BAZ1B

BAZ1B (WSTF) is a multifunctional chromatin regulatory protein that integrates nucleosome remodeling, histone modification, and DNA damage signaling across diverse nuclear processes. It serves as an essential shared subunit of two distinct chromatin remodeling complexes—the ISWI-type WICH complex (with SNF2H) that assembles regularly spaced nucleosomal arrays and maintains heterochromatin through replication, and the SWI/SNF-type WINAC complex that mediates vitamin D receptor-dependent transcription—with its bromodomain directly binding acetylated histones to anchor these complexes on chromatin (PMID:11980720, PMID:19470456, PMID:16252006). BAZ1B possesses intrinsic tyrosine kinase activity through a structurally novel domain that constitutively phosphorylates H2A.X Tyr142, a mark that associates with RNA polymerase II at transcribed loci and, upon DNA damage, orchestrates transcription-coupled homologous recombination; this kinase activity is itself regulated by MOF-mediated acetylation at K426 (PMID:19092802, PMID:31045206, PMID:32518374). BAZ1B also participates in DNA repair by interacting with ATAD5 to prevent premature de-ubiquitination of mono-ubiquitinated PCNA under oxidative stress, localizes to mitotic chromosome axes where its loss delays prophase condensation, and is selectively degraded via ATG8-dependent nuclear autophagy during chronic inflammation—opening chromatin at inflammatory gene loci and amplifying NF-κB signaling (PMID:39627214, PMID:30266865, PMID:40604282). BAZ1B haploinsufficiency in Williams syndrome causes transcriptional dysregulation of neurogenesis genes and impaired neural crest stem cell induction, positioning it as a dosage-sensitive regulator of craniofacial development (PMID:26755828, PMID:31840056).

1. 2002 High

   Identifying BAZ1B as a core subunit of the WICH complex with SNF2H established its first molecular function—ATP-dependent nucleosome spacing—and linked it to heterochromatin replication.

   Evidence Biochemical purification to homogeneity from Xenopus egg extract plus Co-IP in mouse cells with in vitro nucleosome assembly activity

   PMID:11980720

   Open questions at the time

   • The mechanism by which WICH is targeted to pericentric heterochromatin was not defined
   • Whether WICH functions are separable from other BAZ1B-containing complexes was unknown
2. 2005 High

   Demonstrating that BAZ1B's bromodomain directly binds acetylated histones and that WICH associates with PCNA at replication foci resolved how BAZ1B-containing complexes are recruited to chromatin and linked BAZ1B to post-replicative chromatin maturation.

   Evidence In vitro chromatin binding with bromodomain deletion mutants acting as dominant negatives; Co-IP with PCNA and chromatin accessibility assays after WSTF depletion

   PMID:15753658 PMID:16252006

   Open questions at the time

   • Whether bromodomain-dependent and PCNA-dependent recruitment operate in the same or distinct contexts was unclear
   • No structural detail of bromodomain–acetyl-histone interaction
3. 2006 High

   Discovery that BAZ1B forms the B-WICH assembly with nuclear myosin 1 and functions at RNA Pol I and Pol III gene loci expanded its role from replication-coupled chromatin remodeling to active transcriptional regulation of ribosomal and non-coding RNA genes.

   Evidence Biochemical fractionation, ChIP at rRNA and 5S/7SL loci, siRNA knockdown reducing Pol I and Pol III transcript levels, in vitro transcription on chromatin templates

   PMID:16514417 PMID:16603771

   Open questions at the time

   • How B-WICH is distinguished from WICH at Pol I vs. Pol III promoters was not resolved
   • No direct demonstration that SNF2H ATPase activity is required at these loci
4. 2008 High

   The unexpected finding that BAZ1B itself possesses intrinsic tyrosine kinase activity—through a domain unrelated to any known kinase fold—and phosphorylates H2A.X Tyr142 fundamentally reframed BAZ1B as both a chromatin remodeler subunit and a signaling kinase in the DNA damage response.

   Evidence In vitro kinase assay with recombinant protein, active-site mutagenesis abolishing activity

   PMID:19092802

   Open questions at the time

   • The structural basis of the novel kinase fold was unknown
   • Whether pTyr142 had functions beyond damage signaling was not addressed
5. 2009 High

   Genetic analysis in WSTF-knockout MEFs proved BAZ1B is an indispensable shared subunit of both WICH and WINAC, with WICH specifically mediating DNA repair (via SNF2H-PCNA recruitment) and WINAC mediating transcription, resolving the question of functional partitioning between the two complexes.

   Evidence WSTF knockout mouse, component-specific rescue overexpression in MEFs, epistasis analysis

   PMID:19470456

   Open questions at the time

   • How BAZ1B partitions between WICH and WINAC in a given cell was not determined
   • In vivo phenotypes beyond MEFs were not fully characterized
6. 2012 Medium

   Temporal analysis showed WICH transiently associates with the inactive X chromosome during late S-phase replication before BRCA1 and γ-H2AX, and separately BAZ1B was found to coactivate the CYP19A1/aromatase and ERα promoters, broadening its transcriptional roles to include hormone-responsive gene regulation.

   Evidence BrdU pulse-labeling with immunofluorescence for Xi timing; ChIP and siRNA with aromatase activity readout in breast cancer cells

   PMID:23085504 PMID:23166813

   Open questions at the time

   • Whether WICH recruitment to Xi is functionally required for X-inactivation maintenance was not tested
   • The CYP19A1 coactivator role was from a single lab without independent replication
7. 2015 High

   Conditional knockout in spermatocytes demonstrated that BAZ1B is the primary somatic H2AX-Tyr142 kinase but is dispensable for this modification during meiosis, establishing tissue-specific redundancy for this signaling mark.

   Evidence Spermatocyte-specific conditional BAZ1B knockout in mice, immunostaining for pTyr142, MDC1 epistasis

   PMID:25979708

   Open questions at the time

   • The alternative meiotic Tyr142 kinase was not identified
   • Whether BAZ1B has meiosis-specific functions independent of pTyr142 was not addressed
8. 2016 High

   ChIP-seq and transcriptional analysis in Williams syndrome patient-derived neural cells revealed that BAZ1B haploinsufficiency directly dysregulates neurogenesis and neuron differentiation genes, with Wnt inhibitor rescue establishing a causal signaling axis downstream of BAZ1B dosage.

   Evidence iPSC-derived neurons from WS patients, BAZ1B ChIP-seq identifying direct targets, shRNA knockdown, Wnt inhibitor rescue

   PMID:26755828

   Open questions at the time

   • Which specific BAZ1B-containing complex mediates the Wnt-linked neurodevelopmental function was not determined
   • No in vivo rescue in animal models
9. 2018 High

   Proteomics and knockout studies placed BAZ1B on mitotic chromosome axes as a scaffold component required for timely prophase condensation, revealing a cell-cycle function distinct from its interphase chromatin remodeling and kinase roles.

   Evidence Quantitative MS of mitotic chromosome scaffold, BAZ1B knockout in DT40 cells, BAZ1A double-knockout epistasis

   PMID:30266865

   Open questions at the time

   • Whether the mitotic function requires BAZ1B's kinase activity or its remodeling complex association was not dissected
   • The condensation target substrates on chromosome axes are unknown
10. 2019 High

    Functional dissection in neural crest cells from patients with 7q11.23 CNVs established BAZ1B as a master regulator of neural crest induction and migration, and the pTyr142-H2AX mark was shown to associate with RNAPII at transcribed loci and facilitate transcription-coupled HR in G1, unifying BAZ1B's kinase and transcriptional functions at damage sites.

    Evidence NCSC induction/migration assays with patient-derived cells; Co-IP of pTyr142-H2AX with RNAPII, WSTF depletion, HR reporter assays, RAD51/RPA foci

    PMID:31045206 PMID:31840056

    Open questions at the time

    • Whether TC-HR-promoting function of BAZ1B is relevant in neural crest cells specifically was not tested
    • The structural basis for the novel kinase fold remained unresolved
11. 2020 High

    Identification of MOF as the K426 acetyltransferase and SIRT1 as its eraser, with acetylation promoting Ser158 phosphorylation and kinase/transcriptional activity, revealed a hierarchical post-translational regulatory code governing BAZ1B function.

    Evidence In vitro acetylation assay with MOF, K426R mutagenesis, Co-IP of MSL1v1/MOF/WSTF, mass spectrometry

    PMID:32518374

    Open questions at the time

    • The kinase responsible for Ser158 phosphorylation was not identified
    • Whether K426 acetylation differentially regulates WICH vs. WINAC activities was untested
12. 2024 High

    The discovery that BAZ1B interacts with ATAD5 to prevent premature de-ubiquitination of mono-Ub-PCNA during oxidative stress established a new repair-specific function distinct from WICH-mediated SNF2H recruitment, acting through competition with UAF1 for ATAD5 binding.

    Evidence Reciprocal Co-IP, domain mapping showing overlap with UAF1-binding site, PCNA ubiquitination dynamics, H2O2 sensitivity

    PMID:39627214

    Open questions at the time

    • Whether this function requires BAZ1B's kinase activity or its remodeling complex context was not determined
    • In vivo consequences of disrupting ATAD5–BAZ1B interaction in animal models are unknown
13. 2025 High

    The finding that BAZ1B is selectively degraded via ATG8-mediated nuclear autophagy during chronic inflammation—with its loss opening chromatin at NF-κB target genes—revealed a fundamentally new mechanism linking nucleophagy to inflammatory gene regulation and provided a therapeutic target for chronic inflammatory diseases.

    Evidence WSTF nuclear export and lysosomal degradation tracking, WSTF–ATG8 Co-IP, chromatin accessibility assays, cell-penetrating peptide blockade in MASH and osteoarthritis mouse models

    PMID:40604282

    Open questions at the time

    • The signal distinguishing chronic from acute inflammation for BAZ1B degradation was not identified
    • Whether other WICH/WINAC subunits are co-degraded or remain intact is unknown

• The structural basis of BAZ1B's novel kinase fold, the mechanisms partitioning BAZ1B between its multiple complexes and functions in vivo, and the upstream signals that trigger its selective nuclear autophagic degradation remain unresolved.
• No crystal or cryo-EM structure of the atypical kinase domain exists
• How cells allocate BAZ1B between WICH, WINAC, B-WICH, and scaffold functions is unknown
• The chronic inflammation–specific degradation signal has not been identified