title: A human Polycomb isoform lacking the Pc box does not participate to PRC1 complexes but forms protein assemblies and represses transcription.
abstract: Polycomb repression controls the expression of hundreds of genes involved in development and is mediated by essentially two classes of chromatin-associated protein complexes. The Polycomb repressive complex 2 (PRC2) trimethylates histone H3 at lysine 27, an epigenetic mark that serves as a docking site for the PRC1 protein complex. Drosophila core PRC1 is composed of four subunits: Polycomb (Pc), Posterior sex combs (Psc), Polyhomeotic (Ph) and Sex combs extra (Sce). Each of these proteins has multiple orthologs in vertebrates, thus generating an enormous scope for potential combinatorial diversity. In particular, mammalian genomes encode five Pc family members: CBX2, CBX4, CBX6, CBX7 and CBX8. To complicate matters further, distinct isoforms might arise from single genes. Here, we address the functional role of the two human CBX2 isoforms. Owing to different polyadenylation sites and alternative splicing events, the human CBX2 locus produces two transcripts: a 5-exon transcript that encodes the 532-amino acid CBX2-1 isoform that contains the conserved chromodomain and Pc box and a 4-exon transcript encoding a shorter isoform, CBX2-2, lacking the Pc box but still possessing a chromodomain. Using biochemical approaches and a novel in vivo imaging assay, we show that the short CBX2-2 isoform lacking the Pc box, does not participate in PRC1 protein complexes, but self-associates in vivo and forms complexes of high molecular weight. Furthermore, the CBX2 short isoform is still able to repress transcription, suggesting that Polycomb repression might occur in the absence of PRC1 formation.
authors.affiliation: Chromatinomics, Interdisciplinary Research Institute, CNRS USR 3078, Université de Lille 1 Sciences et Technologies, Villeneuve d'Ascq Cedex, France.
authors.forename: Pamela
authors.initials: P
authors.lastname: Völkel
authors.forename: Perrine
authors.initials: P
authors.lastname: Le Faou
authors.forename: Julien
authors.initials: J
authors.lastname: Vandamme
authors.forename: Dorcas
authors.initials: D
authors.lastname: Pira
authors.forename: Pierre-Olivier
authors.initials: PO
authors.lastname: Angrand
mesh_terms: D017398:Alternative Splicing; D000818:Animals; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics / Q000378:metabolism*; D017124:Conserved Sequence; D004591:Electrophoresis, Polyacrylamide Gel; D005091:Exons; D017930:Genes, Reporter; D056426:Genetic Loci; D049452:Green Fluorescent Proteins / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006570:Heterochromatin / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008025:Ligases; D051379:Mice; D033681:Mitochondrial Membrane Transport Proteins; D008970:Molecular Weight; D026723:Polyadenylation; D063150:Polycomb Repressive Complex 1; D063146:Polycomb-Group Proteins; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D040901:Proteomics / Q000379:methods*; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D037941:Silencer Elements, Transcriptional; D014158:Transcription, Genetic; D014162:Transfection; D044767:Ubiquitin-Protein Ligases
chemical_list: C570479:CBX2 protein, human; C570428:CBX8 protein, human; C570426:Cbx2 protein, mouse; D002868:Chromosomal Proteins, Non-Histone; D006570:Heterochromatin; D033681:Mitochondrial Membrane Transport Proteins; D063146:Polycomb-Group Proteins; D020033:Protein Isoforms; D012097:Repressor Proteins; D049452:Green Fluorescent Proteins; C570429:Cbx8 protein, mouse; D063150:Polycomb Repressive Complex 1; D044767:Ubiquitin-Protein Ligases; D008025:Ligases; C570427:CBX4 protein, human