SPOCK2 (testican-2) is a secreted calcium-binding proteoglycan highly expressed in brain that contains chondroitin sulfate and heparan sulfate glycosaminoglycan chains. It has a modular domain structure comprising a follistatin-like domain, EC calcium-binding domain, Kazal-like domain, and thyroglobulin type-1 domain. Unlike other testican family members (testican-1/SPOCK1 and testican-3/SPOCK3) which inhibit membrane-type matrix metalloproteinases (MT-MMPs), testican-2 uniquely abrogates this inhibition by binding to and inactivating other testican proteins, thereby promoting cell motility and ECM remodeling. Calcium binding was experimentally demonstrated through reversible calcium-dependent conformational changes. SPOCK2 participates in neurogenesis and is widely expressed in multiple neuronal cell types throughout the CNS, with additional expression in lung and testis. Core functions include ECM organization, calcium ion binding, modulation of cell-substrate adhesion, and regulation of metalloendopeptidase activity through a unique mechanism of counter-inhibition.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0031012 extracellular matrix | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK2 is a secreted proteoglycan that localizes to the extracellular matrix. UniProt annotation confirms "Secreted, extracellular space, extracellular matrix" and PMID:10386950 describes it as an ECM proteoglycan. This is a core cellular component annotation for SPOCK2. Reason: Well-supported ECM localization by both phylogenetic inference (IBA) and experimental evidence. This represents the primary subcellular localization of this secreted proteoglycan and is essential for its biological function. Supporting Evidence: PMID:10386950 We have screened a human cDNA library using an expressed sequence tag related to the BM-40/secreted protein, acidic and rich in cysteine (SPARC)/osteonectin family of proteins and isolated a novel cDNA. It encodes a protein precursor of 424 amino acids that consists of a signal peptide, a follistatin-like domain, a Ca2+-binding domain, a thyroglobulin-like domain, and a C-terminal region with two putative glycosaminoglycan attachment sites. file:human/SPOCK2/SPOCK2-deep-research-falcon.md model: Edison Scientific Literature |
| GO:0008191 metalloendopeptidase inhibitor activity | IBA GO_REF:0000033 | MODIFY | Summary: This annotation requires careful consideration. While SPOCK2 contains a Kazal-like domain (known for protease inhibition), PMID:12810672 shows that testican-2 uniquely LACKS the MT-MMP inhibitory activity of other testican family members. Instead, it acts as a counter-inhibitor by binding to and inactivating other testicans that DO inhibit metalloendopeptidases. The Kazal domain may retain some direct inhibitory activity, but the dominant biological function is abrogating inhibition by other testicans. Reason: The term is misleading because it suggests direct metalloendopeptidase inhibition as the primary function. However, PMID:12810672 demonstrates that SPOCK2's unique role is to ABOLISH inhibition of MT-MMPs by binding to other testican family members. A more accurate annotation would be "negative regulation of endopeptidase activity" or "regulation of metalloendopeptidase activity" to capture this counter-regulatory function. Proposed replacements: negative regulation of endopeptidase activity endopeptidase regulator activity 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. 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:0010810 regulation of cell-substrate adhesion | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK2 modulates cell-substrate adhesion through its effects on the ECM and by regulating MT-MMP activity. PMID:12810672 demonstrates that testican-2 promotes cell motility on collagen by abrogating the inhibitory effects of other testican family members on MT1-MMP, which is essential for cell migration on ECM substrates. This represents a core regulatory function. Reason: Well-supported by phylogenetic inference and experimental evidence showing that SPOCK2 regulates cell-substrate interactions through modulation of MT-MMP activity. Cell migration on collagen (cell-substrate adhesion) was directly demonstrated to depend on testican-2's counter-inhibitory 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. |
| GO:0005615 extracellular space | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK2 is a secreted protein that localizes to the extracellular space, specifically to the extracellular matrix. UniProt confirms secretion and ECM localization. This is accurate but somewhat redundant with the more specific GO:0031012 (extracellular matrix) annotation. Reason: Correct localization annotation supported by phylogenetic inference and direct evidence. While broader than "extracellular matrix", it is still accurate and represents the primary cellular compartment where SPOCK2 functions. Both annotations are valid. |
| GO:0005509 calcium ion binding | IBA GO_REF:0000033 | ACCEPT | Summary: SPOCK2 contains EC (extracellular) calcium-binding domains and calcium binding was experimentally demonstrated by PMID:10386950 using circular dichroism spectroscopy showing reversible calcium-dependent conformational changes. This is a core molecular function annotation with strong experimental support. Reason: Calcium ion binding is experimentally verified (IDA in PMID:10386950) and supported by phylogenetic inference. The protein contains canonical EF-hand calcium-binding domains and calcium binding is essential for its structural conformation and interactions with other testican family members. Supporting Evidence: PMID:10386950 A recombinant fragment consisting of the Ca2+-binding EF-hand domain and the thyroglobulin-like domain of testican-2 showed a reversible Ca2+-dependent conformational change in circular dichroism studies. |
| GO:0005509 calcium ion binding | IEA GO_REF:0000002 | ACCEPT | Summary: Duplicate annotation for calcium ion binding based on InterPro domain assignment (IPR019577 SPARC/Testican Ca-binding domain). This is redundant with the IBA and IDA annotations for the same term but provides independent computational support. Reason: While duplicate, this IEA annotation from InterPro domain analysis provides additional independent evidence for calcium binding function. It is acceptable to have multiple evidence codes supporting the same well-established function. |
| GO:0008191 metalloendopeptidase inhibitor activity | IEA GO_REF:0000117 | MODIFY | Summary: This IEA annotation from ARBA machine learning is based on the Kazal-like domain, which typically confers protease inhibitor activity. However, functional studies show that testican-2 LACKS the MT-MMP inhibitory activity characteristic of other testican family members and instead acts as a counter-inhibitor. Reason: Same issue as the IBA annotation for GO:0008191. The computational prediction is reasonable based on domain content but contradicts experimental evidence showing SPOCK2's unique counter-inhibitory function. Should be replaced with regulatory terms. Proposed replacements: negative regulation of endopeptidase activity endopeptidase regulator activity 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:0005539 glycosaminoglycan binding | IEA GO_REF:0000107 | MODIFY | Summary: SPOCK2 IS a proteoglycan with chondroitin sulfate and heparan sulfate GAG chains attached. However, "glycosaminoglycan binding" suggests binding to GAGs as ligands rather than CARRYING GAGs as post-translational modifications. UniProt states SPOCK2 "Contains chondroitin sulfate and heparan sulfate O-linked oligosaccharides" indicating it IS a GAG-modified proteoglycan. Reason: The term is misleading. SPOCK2 is not primarily a GAG-binding protein but rather a GAG-carrying proteoglycan. The more appropriate annotation would be related to proteoglycan core protein function or ECM structural molecule activity. While SPOCK2 may have some GAG-binding activity, the dominant characteristic is being a proteoglycan itself. Proposed replacements: extracellular matrix structural constituent |
| GO:0050840 extracellular matrix binding | IEA GO_REF:0000107 | ACCEPT | Summary: SPOCK2 is an ECM proteoglycan that likely interacts with other ECM components. PMID:12810672 shows testican-2 binds to N-Tes (testican-3) deposited on collagen, demonstrating ECM protein-protein interactions. As an ECM structural component itself, SPOCK2 would be expected to bind to ECM. Reason: Reasonable inference from orthology. SPOCK2 is an ECM proteoglycan that interacts with other ECM proteins including other testican family members and likely collagen and other matrix components. This represents a legitimate molecular function even though it is computationally predicted. 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. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding annotation from a high-throughput interactome study showing interaction with ENKD1 (Q9H0I2). While technically correct, this provides minimal functional information as most proteins bind other proteins. The more informative binding activities are the specific interactions with other testican family members. Reason: Following curation guidelines: "Avoid the term protein binding, this doesn't tell us anything about the actual function." This is a generic annotation from a binary interactome screen that provides no meaningful functional insight. More specific binding annotations (ECM binding, interactions with testicans) are more informative. Supporting Evidence: PMID:32296183 Apr 8. A reference map of the human binary protein interactome. |
| GO:0008191 metalloendopeptidase inhibitor activity | TAS PMID:12810672 Testican 2 abrogates inhibition of membrane-type matrix meta... | MODIFY | Summary: This TAS annotation from PMID:12810672 is actually CONTRADICTED by the same paper. The paper explicitly states that testican-2 LACKS the inhibitory function common to other testican family members and instead ABOLISHES inhibition of MT-MMPs. This appears to be a mis-curation. Reason: The cited paper PMID:12810672 demonstrates the opposite - that testican-2 uniquely lacks MT-MMP inhibitory activity and instead acts as a counter-inhibitor. This is a clear example of mis-annotation where the evidence actually contradicts the assigned term. Should be replaced with regulatory terms reflecting the actual function. Proposed replacements: negative regulation of endopeptidase activity positive regulation of cell motility 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:2000147 positive regulation of cell motility | TAS PMID:12810672 Testican 2 abrogates inhibition of membrane-type matrix meta... | ACCEPT | Summary: PMID:12810672 directly demonstrates that testican-2 promotes cell migration on collagen by abrogating the inhibitory effects of other testican family members on MT1-MMP activity. This is a well-supported core biological process annotation. Reason: Strong experimental support from PMID:12810672 showing that SPOCK2 positively regulates cell motility through its unique counter-inhibitory mechanism. Cell migration assays demonstrated that testican-2 expression allowed migration of MT1-MMP-expressing cells on collagen that was otherwise blocked by other testicans. 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:0005509 calcium ion binding | IDA PMID:10386950 Molecular cloning of testican-2: defining a novel calcium-bi... | ACCEPT | Summary: This is the gold-standard experimental demonstration of calcium binding by SPOCK2 using circular dichroism spectroscopy. PMID:10386950 showed reversible calcium-dependent conformational changes in a recombinant fragment containing the EF-hand calcium-binding domain. This is a third annotation for the same term with direct experimental evidence. Reason: Strong experimental evidence (IDA) from the original characterization paper. This represents direct biochemical demonstration of calcium binding activity and is a core molecular function. While duplicate with other calcium binding annotations, this has the strongest evidence code (IDA) and should be retained. Supporting Evidence: PMID:10386950 A recombinant fragment consisting of the Ca2+-binding EF-hand domain and the thyroglobulin-like domain of testican-2 showed a reversible Ca2+-dependent conformational change in circular dichroism studies. |
| GO:0007416 synapse assembly | NAS PMID:10386950 Molecular cloning of testican-2: defining a novel calcium-bi... | KEEP AS NON CORE | Summary: PMID:10386950 states that testican-2 has "the potential to participate in diverse steps of neurogenesis" based on its brain expression pattern and domain structure, but does not provide direct evidence for synapse assembly. This is inferred from expression in neurons and similarity to other neuronal ECM proteins. The evidence is quite indirect. Reason: While SPOCK2 is highly expressed in brain and may contribute to neurogenesis, there is no direct experimental evidence for synapse assembly specifically. The NAS evidence code indicates this is inferred from expression pattern and general neurogenesis potential. This represents a plausible but peripheral function compared to the core ECM/proteoglycan and cell motility regulation activities that are experimentally demonstrated. Supporting Evidence: PMID:10386950 Testican-1 and -2 form a novel Ca2+-binding proteoglycan family built of modular domains with the potential to participate in diverse steps of neurogenesis. |
| GO:0030198 extracellular matrix organization | NAS PMID:10386950 Molecular cloning of testican-2: defining a novel calcium-bi... | ACCEPT | Summary: As an ECM proteoglycan, SPOCK2 is a structural component of the extracellular matrix and contributes to ECM organization. PMID:10386950 characterizes it as an ECM proteoglycan with modular domains typical of ECM structural proteins. PMID:12810672 shows it modulates ECM-mediated cell migration. This is a core biological process. Reason: Well-supported core function. SPOCK2 is an ECM structural proteoglycan that participates in ECM organization both as a structural component and through regulation of ECM remodeling via its effects on MT-MMP activity. While the NAS evidence from PMID:10386950 is based on structure and localization, the function is further supported by PMID:12810672 showing ECM-dependent effects. Supporting Evidence: PMID:10386950 We have screened a human cDNA library using an expressed sequence tag related to the BM-40/secreted protein, acidic and rich in cysteine (SPARC)/osteonectin family of proteins [ECM proteins] PMID:12810672 Testican family proteins are putative extracellular heparan/chondroitin sulfate proteoglycans |
| GO:0031012 extracellular matrix | NAS PMID:10386950 Molecular cloning of testican-2: defining a novel calcium-bi... | ACCEPT | Summary: Duplicate annotation for extracellular matrix localization with NAS evidence from PMID:10386950. This is the same term as the IBA annotation reviewed earlier. Both are correct and supported. Reason: Correct ECM localization annotation with NAS evidence from the original characterization paper. Duplicate of the IBA annotation but both are valid. Multiple evidence codes for the same well-established localization are acceptable. Supporting Evidence: PMID:10386950 Molecular cloning of testican-2: defining a novel calcium-binding proteoglycan family expressed in brain. |
| GO:0045595 regulation of cell differentiation | NAS PMID:10386950 Molecular cloning of testican-2: defining a novel calcium-bi... | KEEP AS NON CORE | Summary: PMID:10386950 mentions "the potential to participate in diverse steps of neurogenesis" which would include regulation of neuronal cell differentiation. However, this is a very broad inference based on brain expression pattern and domain structure without direct experimental evidence for regulating differentiation. Reason: While plausible given SPOCK2's role in neurogenesis and brain development, there is no direct experimental evidence for regulation of cell differentiation. The annotation is based on inference from expression pattern and general neurogenesis potential (NAS evidence). This is a peripheral function compared to the experimentally demonstrated core activities in ECM organization and cell motility regulation. Supporting Evidence: PMID:10386950 Testican-1 and -2 form a novel Ca2+-binding proteoglycan family built of modular domains with the potential to participate in diverse steps of neurogenesis. |
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