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The Arabidopsis LHP1 Protein Is a Component of Euchromatin

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The Arabidopsis LHP1 Protein Is a Component of Euchromatin Planta (2005) 222: 910–925 DOI 10.1007/s00425-005-0129-4 ORIGINAL ARTICLE Marc Libault Æ Federico Tessadori Æ Sophie Germann Berend Snijder Æ Paul Fransz Æ Vale´rie Gaudin The Arabidopsis LHP1 protein is a component of euchromatin Received: 11 June 2005 / Accepted: 29 August 2005 / Published online: 22 October 2005 Ó Springer-Verlag 2005 Abstract The HP1 family proteins are involved in several A. thaliana HP1 homolog with the mammal HP1c iso- aspects of chromatin function and regulation in Dro- form, besides specific plant properties. sophila, mammals and the fission yeast. Here we inves- tigate the localization of LHP1, the unique Arabidopsis Keywords Arabidopsis Æ Chromatin Æ Heterochromatin thaliana HP1 homolog known at present time, to ap- protein 1 Æ Mitosis Æ Nucleolus proach its function. A functional LHP1–GFP fusion protein, able to restore the wild-type phenotype in the Abbreviations PI: Propidium iodide Æ DAPI: lhp1 mutant, was used to analyze the subnuclear distri- 4¢,6-diamidino-2-phenylindole Æ HP1: Heterochromatin bution of LHP1 in both A. thaliana and Nicotiana protein 1 Æ CD: Chromo domain Æ CSD: Chromo tabacum.InA. thaliana interphase nuclei, LHP1 was shadow domain Æ HR: Hinge region Æ RHF: Relative predominantly located outside the heterochromatic heterochromatin fraction Æ FISH: Fluorescent in-situ chromocenters. No major aberrations were observed in hybridization Æ NLS: Nuclear localization heterochromatin content or chromocenter organization signal Æ NoLS: Nucleolar localization signal in lhp1 plants. These data indicate that LHP1 is mainly involved in euchromatin organization in A. thaliana.In tobacco BY-2 cells, the LHP1 distribution, although in foci, slightly differed suggesting that LHP1 localization Introduction is determined by the underlying genome organization of plant species. Truncated LHP1 proteins expressed in Heterochromatin protein 1 (HP1) was originally identi- vivo allowed us to determine the function of the different fied as a non-histone component of heterochromatin segments in the localization. The in foci distribution is associated with position effect variegation control. Since dependent on the presence of the two chromo domains, then, HP1-like proteins were identified from fission yeast whereas the hinge region has some nucleolus-targeting to drosophila mammals and plants, each organism properties. Furthermore, like the animal HP1b and containing one to three isoforms. The HP1-like family is HP1c subtypes, LHP1 dissociates from chromosomes involved in chromosome condensation and segregation, during mitosis. In transgenic plants expressing the in telomere organization, in nuclear architecture, in LHP1–GFP fusion protein, two major localization pat- control of development and cell cycle by generating and terns were observed according to cell types suggesting maintaining a silent chromatin structure (Li et al. 2002; that localization evolves with age or differentiation Maison and Almouzni 2004). states. Our results show conversed characteristics of the This functional diversity arises from the specific and conserved structure of the HP1-like proteins. They are characterized by the presence of two sequence-related M. Libault Æ S. Germann Æ V. Gaudin (&) modules, the amino-terminal chromo domain (CD) and Laboratoire de Biologie Cellulaire, IJPB, INRA, route de St Cyr, the carboxy-terminal chromo shadow domain (CSD), 78026 Versailles Cedex, France separated by a variable intervening ‘‘hinge’’ region E-mail: [email protected] Tel.: +33-1-30833522 (HR). These modules participate in protein–protein Fax: +33-1-30833099 interactions and in nucleic acid binding (Li et al. 2002; Maison and Almouzni 2004). Thus HP1-like proteins F. Tessadori Æ B. Snijder Æ P. Fransz are molecular adaptors that participate in multiple dis- Swammerdam Institute for Life Sciences, University of Amsterdam, Kruislaan 318, 1098 Amsterdam, tinct nucleoprotein complexes with various structural The Netherlands and regulatory functions. 911 In Arabidopsis thaliana, a HP1 homolog, LHP1, was tures), or long-day (LD; 16-h light/8-h dark, at 20°C isolated independently in screens for early flowering constant temperature) conditions. The N. tabacum L. cv. plants (Gaudin et al. 2001), for alterations of inflores- Bright Yellow 2 (tobacco BY-2 or TBY-2) cell line was cence morphology (Larsson et al. 1998; Kotake et al. obtained from C. Bergounioux (IBP, Orsay, France). 2003) or for altered leaf glucosinolate levels (Kim et al. The cell line was grown in the dark, at 24°C, under 2004). The lhp1/tfl2/TU8 allelic mutations have pleio- constant shaking and maintained by weekly subcultures tropic effects, and affect flowering time, photoperiod and (Nagata et al. 1992). temperature sensitivity, plant architecture, leaf mor- phology, inflorescence determinacy and hormone levels (Larsson et al. 1998; Ludwig-Mu¨ ller et al. 1999, 2000; TBY-2 cell synchronization and mitotic events Gaudin et al. 2001; Kotake et al. 2003; Takada and Goto 2003). Furthermore, LHP1 was shown to interact For synchronization, 4 ml of stationary phase cell culture in vitro with the histone H3 dimethylated at lysine 9 was transferred to 40 ml fresh medium supplemented by (Jackson et al. 2002), a hallmark of silent chromatin 2 lg/ml aphidicolin (Sigma) (Planchais et al. 1997). After recognized by the CD of HP1 proteins (see references in 24 h at 24°C, aphidicolin was removed by centrifugation, (Berger and Gaudin 2003)). In vitro interaction between the pelleted cells were washed and resuspended in fresh LHP1 and NtSET1, a tobacco histone methyltransferase medium. The mitotic peak could be observed at 9–10 h and their colocalization were also reported (Yu et al. after the aphidicolin release. To obtain a highly syn- 2004). Taken together, these data suggest that LHP1 chronized cell population starting at the M phase, the plays a role in gene regulation and developmental con- aphidicolin-treated cells were subsequently cultivated trol by establishing and maintaining specific silent with 1.6 lg/ml propyzamide (Sigma) for at least 10 h chromatin domains. before analysis. The second inhibitor was removed by On the contrary to A. thaliana, where a unique HP1 centrifugation and two additional washes. Mitosis was homolog was recorded in its sequenced genome (Gaudin visualized with DRAQ5TM (Biostatus). et al. 2001; ChromDB, http://chromdb.org/), mammals have three HP1 isoforms (a, b, c). Despite similarities in structure and sequences, the three mammal isoforms Nuclease assay have non-redundant functions or even opposite effects on gene regulation as described for HP1a and HP1c (Li To prepare nuclei, 5 g of seedlings was ground in a et al. 2002). These three HP1 subtypes present hetero- prechilled mortar on ice, with 20 ml of cold nuclei iso- geneous patterns of nuclear localization and differ in lation buffer [NIB: 10 mM Tris–HCl pH 7.5, 0.7 M their recruitment to euchromatin and heterochromatin sorbitol, 5 mM MgCl2, 5 mM b-mercaptoethanol, both in interphase and during cell cycle (Yamada et al. 1 mM PefablocÒSC (Pentapharm, Basel, Switzerland)]. 1999; Minc et al. 2001; Sugimoto et al. 2001; Hayakawa After filtration through a nylon mesh, the filtrate was et al. 2003). These data highlight various functions for centrifuged (10 min, 200 g,4°C). The nuclei were wa- HP1-like proteins and thus question the function of shed twice and resuspended in NIB. The quality and LHP1 in plants. Here we investigate the nature of the concentration of the nuclei were checked by staining an LHP1-associated chromatin domains for the purpose of aliquot with 30 lg/ml Hoechst 33342 [1 vol in 1 vol of better understanding the LHP1 function. 4% paraformaldehyde in 1·PBS (137 mM, NaCl, We have analyzed the subnuclear distribution of 2.7 mM KCl, 4.3 mM Na2HPO4 2H2O, 1.4 mM LHP1 in A. thaliana and Nicotiana tabacum, two species KH2PO4, pH 7.3)] and by observations on an epifluo- characterized by different genome sizes and different rescence microscope. DNA concentration was estimated heterochromatin contents. The impact of the lhp1 after phenol/chloroform DNA extraction, ethanol pre- mutation on the nuclear organization was studied. cipitation, resuspension in water and OD260 measure- Furthermore, the dynamics of the LHP1 distribution ment. The nuclei were resuspended in NIB supplemented was analyzed throughout the cell cycle and development. with 3 mM CaCl2 to obtain a 0.1 lg/ll equivalent DNA These data highlight some aspects of the LHP1 function concentration. The samples were incubated at 30°Cwith in plants compared to animals. the Micrococcus aureus endonuclease (MNase, Fermen- tas) (0.25 U/ll final concentration). At various time points, 80 ll aliquots were taken, the reaction was Materials and methods stopped by an addition of 329 ll of stop solution (150 mM NaCl, 1% SDS, 20 mM EDTA), and incu- Plant materials bation 10 min at 37°C. After digestion, nuclei were lysed and incubated with proteinase K (0.5 mg/ml), 30 min at The lhp1-1 and lhp1-2 mutants were isolated from the 55°C, then 140 ll of 5 M potassium acetate was added. Versailles T-DNA insertion collection of A. thaliana After 15 min on ice, the tubes were centrifuged (15 min, (Gaudin et al. 2001). For flowering time analyses, plants 12,000 g,4°C). The supernatants were extracted with were grown in growth chambers under short-day (SD; 8- phenol/chloroform/isoamyl alcohol (24:24:1) and DNA h light/16-h dark, with alternate 20°C/16°C tempera- was precipitated. The DNA pellet was resuspended, 912 treated with RNase A and the MNase digestion prod- pericentromeric 5S rDNA (Fransz et al. 1998) and 45S ucts were analyzed in 1.6% agarose gel. rDNA (Gerlach and Bedbrook 1979). Construction of the fusion proteins Staining and fluorescence observations PCR fragments corresponding to LHP1 or truncated Cell walls were visualized after incubating in a propidi- LHP1 (LHPD) coding sequences were amplified using um iodide (PI) solution (5 lg/ml).
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