CBX1 UniProt P83916

5 alternative isoforms · canonical 185 aa · nuclear chromosome; nucleolus; nucleus

CBX1 (HP1-beta) is a chromodomain-containing heterochromatin protein whose chromodomain specifically reads H3K9me3, establishing it as a reader that translates this histone mark into higher-order chromatin organization [PMID:21047797]. At pericentromeric and centromeric heterochromatin it associates with the H3K9 methyltransferase SUV39H1 [PMID:10202156], and its chromocenter localization depends on SUV39h/H3K9 trimethylation, whereas a distinct nucleolar pool localizes independently of SUV39h and HDAC activity [PMID:20033197]. CBX1 has non-redundant developmental functions: its loss in mice causes perinatal lethality with neocortical and neuromuscular junction defects and genomic instability [PMID:19015315]. Beyond constitutive heterochromatin, CBX1 nucleates tissue-specific silencing by forming a trimeric complex with PurB and Sp3 that positions nucleosomes and recruits PRC2 to deposit H3K27me3 at cardiomyocyte loci [PMID:35605661]. CBX1 also acts dynamically in the DNA damage response: CK2-mediated phosphorylation of Thr51 disrupts chromodomain folding around H3K9me, mobilizing HP1-beta from chromatin, an event required to initiate H2AX phosphorylation [PMID:18438399]; KAP-1 Ser473 phosphorylation by Chk2 likewise drives HP1-beta mobilization to enable double-strand break repair within heterochromatin [PMID:22715096]. Heterozygous de novo chromodomain variants that reduce heterochromatin binding cause a dominant-negative neurodevelopmental disorder, with mutant HP1-beta sequestering wild-type protein [PMID:37087635]. In cancer contexts CBX1 promotes proliferation, invasion, EMT, and immune evasion through H3K9me3-mediated repression and signaling axes including Wnt/β-catenin and IGF-1R/AKT/SNAIL [PMID:36310139, PMID:30031230, PMID:38769286].