SPOCK3 (testican-3) is a secreted extracellular matrix proteoglycan that functions as a tumor-suppressive metalloendopeptidase inhibitor. It inhibits membrane-type matrix metalloproteinases (MT1-MMP and MT3-MMP), thereby preventing pro-MMP-2 activation and blocking tumor invasion. The protein contains a modular architecture including follistatin-like domain, acidic region, EC calcium-binding domains, Kazal-like protease inhibitor domain, and thyroglobulin type-1 domain. SPOCK3 carries chondroitin sulfate and heparan sulfate glycosaminoglycan chains and is highly expressed in normal brain, with expression decreasing in gliomas. A splice variant (N-Tes, isoform 2) lacks the thyroglobulin domain but retains MMP inhibitory activity. Unlike testican-2 (SPOCK2), which counteracts MMP inhibition, SPOCK3 shares with testican-1 (SPOCK1) the key function of suppressing MT-MMP-mediated tumor invasion and cell motility through direct interaction with MT-MMPs.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0031012 extracellular matrix | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK3 is a secreted extracellular matrix proteoglycan. UniProt states "Secreted, extracellular space, extracellular matrix" and this is consistent with its function as an ECM-localized MMP inhibitor. Reason: This is a core cellular component annotation for SPOCK3. The protein is secreted into the ECM where it carries out its primary function of inhibiting membrane-type MMPs. This localization is essential for its tumor-suppressive role and is supported by phylogenetic inference (IBA) as well as experimental evidence from PMID:11751414 showing SPOCK3/N-Tes is deposited on collagen and from PMID:12810672 demonstrating ECM deposition. The annotation is further supported by high-throughput proteomics studies (PMID:32055794, PMID:33266304) identifying SPOCK3 in ECM fractions. Supporting Evidence: PMID:11751414 Expression of N-Tes and testican 3 was detected in normal brain but down-regulated in glioma tissues. Transfection of either the N-Tes or testican 3 gene into U251 glioma cells or Madin-Darby canine kidney cells transformed by erbB2 suppressed their invasive growth in collagen gel. PMID:12810672 Testican 2 was not incorporated into collagen by itself, and was deposited only in the presence of N-Tes, suggesting that testican 2 binds to N-Tes deposited on collagen. file:human/SPOCK3/SPOCK3-deep-research-falcon.md model: Edison Scientific Literature |
| GO:0008191 metalloendopeptidase inhibitor activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is THE core molecular function of SPOCK3. Experimentally demonstrated to inhibit MT1-MMP and MT3-MMP, preventing pro-MMP-2 activation. This is the key tumor-suppressive function of SPOCK3. Reason: This is absolutely a core function annotation that must be retained. SPOCK3 was originally characterized as an inhibitor of membrane-type matrix metalloproteinases (PMID:11751414). The paper demonstrates direct interaction of SPOCK3/N-Tes with MT1-MMP and MT3-MMP, inhibiting their ability to activate pro-MMP-2. PMID:12810672 confirms this function is common among testican family members except testican-2. UniProt states "Inhibits the processing of pro-matrix metalloproteinase 2 (MMP-2) by MT1-MMP and MT3-MMP." This metalloendopeptidase inhibitor activity is the mechanistic basis for SPOCK3's tumor suppression and represents its primary molecular function. The IBA annotation is well-supported by experimental evidence. Supporting Evidence: PMID:11751414 Pro-MMP-2 activation by MT3-MMP was also inhibited by the coexpression of N-Tes. Immunoprecipitation analysis demonstrated direct interaction of N-Tes with either MT1-MMP or MT3-MMP. PMID:12810672 The inhibitory function is common among testican family members except for testican 2, which was shown to uniquely abolish inhibition of MT1-MMP- or MT3-MMP-mediated pro-MMP-2 activation by other testican family members. |
| GO:0010810 regulation of cell-substrate adhesion | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK3 regulates cell-substrate adhesion through its interactions with ECM components and its MMP inhibitory activity. The IBA annotation appears to be based on SPOCK1/testican-1 data. Reason: This annotation is reasonable and supported by the biology of SPOCK3. While the direct experimental evidence for SPOCK3 focuses more on cell motility (see GO:2000146), cell-substrate adhesion and cell motility are mechanistically linked processes. PMID:12810672 shows that "Migration of U251 cells on collagen, which was dependent on MT1-MMP activity under serum-free condition, was inhibited by N-Tes or N-Tes-Delta 122 deposited on collagen." This demonstrates SPOCK3 affects cell-substrate interactions on collagen. The IBA annotation likely reflects conserved function across testican family members, particularly with SPOCK1 (testican-1), which has similar ECM and MMP-regulatory functions. Given SPOCK3's role as an ECM proteoglycan that binds collagen and regulates MT-MMP activity, it is reasonable that it regulates cell-substrate adhesion as part of its biological role. Supporting Evidence: PMID:12810672 Migration of U251 cells on collagen, which was dependent on MT1-MMP activity under serum-free condition, was inhibited by N-Tes or N-Tes-Delta 122 deposited on collagen. |
| GO:0005615 extracellular space | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK3 is a secreted protein located in the extracellular space. UniProt annotation confirms "Secreted, extracellular space, extracellular matrix." Reason: This is an accurate cellular component annotation. SPOCK3 has a signal peptide (residues 1-21) and is secreted into the extracellular space where it localizes to the extracellular matrix. This annotation (GO:0005615 extracellular space) is the parent term of GO:0031012 (extracellular matrix), so both annotations are correct and complementary. The IBA annotation is supported by phylogenetic inference across testican family members, all of which are secreted ECM proteins. UniProt explicitly states the protein is "Secreted, extracellular space, extracellular matrix." Supporting Evidence: file:human/SPOCK3/SPOCK3-uniprot.txt SUBCELLULAR LOCATION: Secreted, extracellular space, extracellular matrix. |
| GO:0005509 calcium ion binding | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK3 contains EC (extracellular) calcium-binding domains. The protein is described in PMID:11751414 as a "calcium-binding proteoglycan" and the N-Tes variant includes the Ca2+-binding domain. Reason: This is a well-supported molecular function annotation. SPOCK3 is explicitly described as a "calcium-binding proteoglycan" in PMID:11751414. The UniProt entry indicates SPOCK3 contains EC calcium-binding domains (InterPro:IPR019577), which are characteristic of the SPARC/testican family. The protein structure includes EF-hand calcium-binding motifs. PMID:12810672 demonstrates that testican-2 inactivates N-Tes "by binding to the COOH-terminal extracellular calcium-binding domain of N-Tes," confirming this domain is present and functionally important. The IBA annotation reflects the conserved calcium-binding capability across all testican family members. Supporting Evidence: PMID:11751414 It encodes the NH(2)-terminal 313-amino acid region of a calcium-binding proteoglycan, testican 3, with a 3-amino acid substitution at the COOH terminus and thus was named N-Tes. N-Tes comprises a signal peptide, a unique domain, a follistatin-like domain, and a Ca(2+)-binding domain PMID:12810672 Testican 2 inactivates N-Tes by binding to the COOH-terminal extracellular calcium-binding domain of N-Tes through its NH(2)-terminal unique domain |
| GO:0004857 enzyme inhibitor activity | IEA GO_REF:0000043 | ACCEPT | Summary: This is a broad parent term of the more specific GO:0008191 (metalloendopeptidase inhibitor activity). SPOCK3 does have enzyme inhibitor activity via its MMP inhibition. Reason: This IEA annotation from UniProtKB keyword mapping is correct but very general. GO:0004857 (enzyme inhibitor activity) is a broad molecular function term that encompasses GO:0008191 (metalloendopeptidase inhibitor activity) and GO:0030414 (peptidase inhibitor activity). Since SPOCK3 is experimentally confirmed to inhibit metalloendopeptidases (MT1-MMP, MT3-MMP), this parent term is accurate. The annotation is less informative than the more specific child terms, but it is not incorrect. It's acceptable to retain both general and specific terms in GO annotation to capture the full hierarchy. Supporting Evidence: PMID:11751414 Using expression cloning to screen a human fetal kidney cDNA library for regulator(s) of pro-matrix metalloproteinase (MMP)-2 processing mediated by membrane-type (MT) 1 MMP, we isolated a cDNA whose product interfered with pro-MMP-2 activation. |
| GO:0005509 calcium ion binding | IEA GO_REF:0000002 | ACCEPT | Summary: Duplicate annotation of GO:0005509 but from InterPro domain mapping (IPR019577 SPARC/Testican Ca-binding domain). Same function, different evidence source. Reason: This is a duplicate GO ID annotation but from a different evidence pipeline (IEA from InterPro vs IBA from phylogenetic inference). Both are correct. The IEA annotation is based on the presence of InterPro domain IPR019577 (SPARC/Testican_Ca-bd-dom), which maps to calcium ion binding function. Having multiple evidence codes for the same function from independent sources (domain-based inference and phylogenetic inference) strengthens confidence in the annotation. This is standard practice in GO annotation - duplicates with different evidence codes are acceptable and informative. Supporting Evidence: file:human/SPOCK3/SPOCK3-uniprot.txt InterPro IPR019577 SPARC/Testican_Ca-bd-dom |
| GO:0008191 metalloendopeptidase inhibitor activity | IEA GO_REF:0000117 | ACCEPT | Summary: Another duplicate of GO:0008191, this time from ARBA machine learning models. Same core function annotation as the IBA version reviewed above. Reason: This is a duplicate annotation of GO:0008191 (metalloendopeptidase inhibitor activity) but from ARBA (Association Rule-Based Annotator) machine learning models. The same GO term appears with IBA evidence code (phylogenetic) reviewed earlier. Having multiple independent evidence sources (IBA phylogenetic inference, IEA from ARBA machine learning, and TAS experimental from PMID:12810672) all converging on the same core function provides very strong support for this annotation. This metalloendopeptidase inhibitor activity is THE defining molecular function of SPOCK3. Multiple evidence codes for the same critical function are valuable and appropriate. Supporting Evidence: PMID:11751414 Pro-MMP-2 activation by MT3-MMP was also inhibited by the coexpression of N-Tes. Immunoprecipitation analysis demonstrated direct interaction of N-Tes with either MT1-MMP or MT3-MMP. |
| GO:0030414 peptidase inhibitor activity | IEA GO_REF:0000043 | ACCEPT | Summary: Parent term of metalloendopeptidase inhibitor activity. SPOCK3 contains Kazal-like and thyroglobulin type-1 domains which are protease inhibitor domains. Reason: This is an accurate annotation based on UniProtKB keyword mapping to "Protease inhibitor" and "Metalloprotease inhibitor" keywords. GO:0030414 (peptidase inhibitor activity) is the parent term of GO:0008191 (metalloendopeptidase inhibitor activity), which is more specific. Since SPOCK3 inhibits metallopeptidases (a subclass of peptidases), the broader peptidase inhibitor activity term is also correct. Additionally, SPOCK3 contains a Kazal-like domain (residues 133-185) and a thyroglobulin type-1 domain (residues 314-380), both of which are protease inhibitor domains found in the MEROPS database (I01.980 and I31.007). This structural evidence supports the general peptidase inhibitor activity annotation. Supporting Evidence: file:human/SPOCK3/SPOCK3-uniprot.txt DOMAIN 133..185 Kazal-like [PROSITE-ProRule:PRU00798]; DOMAIN 314..380 Thyroglobulin type-1 [PROSITE-ProRule:PRU00500]; MEROPS I01.980 [Kazal]; MEROPS I31.007 [thyroglobulin] |
| GO:0005539 glycosaminoglycan binding | IEA GO_REF:0000107 | ACCEPT | Summary: SPOCK3 is a proteoglycan that carries chondroitin sulfate and heparan sulfate GAG chains. The annotation is inferred from mouse ortholog. Reason: This annotation is correct based on SPOCK3's nature as a proteoglycan. UniProt states "Contains chondroitin sulfate and heparan sulfate O-linked oligosaccharides" and indicates putative GAG attachment sites at Ser387 and Ser392. As a proteoglycan, SPOCK3 has glycosaminoglycan (GAG) chains covalently attached to its protein core. These GAG chains can interact with other molecules, giving the protein glycosaminoglycan binding capability. The IEA annotation is transferred from the mouse ortholog (Spock3, UniProtKB:Q8BKV0) via Ensembl Compara. While this is computational inference, it is well-supported by SPOCK3's biochemical nature as a heparan/chondroitin sulfate proteoglycan, which is consistent across species. Supporting Evidence: file:human/SPOCK3/SPOCK3-uniprot.txt PTM: Contains chondroitin sulfate and heparan sulfate O-linked oligosaccharides. [ECO:0000250]; CARBOHYD 387 O-linked (Xyl...) (glycosaminoglycan) serine [ECO:0000255]; CARBOHYD 392 O-linked (Xyl...) (glycosaminoglycan) serine [ECO:0000255] |
| GO:0031012 extracellular matrix | HDA PMID:32055794 3D mapping of native extracellular matrix reveals cellular r... | ACCEPT | Summary: Third annotation of GO:0031012 ECM localization, this time from high-throughput proteomics study mapping native ECM in 3D. Reason: This is another duplicate of GO:0031012 (extracellular matrix) but with HDA (High-throughput Direct Assay) evidence from a proteomics study. PMID:32055794 is a large-scale study titled "3D mapping of native extracellular matrix reveals cellular responses to the microenvironment" that systematically identifies ECM proteins in their native context. The identification of SPOCK3 in this high-throughput ECM proteomics dataset provides independent experimental support for ECM localization beyond the phylogenetic (IBA) inference. Having IBA, HDA, and literature evidence all supporting ECM localization strengthens confidence in this core cellular component annotation for SPOCK3. Supporting Evidence: PMID:32055794 3D mapping of native extracellular matrix reveals cellular responses to the microenvironment. |
| GO:0031012 extracellular matrix | HDA PMID:33266304 Divide-and-Conquer Matrisome Protein (DC-MaP) Strategy: An M... | ACCEPT | Summary: Fourth annotation of GO:0031012 ECM localization, from another high-throughput proteomics study using divide-and-conquer approach for matrisome characterization. Reason: This is yet another duplicate of GO:0031012 (extracellular matrix) with HDA evidence from a different proteomics study. PMID:33266304 titled "Divide-and-Conquer Matrisome Protein (DC-MaP) Strategy - An MS-Friendly Approach to Proteomic Matrisome Characterization" describes a systematic approach to characterize the complete matrisome (all ECM and ECM-associated proteins). SPOCK3's identification in this independent high-throughput matrisome study provides additional experimental validation of its ECM localization. Multiple HDA annotations from different proteomics studies (PMID:32055794 and PMID:33266304) plus phylogenetic inference (IBA) provide very strong, convergent evidence for SPOCK3's ECM localization. Supporting Evidence: PMID:33266304 Divide-and-Conquer Matrisome Protein (DC-MaP) Strategy: An MS-Friendly Approach to Proteomic Matrisome Characterization. |
| GO:0008191 metalloendopeptidase inhibitor activity | TAS PMID:12810672 Testican 2 abrogates inhibition of membrane-type matrix meta... | ACCEPT | Summary: Third annotation of GO:0008191 metalloendopeptidase inhibitor activity, this time with TAS (Traceable Author Statement) evidence from key experimental paper. Reason: This is the third annotation of GO:0008191 (metalloendopeptidase inhibitor activity), now with TAS evidence code citing PMID:12810672. This is one of the key experimental papers characterizing SPOCK3's MMP inhibitory function. The paper states "The inhibitory function is common among testican family members except for testican 2" and demonstrates that SPOCK3 (along with testican-1 and N-Tes) inhibits MT1-MMP and MT3-MMP mediated pro-MMP-2 activation. Having TAS experimental evidence in addition to IBA (phylogenetic) and IEA (computational) annotations provides the strongest possible support for this core molecular function. This is THE defining activity of SPOCK3 and is crucial for its tumor-suppressive role. Supporting Evidence: PMID:12810672 The inhibitory function is common among testican family members except for testican 2, which was shown to uniquely abolish inhibition of MT1-MMP- or MT3-MMP-mediated pro-MMP-2 activation by other testican family members. |
| GO:2000146 negative regulation of cell motility | TAS PMID:12810672 Testican 2 abrogates inhibition of membrane-type matrix meta... | ACCEPT | Summary: SPOCK3 negatively regulates cell motility through its MMP inhibitory activity. This is a key tumor-suppressive biological process. Reason: This is an important biological process annotation with TAS experimental evidence from PMID:12810672. The paper demonstrates that "Migration of U251 cells on collagen, which was dependent on MT1-MMP activity under serum-free condition, was inhibited by N-Tes or N-Tes-Delta 122 deposited on collagen." PMID:11751414 also shows that "Transfection of either the N-Tes or testican 3 gene into U251 glioma cells or Madin-Darby canine kidney cells transformed by erbB2 suppressed their invasive growth in collagen gel." These data clearly demonstrate SPOCK3's role in negatively regulating cell motility, which is mechanistically linked to its metalloendopeptidase inhibitor activity. By inhibiting MT-MMPs, SPOCK3 prevents cells from degrading ECM components necessary for migration, thereby blocking cell motility and tumor invasion. This is a core biological process function. Supporting Evidence: PMID:12810672 Migration of U251 cells on collagen, which was dependent on MT1-MMP activity under serum-free condition, was inhibited by N-Tes or N-Tes-Delta 122 deposited on collagen. PMID:11751414 Transfection of either the N-Tes or testican 3 gene into U251 glioma cells or Madin-Darby canine kidney cells transformed by erbB2 suppressed their invasive growth in collagen gel. |
| GO:0005518 collagen binding | IBA GO_REF:0000033 | NEW | Summary: SPOCK3 binds to collagen in the ECM. This annotation is present in UniProt GO lines but missing from the GOA file reviewed above. Reason: This annotation is present in the UniProt entry (line 284 - "GO:0005518 F:collagen binding; IBA:GO_Central") but was not included in the GOA file provided for review. SPOCK3's collagen binding is experimentally supported by PMID:12810672, which demonstrates that N-Tes is "deposited on collagen" and that cells can migrate on "collagen" where N-Tes is present. PMID:11751414 shows that SPOCK3 expression "suppressed their invasive growth in collagen gel," indicating interaction with collagen. As an ECM proteoglycan, SPOCK3 binds to collagen fibers where it exerts its MMP inhibitory activity to regulate cell motility. This is a core molecular function that should be included in the annotation set. Supporting Evidence: PMID:12810672 Testican 2 was not incorporated into collagen by itself, and was deposited only in the presence of N-Tes, suggesting that testican 2 binds to N-Tes deposited on collagen. Binding of testican 2 to N-Tes deposited on collagen allowed migration of cells expressing MT1-MMP. PMID:11751414 Transfection of either the N-Tes or testican 3 gene into U251 glioma cells or Madin-Darby canine kidney cells transformed by erbB2 suppressed their invasive growth in collagen gel. |
| GO:0050840 extracellular matrix binding | IBA GO_REF:0000033 | NEW | Summary: SPOCK3 binds to ECM components. This annotation is present in UniProt GO lines but missing from the GOA file reviewed above. Reason: This annotation is present in the UniProt entry (line 285 - "GO:0050840 F:extracellular matrix binding; IBA:GO_Central") but was not in the GOA file. As an ECM proteoglycan, SPOCK3 binds to various ECM components including collagen (GO:0005518). GO:0050840 (extracellular matrix binding) is the parent term of collagen binding and represents SPOCK3's broader capacity to interact with ECM molecules. The experimental evidence shows SPOCK3 is deposited on and interacts with collagen matrices (PMID:12810672, PMID:11751414). This ECM binding function is essential for SPOCK3 to localize properly and exert its MMP inhibitory and cell motility regulatory functions in the extracellular space. Supporting Evidence: PMID:12810672 N-Tes deposited on collagen PMID:11751414 suppressed their invasive growth in collagen gel |
| GO:0010951 negative regulation of endopeptidase activity | IDA PMID:11751414 Suppression of membrane-type 1 matrix metalloproteinase (MMP... | NEW | Summary: SPOCK3 negatively regulates endopeptidase activity by inhibiting MT-MMPs. This annotation is present in UniProt GO lines but missing from the GOA file. Reason: This annotation is present in the UniProt entry (line 289 - "GO:0010951 P:negative regulation of endopeptidase activity; IDA:UniProtKB") but was not in the GOA file. This is a biological process term describing the regulatory role of SPOCK3's molecular function (metalloendopeptidase inhibitor activity). PMID:11751414 provides direct experimental evidence that SPOCK3/N-Tes "interfered with pro-MMP-2 activation" by inhibiting the endopeptidase activity of MT1-MMP and MT3-MMP. This is a more specific biological process term than just "negative regulation of cell motility" and directly describes the mechanistic process by which SPOCK3 acts as an MMP inhibitor. This should be included as it captures the regulatory aspect of SPOCK3's core function. Supporting Evidence: PMID:11751414 Using expression cloning to screen a human fetal kidney cDNA library for regulator(s) of pro-matrix metalloproteinase (MMP)-2 processing mediated by membrane-type (MT) 1 MMP, we isolated a cDNA whose product interfered with pro-MMP-2 activation. [...] Pro-MMP-2 activation by MT3-MMP was also inhibited by the coexpression of N-Tes. |
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