ITGB3BP

ITGB3BP (NRIF3/TAP20/CENP-R) is a multifunctional protein operating in two largely distinct arenas: nuclear receptor coregulation and mitotic kinetochore function (PMID:10490654, PMID:36069839). As a nuclear coactivator it localizes to the nucleus and uses a bivalent interaction mechanism—a C-terminal LXXIL module (RID1) as the primary receptor-contact and an N-terminal LXXLL motif (RID2) as a modulatory contact—to engage thyroid hormone receptor and RXR dimers in a ligand-dependent manner, with inter-module spacing governing affinity (PMID:10490654, PMID:11713274). Its C-terminus harbors an autonomous transactivation domain while an N-terminal RepD1 (residues 20–50) confers transrepression that is abolished by Ser28Ala, defining this residue as a regulatory phospho-site (PMID:11713274). Pak1 phosphorylates Ser28, enhancing nuclear localization, ERα binding, recruitment to ERα target promoters, and estrogen responsiveness, thereby extending coactivator function to ERα (PMID:18521086); MTA1 binds the N-terminal LXXLL region and represses NRIF3-driven ERE transcription by blocking its chromatin association (PMID:15254226). A separate 30-amino-acid death domain (DD1) drives caspase 2-dependent, mitochondria-involving apoptosis selectively in breast cancer cells (PMID:15082778). In mitosis, ITGB3BP is a constitutive kinetochore component of the CENP-O complex (CENP-O/P/Q/R/U) that regulates kinetochore–microtubule attachment stability; Aurora B phosphorylation weakens its kinetochore localization and disrupts its binding to CENP-U to correct erroneous attachments (PMID:36069839, PMID:28795467), and co-condensation with EB1 via its N-terminal disordered region links the inner kinetochore to dynamic microtubule plus-ends to drive chromosome oscillations (PMID:40349345). Independently, TAP20 acts downstream of PKCθ in endothelial cells, binding the β5 integrin cytoplasmic domain to reduce adhesion and promote migration and tube formation (PMID:10579726), and its protein levels are controlled by MDM2-promoted ubiquitin-mediated degradation (PMID:39674578).

1. 1999 High

   Established ITGB3BP as a nuclear receptor coactivator with specificity, answering whether it acts broadly or selectively across receptors.

   Evidence Yeast two-hybrid, in vitro binding, mutagenesis and transactivation assays mapping a C-terminal LXXIL (RID1) module

   PMID:10490654

   Open questions at the time

   • Did not resolve stoichiometry of receptor-dimer engagement
   • No structural model of RID1 binding
   • Endogenous target genes not defined
2. 1999 High

   Defined a separate cytoplasmic identity for the same protein, showing it links PKCθ signaling to integrin-dependent endothelial behavior.

   Evidence Coprecipitation with β5 integrin cytoplasmic domain plus adhesion, migration, tube-formation and antibody-blocking assays

   PMID:10579726

   Open questions at the time

   • Molecular basis of integrin-tail binding not mapped
   • Relationship to the nuclear coactivator role unexplained
   • Downstream signaling effectors unidentified
3. 2001 High

   Resolved how the coregulator engages receptors and how its activity is partitioned into activating and repressing domains, defining Ser28 as a regulatory node.

   Evidence Deletion/mutagenesis mapping of bivalent RID modules, dimerization domain, AD1, and RepD1 with Gal4 reporter assays

   PMID:11713274

   Open questions at the time

   • Kinase responsible for Ser28 phosphorylation not yet identified
   • Physiological context of repression versus activation unclear
4. 2004 High

   Revealed a coregulatory antagonism, showing MTA1 represses ITGB3BP-driven ER signaling by displacing it from chromatin.

   Evidence Y2H, in vitro and in vivo binding, ChIP, and ERE reporter assays

   PMID:15254226

   Open questions at the time

   • Mechanism of chromatin displacement not structurally defined
   • Generality across other receptor target genes untested
5. 2004 High

   Identified a pro-apoptotic activity distinct from transcription, defining a death domain that selectively kills breast cancer cells.

   Evidence Domain mapping, caspase inhibitor studies, and mitochondrial permeability assays in cell lines

   PMID:15082778

   Open questions at the time

   • Molecular basis of cell-type selectivity unknown
   • Upstream trigger of DD1-mediated apoptosis unidentified
   • Endogenous relevance versus overexpression unclear
6. 2008 High

   Connected upstream signaling to coregulator output, showing Pak1 phosphorylation of Ser28 activates ITGB3BP toward ERα.

   Evidence In vitro kinase assay, phospho-mimic/non-phosphorylatable mutants, ChIP, and transactivation assays

   PMID:18521086

   Open questions at the time

   • Interplay between Ser28 activation here and RepD1 repression unresolved
   • Other phospho-regulated receptors not surveyed
7. 2017 Medium

   Placed ITGB3BP in the constitutive kinetochore CENP-O complex and tested organismal requirement, showing it is dispensable for development but modifies tumor formation.

   Evidence Cenp-r−/− knockout mouse with DMBA/TPA carcinogenesis and histological proliferation/apoptosis analysis

   PMID:28795467

   Open questions at the time

   • Tumor phenotype mechanism inferred from in vivo readout without biochemical dissection
   • Link between kinetochore role and tumorigenesis not defined
8. 2022 High

   Defined how the kinetochore role is regulated, showing Aurora B phosphorylation disengages CENP-R from CENP-U to correct erroneous attachments.

   Evidence Phospho-mimic mutagenesis, kinetochore localization, CENP-R/CENP-U co-IP, and chromosome segregation assays

   PMID:36069839

   Open questions at the time

   • Structural basis of CENP-R/CENP-U interface not solved
   • Aurora B phospho-sites not enumerated here
9. 2025 High

   Established a physical mechanism coupling the inner kinetochore to microtubule plus-ends, showing CENP-R co-condenses with EB1 to drive chromosome oscillations.

   Evidence NMR interaction mapping, co-condensation assays, and EB1-binding-defective mutant with live-cell oscillation imaging

   PMID:40349345

   Open questions at the time

   • Regulation of the condensation in vivo not defined
   • Interplay with Aurora B-controlled CENP-U binding unexplored
10. 2025 Medium

    Identified post-translational control of ITGB3BP abundance and a metabolic/differentiation role in stem cells.

    Evidence Ubiquitination and stability assays, MDM2 interaction, osteogenic and migration assays, Seahorse respiration, and calvarial defect model

    PMID:39674578

    Open questions at the time

    • MDM2-mediated degradation linked indirectly via lncRNA context
    • Single lab, mechanism of mitochondrial enhancement undefined

• How the protein's nuclear coregulator, apoptotic, kinetochore, and integrin-signaling roles are coordinated within one cell, and whether they share a common regulatory logic, remains unresolved.
• No unifying model linking the distinct functional modules
• Isoform/context determinants of which role is active are unknown