IBSP encodes bone sialoprotein, a secreted, highly phosphorylated, sulfated, and glycosylated SIBLING-family protein of mineralized extracellular matrices. Acidic regions bind hydroxyapatite and influence mineral crystal formation, while a C-terminal Arg-Gly-Asp motif binds alpha-V-containing integrins to support cell attachment and migration. IBSP is produced chiefly by mature osteoblast-lineage cells and osteoclasts and is deposited in bone, dentin, and cementum, where it contributes to matrix organization, primary ossification, and bone mineralization. Outside the skeleton, integrin engagement can promote tumor-cell migration, and IBSP-bound complement factor H can protect attached cells from complement-mediated lysis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030198 extracellular matrix organization | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference assigns IBSP a conserved role in extracellular matrix organization. Reason: IBSP is directly deposited as a major noncollagenous component of human bone matrix, and mouse knockout disrupts bone formation and mineralization. These independent data validate the conserved annotation. Supporting Evidence: PMID:1818768 BSP is primarily an osteoblast-derived component of the bone matrix expressed at late stages of differentiation. file:human/IBSP/IBSP-deep-research-manual.md IBSP is best understood as a mineralized-matrix ligand with two coupled molecular activities: hydroxyapatite binding and RGD-dependent integrin binding. |
| GO:0030282 bone mineralization | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference assigns a conserved bone-mineralization role. Reason: Human BSP directly binds hydroxyapatite, and Ibsp-null mice have reduced cortical mineral density and delayed primary mineralization. This is a central, evolutionarily conserved process. Supporting Evidence: PMID:11459848 The affinity for hydroxyapatite was higher for bone-derived BSP than for recombinant BSP. PMID:24816232 lack of BSP alters long bone growth and membranous/cortical primary bone formation and mineralization. |
| GO:0001503 ossification | IEA GO_REF:0000002 | MODIFY | Summary: InterPro transfers the broad ossification process from the IBSP family. Reason: The biological essence is correct, but bone mineralization is more precise for the direct hydroxyapatite-binding function and conserved knockout phenotype. The broader ossification term obscures that specificity. Proposed replacements: bone mineralization Supporting Evidence: PMID:24816232 lack of BSP alters long bone growth and membranous/cortical primary bone formation and mineralization. |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: Combined automated methods place signal-peptide-containing IBSP extracellularly. Reason: The annotation is correct: the reviewed sequence contains a signal peptide, and direct human tissue localization places mature IBSP in bone extracellular matrix. The broad term remains useful across skeletal, placental, and tumor contexts. Supporting Evidence: PMID:1818768 BSP is primarily an osteoblast-derived component of the bone matrix expressed at late stages of differentiation. |
| GO:0007155 cell adhesion | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro transfers a broad role in cell adhesion from the IBSP family. Reason: Direct RGD mutation and cell-attachment assays independently validate that extracellular IBSP participates in integrin-mediated cell adhesion. The more specific positive-regulation term is already represented by a separate experimental annotation. Supporting Evidence: PMID:24103036 replacement of the RGD (arg-gly-asp) peptide moiety with peptide KAE (lys-ala-glu) resulted in a dramatic loss of cell-attachment activity |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: HuRI binary screening reports IBSP interactions with FAM9B and FXYD3. Reason: The generic term is uninformative, and no endogenous mechanism validates either screening interaction. Mature IBSP is secreted, making the cellular opportunity for these particular interactions unclear. Supporting Evidence: PMID:32296183 reference interactome map of human binary protein interactions |
| GO:0140149 non-collagenous component of interstitial matrix | TAS PMID:36399478 MatrisomeDB 2.0: 2023 updates to the ECM-protein knowledge d... | ACCEPT | Summary: MatrisomeDB classifies IBSP as a noncollagenous interstitial-matrix component. Reason: Human developmental tissue evidence independently places abundant, noncollagenous BSP protein in bone matrix. The term accurately captures its mature structural context. Supporting Evidence: PMID:1818768 BSP is primarily an osteoblast-derived component of the bone matrix expressed at late stages of differentiation. |
| GO:0005198 structural molecule activity | IDA PMID:24103036 Structure-activity relationship of human bone sialoprotein p... | MODIFY | Summary: The cited study tests attachment activity of RGD-containing human BSP peptides. Reason: The experiment does not measure generic structural integrity. It directly maps integrin-dependent attachment activity to the RGD region, so integrin binding is the specific molecular function supported by this source. Proposed replacements: integrin binding Supporting Evidence: PMID:24103036 replacement of the RGD (arg-gly-asp) peptide moiety with peptide KAE (lys-ala-glu) resulted in a dramatic loss of cell-attachment activity |
| GO:0036094 small molecule binding | IPI PMID:9258751 Characterization of native and recombinant bone sialoprotein... | MODIFY | Summary: The annotation records physical interaction with ChEBI:52254, hydroxyapatite. Reason: Native BSP binds hydroxyapatite with approximately nanomolar affinity. Hydroxyapatite binding is an existing, informative GO molecular function and should replace the overly broad small-molecule term. Proposed replacements: hydroxyapatite binding Supporting Evidence: PMID:9258751 hydroxyapatite Kd approximately 2.6 x 10(-9) M PMID:11459848 The affinity for hydroxyapatite was higher for bone-derived BSP than for recombinant BSP. |
| GO:0005515 protein binding | IPI PMID:10747989 Factor H binding to bone sialoprotein and osteopontin enable... | MODIFY | Summary: BSP binds alpha-V/beta-3 integrin and complement factor H in a cell-surface complement-evasion mechanism. Reason: Protein binding is uninformative. The same study directly identifies initial alpha-V/beta-3 engagement, which can be represented by integrin binding. Factor-H recruitment is captured by the proposed complement- regulation process because no specific factor-H-binding MF exists. Proposed replacements: integrin binding Supporting Evidence: PMID:10747989 initial binding to alpha(V)beta(3) integrin |
| GO:0001649 osteoblast differentiation | HDA PMID:16210410 Differential expression profiling of membrane proteins by qu... | UNDECIDED | Summary: A membrane-enriched quantitative proteomic survey detected IBSP during short-term osteoblast differentiation of a human mesenchymal stem-cell line. Reason: The accessible abstract does not expose the IBSP-specific abundance result or show that IBSP functionally drives differentiation. Full text is unavailable, so the experimental curator's evidence cannot be adjudicated confidently and is not overruled. Supporting Evidence: PMID:16210410 used MS to characterize changes in expression of membrane protein markers before and after short-term induction of osteoblast |
| GO:0016020 membrane | HDA PMID:16210410 Differential expression profiling of membrane proteins by qu... | UNDECIDED | Summary: IBSP was recovered in a membrane-associated proteomic fraction during human mesenchymal-stem-cell differentiation. Reason: Secreted BSP can remain cell-surface-associated through integrins, but the abstract does not expose the IBSP-specific fractionation evidence or distinguish membrane association from co-purification. Full text is unavailable, so the HDA is retained without a confident decision. Supporting Evidence: PMID:16210410 We identified 463 unique proteins with extremely high confidence |
| GO:0005178 integrin binding | IMP PMID:24103036 Structure-activity relationship of human bone sialoprotein p... | ACCEPT | Summary: RGD-region substitutions and truncations alter human BSP peptide attachment activity toward osteoblast-like cells. Reason: The RGD-to-KAE loss of attachment and structural analysis directly support integrin binding. Independent full-length human BSP and alpha-V/beta-3 cell assays corroborate this core molecular function. Supporting Evidence: PMID:24103036 replacement of the RGD (arg-gly-asp) peptide moiety with peptide KAE (lys-ala-glu) resulted in a dramatic loss of cell-attachment activity |
| GO:0045785 positive regulation of cell adhesion | IMP PMID:24103036 Structure-activity relationship of human bone sialoprotein p... | ACCEPT | Summary: The human BSP RGD peptide promotes attachment of osteoblast-like cells. Reason: Loss of attachment after RGD replacement directly shows that BSP promotes rather than merely participates in cell adhesion, with full-length BSP assays providing independent support. Supporting Evidence: PMID:24103036 attachment activity toward osteoblast-like (MC3T3) cells |
| GO:0005576 extracellular region | ISS GO_REF:0000024 | ACCEPT | Summary: Curator-reviewed orthology transfers extracellular localization from rat BSP. Reason: The transfer is independently validated by direct secretion and human bone- matrix localization of IBSP and by its N-terminal signal peptide. Supporting Evidence: PMID:1818768 BSP is primarily an osteoblast-derived component of the bone matrix expressed at late stages of differentiation. |
| GO:0007155 cell adhesion | ISS GO_REF:0000024 | ACCEPT | Summary: Curator-reviewed orthology transfers a cell-adhesion role from rat BSP. Reason: Direct human BSP peptide, full-length protein, and alpha-V/beta-3 cell assays independently establish integrin-dependent cell attachment. Supporting Evidence: PMID:10640428 markedly enhanced alpha(V)beta(3)-dependent adhesion to BSP. |
| GO:0030198 extracellular matrix organization | ISS GO_REF:0000024 | ACCEPT | Summary: Curator-reviewed orthology transfers extracellular-matrix organization from mouse BSP. Reason: Human BSP is directly localized to bone matrix, binds its mineral phase, and supports matrix-cell adhesion. Mouse knockout phenotypes establish conserved functional importance for matrix formation and mineralization. Supporting Evidence: PMID:24816232 lack of BSP alters long bone growth and membranous/cortical primary bone formation and mineralization. |
| GO:0031982 vesicle | ISS GO_REF:0000024 | MODIFY | Summary: Orthology transfer traces to direct Western-blot detection of rat BSP in isolated growth-plate matrix vesicles. Reason: The conserved localization is plausible, but generic vesicle is less informative than extracellular vesicle for secreted mineralizing matrix vesicles. The annotation is not treated as a core resident compartment. Proposed replacements: extracellular vesicle Supporting Evidence: PMID:18758911 BSP, ON, OPN, and OC were all detected by Western blot analyses. |
| GO:0071363 cellular response to growth factor stimulus | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed orthology transfers a growth-factor-response annotation from experimentally regulated mouse Ibsp expression. Reason: Growth factors regulate IBSP transcription, so the transferred response is biologically plausible. It describes regulation of IBSP abundance rather than the mature protein's direct mineral- or integrin-binding function and is therefore non-core. |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-265427 | ACCEPT | Summary: Reactome places IBSP extracellularly during binding to cell-surface integrin alpha-V/beta-3. Reason: The event topology is correct for a secreted matrix ligand and is independently supported by human bone-matrix localization and direct integrin-binding assays. Supporting Evidence: Reactome:R-HSA-265427 BSP is considered as an important physiological ligand of alphaVbeta3 |
| GO:0030335 positive regulation of cell migration | IDA PMID:10640428 Activation of integrin alpha(V)beta(3) regulates cell adhesi... | NEW | Summary: BSP-coated substrates support alpha-V/beta-3-dependent migration of human endothelial and osteoblast-like cells. Reason: Direct migration assays establish a positive extracellular-ligand effect, and independent glioblastoma experiments show that IBSP promotes vascular tumor-cell migration. Supporting Evidence: PMID:10640428 alpha(V)beta(3)-mediated migration of HUVEC or osteoblastic cells to BSP was substantially enhanced PMID:36261010 IBSP, enhances tumor growth and promotes the migration of GTCs along the vasculature. |
| GO:1903660 negative regulation of complement-dependent cytotoxicity | IDA PMID:10747989 Factor H binding to bone sialoprotein and osteopontin enable... | NEW | Summary: Recombinant BSP recruits factor H and protects attached cells from complement-mediated lysis. Reason: The cell-survival assay directly demonstrates negative regulation of complement-dependent cytotoxicity. This is experimentally strong but retained outside the skeletal core because it was studied in tumor models. Supporting Evidence: PMID:10747989 Recombinant OPN and BSP can protect murine erythroleukemia cells from attack by human complement |
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Download this section (compressed HTML)Q: Which IBSP phosphorylation and glycosylation patterns determine mineral nucleation versus crystal-growth inhibition in human bone?
Q: How much of the skeletal phenotype requires RGD-integrin signaling rather than direct hydroxyapatite binding?
Q: Is IBSP actively packaged into human mineralizing extracellular vesicles?
Q: Does factor-H recruitment by IBSP have a normal placental or skeletal role, or is it primarily exploited by tumors?
Experiment: Generate site-specifically phosphorylated human IBSP proteoforms and measure hydroxyapatite binding, nucleation lag, crystal-growth kinetics, and crystal morphology in collagen-containing bone-like matrices.
Hypothesis: Distinct phosphorylation patterns convert IBSP between a hydroxyapatite nucleator and a crystal-growth inhibitor.
Type: Proteoform-resolved mineralization biophysics
Experiment: Rescue IBSP-null human osteogenic organoids with wild-type, RGD-to-KAE, or acidic-cluster mutants and quantify integrin signaling, cell attachment, osteoblast maturation, mineral deposition, and matrix ultrastructure.
Hypothesis: IBSP's RGD motif and acidic mineral-binding regions make separable contributions to osteoblast differentiation and matrix mineralization.
Type: Separation-of-function rescue in human osteogenic organoids
Experiment: Isolate vesicles from primary human osteoblast cultures using density and size-exclusion methods, confirm luminal or surface topology by protease protection, and track IBSP-dependent mineral nucleation by live imaging.
Hypothesis: Human osteoblasts package IBSP into extracellular matrix vesicles to deliver it to nascent mineralization foci.
Type: Extracellular-vesicle localization and mineralization assay
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The modification-dependent switch between hydroxyapatite nucleation, crystal-growth inhibition, and passive mineral binding is unresolved.
OPEN BIOLOGY MF_DARK
What is known: Native and recombinant proteins differ in hydroxyapatite affinity, and several BSP preparations inhibit or nucleate mineral in vitro, but the physiological state responsible for each behavior is not defined.
Significance: Resolving this switch is essential for a mechanistic explanation of IBSP's positive in vivo contribution to bone mineralization.
What would resolve it: Produce site-specifically phosphorylated and glycosylated human IBSP proteoforms and compare mineral nucleation, growth, and binding under bone-like ionic and matrix conditions.
Provenance (the field's own admissions):
Gap: The relative contributions of direct mineral binding and RGD-integrin signaling to skeletal phenotypes are not fully separated.
OPEN BIOLOGY BP_DARK
What is known: Whole-gene knockout affects mineralization and bone-cell programs, while peptide and cell assays establish integrin binding, but human separation-of-function evidence is lacking.
Significance: Distinguishing the mechanisms would clarify whether IBSP is primarily a mineral nucleator, a matrix signal, or a coordinated bifunctional regulator.
What would resolve it: Compare hydroxyapatite-binding-deficient and RGD-deficient human IBSP knock-in alleles in mineralizing organoids and skeletal animal models.
Provenance (the field's own admissions):
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