title: Kicking against the PRCs - A Domesticated Transposase Antagonises Silencing Mediated by Polycomb Group Proteins and Is an Accessory Component of Polycomb Repressive Complex 2.
abstract: The Polycomb group (PcG) and trithorax group (trxG) genes play crucial roles in development by regulating expression of homeotic and other genes controlling cell fate. Both groups catalyse modifications of chromatin, particularly histone methylation, leading to epigenetic changes that affect gene activity. The trxG antagonizes the function of PcG genes by activating PcG target genes, and consequently trxG mutants suppress PcG mutant phenotypes. We previously identified the ANTAGONIST OF LIKE HETEROCHROMATIN PROTEIN1 (ALP1) gene as a genetic suppressor of mutants in the Arabidopsis PcG gene LIKE HETEROCHROMATIN PROTEIN1 (LHP1). Here, we show that ALP1 interacts genetically with several other PcG and trxG components and that it antagonizes PcG silencing. Transcriptional profiling reveals that when PcG activity is compromised numerous target genes are hyper-activated in seedlings and that in most cases this requires ALP1. Furthermore, when PcG activity is present ALP1 is needed for full activation of several floral homeotic genes that are repressed by the PcG. Strikingly, ALP1 does not encode a known chromatin protein but rather a protein related to PIF/Harbinger class transposases. Phylogenetic analysis indicates that ALP1 is broadly conserved in land plants and likely lost transposase activity and acquired a novel function during angiosperm evolution. Consistent with this, immunoprecipitation and mass spectrometry (IP-MS) show that ALP1 associates, in vivo, with core components of POLYCOMB REPRESSIVE COMPLEX 2 (PRC2), a widely conserved PcG protein complex which functions as a H3K27me3 histone methyltransferase. Furthermore, in reciprocal pulldowns using the histone methyltransferase CURLY LEAF (CLF), we identify not only ALP1 and the core PRC2 components but also plant-specific accessory components including EMBRYONIC FLOWER 1 (EMF1), a transcriptional repressor previously associated with PRC1-like complexes. Taken together our data suggest that ALP1 inhibits PcG silencing by blocking the interaction of the core PRC2 with accessory components that promote its HMTase activity or its role in inhibiting transcription. ALP1 is the first example of a domesticated transposase acquiring a novel function as a PcG component. The antagonistic interaction of a modified transposase with the PcG machinery is novel and may have arisen as a means for the cognate transposon to evade host surveillance or for the host to exploit features of the transposition machinery beneficial for epigenetic regulation of gene activity.
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Shih Chieh
authors.initials: SC
authors.lastname: Liang
authors.affiliation: Department of Plant Developmental Biology, Max Planck Institute for Plant Breeding Research, Köln, Germany.
authors.forename: Ben
authors.initials: B
authors.lastname: Hartwig
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Pumi
authors.initials: P
authors.lastname: Perera
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Santiago
authors.initials: S
authors.lastname: Mora-García
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Erica
authors.initials: E
authors.lastname: de Leau
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Harry
authors.initials: H
authors.lastname: Thornton
authors.affiliation: Centre for Microbial Chemical Biology, Department of Biochemistry, McMaster University, Ontario, Canada.
authors.forename: Flavia
authors.initials: F
authors.lastname: de Lima Alves
authors.affiliation: Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Flavia Lima
authors.initials: FL
authors.lastname: de Alves
authors.affiliation: Department of Bioanalytics, Institute of Biotechnology, Technische Universität Berlin, Berlin, Germany, 2. Wellcome Trust Centre for Cell Biology, University of Edinburgh,  Edinburgh, United Kingdom.
authors.forename: Juri
authors.initials: J
authors.lastname: Rappsilber
authors.affiliation: Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Juri
authors.initials: J
authors.lastname: Rapsilber
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Suxin
authors.initials: S
authors.lastname: Yang
authors.affiliation: Department of Plant Developmental Biology, Max Planck Institute for Plant Breeding Research, Köln, Germany.
authors.forename: Geo Velikkakam
authors.initials: GV
authors.lastname: James
authors.affiliation: Department of Plant Developmental Biology, Max Planck Institute for Plant Breeding Research, Köln, Germany.
authors.forename: Korbinian
authors.initials: K
authors.lastname: Schneeberger
authors.affiliation: CSIRO Plant Industry, Canberra, Australia.
authors.forename: E Jean
authors.initials: EJ
authors.lastname: Finnegan
authors.affiliation: Department of Plant Developmental Biology, Max Planck Institute for Plant Breeding Research, Köln, Germany.
authors.forename: Franziska
authors.initials: F
authors.lastname: Turck
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Justin
authors.initials: J
authors.lastname: Goodrich
mesh_terms: D017360:Arabidopsis / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D002843:Chromatin / Q000235:genetics; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics* / Q000378:metabolism; D044127:Epigenesis, Genetic; D035264:Flowers / Q000235:genetics; D018506:Gene Expression Regulation, Plant; D011495:Histone-Lysine N-Methyltransferase / Q000235:genetics; D006657:Histones / Q000235:genetics; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D010802:Phylogeny; D063151:Polycomb Repressive Complex 2 / Q000235:genetics* / Q000378:metabolism; D063146:Polycomb-Group Proteins / Q000235:genetics* / Q000378:metabolism; D036226:Seedlings / Q000235:genetics; D019895:Transposases / Q000096:biosynthesis / Q000235:genetics
chemical_list: C000608104:ALP1 protein, Arabidopsis; D029681:Arabidopsis Proteins; C104597:CURLY LEAF protein, Arabidopsis; D002843:Chromatin; D002868:Chromosomal Proteins, Non-Histone; D006657:Histones; D018398:Homeodomain Proteins; D063146:Polycomb-Group Proteins; C443901:like heterochromatin protein 1; D011495:Histone-Lysine N-Methyltransferase; D063151:Polycomb Repressive Complex 2; D019895:Transposases