Spy (spheroplast protein Y) is a stress-induced, ATP-independent chaperone in the Escherichia coli periplasm. Its elongated antiparallel homodimer forms a flexible cradle that binds non-native client proteins, suppresses their aggregation, and increases productive refolding. Spy is unusual in allowing the model client Im7 to traverse unfolded, intermediate, and native conformations while remaining associated with the chaperone surface. Bae and Cpx envelope-stress pathways strongly induce Spy during conditions such as tannin, butanol, and ethanol exposure.
Definition: Binding to an unfolded or misfolded protein to prevent its aggregation without actively catalyzing refolding. The holdase maintains the client protein in a soluble, folding-competent state.
Justification: Spy directly prevents aggregation of non-native clients in the periplasm. Obsolete GO:0051082 captures binding only, while carrier-specific GO:0140309 requires escort to an acceptor or location not demonstrated for Spy. Spy's separate ability to support folding is represented by GO:0044183.
Parent term: molecular_function
Supporting Evidence:
| 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). 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. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002445564 Β· PANTHER:PTN002445564 SUPPORTS TRANSFER Current PTHR38102 PAINT retains GO:0030288 at this ancestral node, seeded by direct localization evidence on Spy and CpxP. UniProtKB:P0AE85 Β· CpxP SUPPORTS TRANSFER CpxP is a periplasmic member of the same family and supports the ancestral localization assertion. UniProtKB:P77754 Β· Spy SUPPORTS TRANSFER Spy's own experimental localization is valid descendant evidence for the node placement and is not circular propagation. 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 now formally obsolete 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. 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. Propagation Review Root cause: SOURCE STALE OR MISSING Sources checked: PANTHER:PTN002445564 Β· PANTHER:PTN002445564 SOURCE STALE OR MISSING Current PTHR38102 PAINT retains only GO:0030288 at this node; the historical GO:0051082 node assertion is absent. UniProtKB:P0AE85 Β· CpxP SOURCE STALE OR MISSING P0AE85 occurs in the historical WITH/FROM, but current PAINT no longer exposes GO:0051082 at the node. UniProtKB:P77754 Β· Spy SOURCE STALE OR MISSING P77754 occurs in the historical WITH/FROM, but current PAINT no longer exposes GO:0051082 at the node. Spy's direct holdase evidence supports the biological reason below, not a current propagation claim. Proposed replacements: holdase chaperone activity 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 At high Spy concentrations, the vast majority of these unfolded proteins are bound to Spy, preventing their aggregation. |
| 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). 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... | KEEP AS NON CORE | 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: The IPI correctly records a directly observed Spy-Im7 interaction, with Q03708 as the exact interacting entity, and the full cached article provides quantitative binding measurements. GO:0005515 is nevertheless uninformative about the chaperone mechanism, so it is retained as non-core rather than promoted as a function or forced into a single replacement term. Supporting Evidence: PMID:26619265 We found that Spy binds to all three variants of Im7, with affinities of 10.4 ΞΌM, 3.5 ΞΌM, and 20.5 ΞΌM 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 ... | MODIFY | 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 refolding 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. GO:0006457 is correct but too general; GO:0042026 protein refolding more precisely captures the demonstrated recovery of unfolded clients without implying a distinct catalytic chemistry. Proposed replacements: protein refolding 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. file:ECOLI/Spy/Spy-deep-research-falcon.md Spy is best defined as a **stress-induced periplasmic chaperone** that binds unfolded or unstable proteins, **suppresses aggregation**, and can **promote refolding** without ATP. |
| 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 now formally obsolete. 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. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:26619265 We found that Spy binds to all three variants of Im7, with affinities of 10.4 ΞΌM, 3.5 ΞΌM, and 20.5 ΞΌM 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 ΞΌs) |
| 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 now formally obsolete and Spy is an in-situ holdase, not a carrier. Proposed replacements: holdase chaperone activity 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. | KEEP AS NON CORE | 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), with independent structural and solution-biophysical support from PMID:21317898. Dimerization supplies the cradle architecture used in mechanistic models, but it is a structural property supporting Spy's chaperone activity rather than the molecular function itself, so it is retained as non-core. Supporting Evidence: PMID:20799348 which reveals a long kinked hairpin-like structure of four Ξ±-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 subsequent work (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 ... | KEEP AS NON CORE | 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: Crystallography, size-exclusion chromatography, and analytical ultracentrifugation independently confirm the Spy homodimer. This row is correct and complements the PMID:20799348 structure, but homodimerization is supporting architecture for the chaperone surface rather than Spy's core molecular function; it is therefore retained as non-core. 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 βΌ1850 Γ
2 per monomer upon dimerization and suggesting high dimeric stability |
| GO:0050821 protein stabilization | IDA PMID:21317898 Genetic selection designed to stabilize proteins uncovers a ... | NEW | Summary: Spy directly suppresses aggregation of chemically and thermally unfolded clients and increases soluble accumulation of unstable Im7 variants. Reason: Direct in vitro aggregation-suppression assays and in vivo stabilization of unstable periplasmic clients establish involvement in protein stabilization beyond the broader folding-process annotation. 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:21317898 it massively overproduced a periplasmic protein called Spy, which increases the steady-state levels of a set of unstable protein mutants up to 700-fold |
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Download this section (compressed HTML)Q: Which native periplasmic proteins are the principal Spy clients under each Bae- or Cpx-inducing stress, rather than model substrates used in vitro?
Q: Does Spy ever hand a client to a defined downstream acceptor, or do clients fold and dissociate without a directed delivery step?
Experiment: Use substrate-binding and release-altered Spy variants with in vivo periplasmic crosslinking during distinct envelope stresses to identify direct clients and measure their aggregation and recovery.
Type: native-client trapping proteomics
Experiment: Monitor conformational state, Spy occupancy, aggregation, and release for native periplasmic clients to separate in-situ holdase protection from folding assistance and test for any defined acceptor-mediated handoff.
Type: single-client folding and release kinetics
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