort
proteins between compartments, so GO:0140309 (unfolded protein carrier activity)
does not apply. The ideal replacement is a proposed "holdase chaperone activity"
NTR. Until that NTR exists, GO:0051082 should be retained. Spy also has some
foldase-like activity (PMID:26619265), but its primary mechanism is holdase.
GO:0044183 (protein folding chaperone, already annotated separately) captures
the foldase aspect.
proposed_replacement_terms:
- id: GO:0051082
label: unfolded protein binding (retain until holdase NTR created)
supported_by:
- reference_id: PMID:21317898
supporting_text: In vitro studies demonstrate that the Spy protein is an effective
ATP-independent chaperone that suppresses protein aggregation and aids protein
refolding.
- reference_id: PMID:26619265
supporting_text: It has previously been thought that only ATP dependent "foldase"
chaperones can actively facilitate protein folding. The ATP independent "holdase"
chaperones were thought to play a more passive role, holding onto aggregation-sensitive
folding intermediates
- term:
id: GO:0042597
label: periplasmic space
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: IEA annotation for periplasmic space based on InterPro and UniProt subcellular
location mappings. This is a broader term than GO:0030288 (outer membrane-bounded
periplasmic space), which is also annotated. Both are correct.
action: ACCEPT
reason: The IEA mapping to periplasmic space is consistent with all experimental
evidence. While GO:0030288 is more specific, the broader GO:0042597 is not incorrect
for an IEA annotation. Spy is unambiguously a periplasmic protein (PMID:9068658,
PMID:21317898, PMID:9694902).
supported_by:
- reference_id: PMID:9068658
supporting_text: It encodes a precursor of a so far unknown 139-residue, rather
basic periplasmic protein.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:26619265
review:
summary: IPI annotation for protein binding based on physical interaction evidence
from IntAct (Spy interacts with immunity protein Imm). The PMID:26619265 study
used ITC and stopped-flow fluorescence to demonstrate direct binding of Spy
to Im7 in multiple conformational states (Kd values of 3.5-20.5 uM).
action: REMOVE
reason: GO:0005515 (protein binding) is uninformative for a protein whose core
function is binding unfolded proteins as a chaperone. The chaperone-substrate
interaction is already captured by GO:0051082 (unfolded protein binding) and
GO:0044183 (protein folding chaperone). Per curation guidelines, protein binding
should be avoided in favor of more specific MF terms.
supported_by:
- reference_id: PMID:26619265
supporting_text: We found that Spy binds to all three variants of Im7, with
affinities of 10.4 ΞM, 3.5 ΞM, and 20.5 ΞM for Im7-L18A L19A L37A (Im7U),
Im7-L53A I54A (Im7I), and Im7-WT (Im7N), respectively
- term:
id: GO:0006457
label: protein folding
evidence_type: IDA
original_reference_id: PMID:21317898
review:
summary: IDA annotation for involvement in protein folding, based on the discovery
paper showing that Spy suppresses protein aggregation and aids protein refolding
in vitro. Spy increased refolding yield of chemically and thermally unfolded
substrates in the absence of ATP or cofactors (PMID:21317898). Subsequent work
showed that substrates can actually fold while bound to Spy's surface (PMID:26619265).
action: ACCEPT
reason: Spy is directly involved in protein folding in the periplasm. The original
discovery paper demonstrated that Spy suppresses aggregation of multiple substrates
(MDH, aldolase, GAPDH) and significantly increases refolding yield (PMID:21317898).
The follow-up study (PMID:26619265) provided the remarkable finding that Im7
folds while bound to Spy. This BP annotation accurately captures Spy's role.
supported_by:
- reference_id: PMID:21317898
supporting_text: To test if Spy can support protein folding, even though it
is localized to the ATP-devoid environment of the bacterial periplasm, we
analyzed its influence on the refolding yield of chemically and thermally
unfolded proteins. We found that Spy significantly increased the refolding
yield of a number of substrates
- reference_id: PMID:26619265
supporting_text: Spy then allows Im7 to fully fold into its native state while
it remains bound to the surface of the chaperone.
- term:
id: GO:0044183
label: protein folding chaperone
evidence_type: IDA
original_reference_id: PMID:26619265
review:
summary: IDA annotation for protein folding chaperone activity, based on the landmark
study demonstrating that Im7 substrate folds while bound to Spy (PMID:26619265).
Global kinetic fitting showed that the only model consistent with the data requires
Im7 to fold completely (U -> I -> N) while remaining associated with Spy. This
is unusual for an ATP-independent chaperone.
action: ACCEPT
reason: Spy is demonstrated to function as a protein folding chaperone, as the
substrate Im7 folds through all intermediates while continuously bound to Spy's
surface (PMID:26619265). Although Spy is primarily considered a holdase (preventing
aggregation), it clearly also promotes folding without ATP. GO:0044183 is appropriate
for this experimentally demonstrated activity. The structural basis for this
was further elucidated by READ crystallography (PMID:27239796).
supported_by:
- reference_id: PMID:26619265
supporting_text: A good fit was only achieved when we globally fit the data
to the kinetic mechanism that allows both folding steps 4 and 5, i.e., complete
folding of Im7 while bound to Spy, making this the simplest kinetic mechanism
that can explain all of the experimental data
- reference_id: PMID:27239796
supporting_text: The ensemble shows that Spy-associated Im7 samples conformations
ranging from unfolded to partially folded to native-like states and reveals
how a substrate can explore its folding landscape while being bound to a chaperone.
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IDA
original_reference_id: PMID:26619265
review:
summary: IDA annotation for unfolded protein binding based on ITC and stopped-flow
kinetics showing Spy binds unfolded, intermediate, and native Im7 with micromolar
affinities (PMID:26619265). Spy bound Im7U with Kd of 10.4 uM and association
rate constant of 1.3 x 10^7 M-1 s-1, indicating rapid capture of unfolded substrate.
action: MODIFY
reason: The experimental evidence robustly supports Spy binding to unfolded proteins.
However, GO:0051082 is proposed for obsoletion. Spy is primarily a holdase --
it prevents aggregation in situ in the periplasm, not a carrier that escorts
proteins between compartments. GO:0140309 (unfolded protein carrier activity)
does NOT fit because Spy acts in situ. The ideal replacement is a proposed "holdase
chaperone activity" NTR. Until the NTR exists, retain GO:0051082.
proposed_replacement_terms:
- id: GO:0051082
label: unfolded protein binding (retain until holdase NTR created)
supported_by:
- reference_id: PMID:26619265
supporting_text: We found that Spy binds to all three variants of Im7, with
affinities of 10.4 ΞM, 3.5 ΞM, and 20.5 ΞM for Im7-L18A L19A L37A (Im7U),
Im7-L53A I54A (Im7I), and Im7-WT (Im7N), respectively
- reference_id: PMID:26619265
supporting_text: At this concentration, association between Spy and Im7U would
be very rapid (occurring with a half-time of 26 Ξs)
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IDA
original_reference_id: PMID:27239796
review:
summary: IDA annotation for unfolded protein binding based on crystallographic
visualization of Spy-Im7 complex using the READ (Residual Electron and Anomalous
Density) technique (PMID:27239796). The structural ensemble captured Im7 in
multiple conformations (unfolded, partially folded, native-like) bound within
Spy's cradle-shaped concave surface.
action: MODIFY
reason: Same rationale as the other GO:0051082 annotations. The READ structural
study provides direct visualization of an unfolded substrate bound to Spy, confirming
unfolded protein binding. However, GO:0051082 is proposed for obsoletion and
Spy is an in-situ holdase, not a carrier. Retain GO:0051082 until the holdase
NTR is created.
proposed_replacement_terms:
- id: GO:0051082
label: unfolded protein binding (retain until holdase NTR created)
supported_by:
- reference_id: PMID:27239796
supporting_text: The ensemble shows that Spy-associated Im7 samples conformations
ranging from unfolded to partially folded to native-like states and reveals
how a substrate can explore its folding landscape while being bound to a chaperone.
- reference_id: PMID:27239796
supporting_text: we observed that Im76-45 takes on several different conformations
while bound. We found these conformations to be highly heterogeneous and to
include unfolded, partially folded, and native-like states
- term:
id: GO:0042803
label: protein homodimerization activity
evidence_type: IDA
original_reference_id: PMID:20799348
review:
summary: IDA annotation for protein homodimerization based on the crystal structure
of Spy (PMID:20799348), which revealed an antiparallel homodimer with a curved
oval shape. The dimer interface buries approximately 1850 A^2 per monomer, indicating
a stable dimeric assembly.
action: ACCEPT
reason: The crystal structure unambiguously shows Spy as a homodimer (PMID:20799348),
confirmed independently by a second crystal structure (PMID:21317898), size
exclusion chromatography, and analytical ultracentrifugation. Homodimerization
is functionally essential for forming the cradle-shaped substrate binding surface.
This is a core structural feature of Spy.
supported_by:
- reference_id: PMID:20799348
supporting_text: which reveals a long kinked hairpin-like structure of four
Îą-helices that form an antiparallel dimer
- reference_id: PMID:21317898
supporting_text: The crystal structure shows that Spy molecules associate into
tightly bound dimers
- term:
id: GO:0030288
label: outer membrane-bounded periplasmic space
evidence_type: IDA
original_reference_id: PMID:9068658
review:
summary: IDA annotation for periplasmic localization based on the original discovery
of Spy (PMID:9068658). Spy was identified as a new periplasmic protein produced
abundantly in spheroplasts but not in intact cells. The protein contains a signal
peptide (residues 1-23) directing it to the periplasm.
action: ACCEPT
reason: The original identification of Spy as a periplasmic protein (PMID:9068658)
is the foundational localization evidence. The signal peptide cleaves at position
23 to generate the mature periplasmic form. This has been confirmed by multiple
subsequent studies (PMID:9694902, PMID:21317898). UniProt also annotates Spy
to the periplasm with multiple experimental evidence codes.
supported_by:
- reference_id: PMID:9068658
supporting_text: It encodes a precursor of a so far unknown 139-residue, rather
basic periplasmic protein. It was not detectable immunologically in intact
cells but was produced abundantly in spheroplasts.
- term:
id: GO:0042803
label: protein homodimerization activity
evidence_type: IDA
original_reference_id: PMID:21317898
review:
summary: IDA annotation for protein homodimerization from the Spy chaperone discovery
paper (PMID:21317898). The crystal structure, size exclusion chromatography,
and analytical ultracentrifugation all confirmed that Spy is a homodimer with
extensive monomer-monomer contacts burying ~1850 A^2 per monomer.
action: ACCEPT
reason: Independent confirmation of Spy homodimerization using multiple biophysical
methods (crystallography, SEC, AUC) in the landmark chaperone discovery paper.
This is a duplicate of the PMID:20799348 annotation (both correct) and reflects
that two independent groups solved the Spy dimer structure. Homodimerization
is essential for the cradle architecture that enables chaperone function.
supported_by:
- reference_id: PMID:21317898
supporting_text: The crystal structure shows that Spy molecules associate into
tightly bound dimers
- reference_id: PMID:21317898
supporting_text: The contacts between the two monomers are extensive, burying
a surface of âž1850 Ã
2 per monomer upon dimerization and suggesting high dimeric
stability
references:
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods
findings: []
- id: PMID:9068658
title: A new periplasmic protein of Escherichia coli which is synthesized in spheroplasts
but not in intact cells.
findings: []
- id: PMID:20799348
title: The crystal structure Escherichia coli Spy.
findings: []
- id: PMID:21317898
title: Genetic selection designed to stabilize proteins uncovers a chaperone called
Spy.
findings: []
- id: PMID:24497545
title: Super Spy variants implicate flexibility in chaperone action.
findings: []
- id: PMID:26619265
title: Substrate protein folds while it is bound to the ATP-independent chaperone
Spy.
findings: []
- id: PMID:27239796
title: Visualizing chaperone-assisted protein folding.
findings: []
core_functions:
- molecular_function:
id: GO:0044183
label: protein folding chaperone
description: Spy is an ATP-independent periplasmic chaperone that suppresses protein
aggregation and aids protein refolding (PMID:21317898). Remarkably, substrate
Im7 folds completely while remaining bound to Spy's cradle-shaped surface (PMID:26619265).
Global kinetic fitting demonstrated that the only consistent model requires folding
while bound. Structural ensembles captured substrates in unfolded, intermediate,
and native-like conformations on Spy (PMID:27239796). Spy is primarily a holdase
(preventing aggregation in situ without ATP), but also has foldase-like activity
as substrates can fold while bound. The ideal MF annotation would be the proposed
holdase chaperone activity NTR in addition to GO:0044183. GO:0140309 (carrier-holdase)
does NOT fit because Spy acts in situ in the periplasm, not as an inter-compartment
carrier.
directly_involved_in:
- id: GO:0006457
label: protein folding
locations:
- id: GO:0030288
label: outer membrane-bounded periplasmic space
supported_by:
- reference_id: PMID:21317898
supporting_text: In vitro studies demonstrate that the Spy protein is an effective
ATP-independent chaperone that suppresses protein aggregation and aids protein
refolding.
- reference_id: PMID:26619265
supporting_text: Spy then allows Im7 to fully fold into its native state while
it remains bound to the surface of the chaperone.
- reference_id: PMID:27239796
supporting_text: The ensemble shows that Spy-associated Im7 samples conformations
ranging from unfolded to partially folded to native-like states and reveals
how a substrate can explore its folding landscape while being bound to a chaperone.