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• W2000863102 abstract "The constitutive photomorphogenesis 1 (COP1) protein of Arabidopsis thaliana accumulates in discrete subnuclear foci. To better understand the role of subnuclear architecture in COP1-mediated gene expression, we investigated the structural motifs of COP1 that mediate its localization to subnuclear foci using mutational analysis with green fluorescent protein as a reporter. In a transient expression assay, a subnuclear localization signal consisting of 58 residues between amino acids 120 and 177 of COP1 was able to confer speckled localization onto the heterologous nuclear NIa protein from tobacco etch virus. The subnuclear localization signal overlaps two previously characterized motifs, a cytoplasmic localization signal and a putative α-helical coiled-coil domain that has been implicated in COP1 dimerization. Moreover, phenotypically lethal mutations in the carboxyl-terminal WD-40 repeats inhibited localization to subnuclear foci, consistent with a functional role for the accumulation of COP1 at subnuclear sites. The constitutive photomorphogenesis 1 (COP1) protein of Arabidopsis thaliana accumulates in discrete subnuclear foci. To better understand the role of subnuclear architecture in COP1-mediated gene expression, we investigated the structural motifs of COP1 that mediate its localization to subnuclear foci using mutational analysis with green fluorescent protein as a reporter. In a transient expression assay, a subnuclear localization signal consisting of 58 residues between amino acids 120 and 177 of COP1 was able to confer speckled localization onto the heterologous nuclear NIa protein from tobacco etch virus. The subnuclear localization signal overlaps two previously characterized motifs, a cytoplasmic localization signal and a putative α-helical coiled-coil domain that has been implicated in COP1 dimerization. Moreover, phenotypically lethal mutations in the carboxyl-terminal WD-40 repeats inhibited localization to subnuclear foci, consistent with a functional role for the accumulation of COP1 at subnuclear sites. cytoplasmic localization signal nuclear localization signal green fluorescent protein subnuclear localization signal In Arabidopsis thaliana, the constitutive photomorphogenesis 1 (COP1) protein mediates diverse developmental adaptations in response to environmental light signals. When grown under light conditions, Arabidopsis seedlings follow a developmental pathway known as photomorphogenesis, during which aerial portions of the seedling are prepared for photoautotrophic metabolism. When germinating in darkness, however, seedlings use their seed storage reserves to follow an alternative pathway, termed etiolation, in apparent adaptation for rapid growth toward a light source (1von Arnim A.G. Deng X.-W. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 215-243Crossref PubMed Scopus (255) Google Scholar). Loss of function mutants in the COP1 gene cause constitutive photomorphogenesis, implicating COP1 as a repressor of photomorphogenesis or an activator of etiolation. In cop1 mutants, photomorphogenesis in darkness is mediated at least in part by the transcriptional derepression of light-inducible nuclear genes, indicating that COP1 functions, directly or indirectly, as a transcriptional repressor (1von Arnim A.G. Deng X.-W. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 215-243Crossref PubMed Scopus (255) Google Scholar, 2Deng X.-W. Matsui M. Wei N. Wagner D. Chu A.M. Feldmann K.A. Quail P.H. Cell. 1992; 71: 791-801Abstract Full Text PDF PubMed Scopus (469) Google Scholar, 3Chattopadhyay S. Puente P. Deng X.-W. Wei N. Plant J. 1998; 15: 69-77Crossref PubMed Scopus (77) Google Scholar).The COP1 protein contains an amino-terminal zinc binding Ring finger domain (Ring), a coiled-coil domain (Helix), a central core domain, and a carboxyl-terminal domain composed of WD-40 repeats (2Deng X.-W. Matsui M. Wei N. Wagner D. Chu A.M. Feldmann K.A. Quail P.H. Cell. 1992; 71: 791-801Abstract Full Text PDF PubMed Scopus (469) Google Scholar, 4McNellis T.W. von Arnim A.G. Araki T. Komeda Y. Miséra S. Deng X.-W. Plant Cell. 1994; 6: 487-500Crossref PubMed Scopus (304) Google Scholar). COP1 protein is expressed under both light and dark conditions, and severe cop1 mutants show a seedling-lethal phenotype even under light conditions, suggesting that COP1, besides regulating photomorphogenesis, plays a second fundamental role during late embryogenesis, seedling, and vegetative development (5Miséra S. Müller A.J. Weiland-Heidecker U. Jürgens G. Mol. Gen. Genet. 1994; 244: 242-252Crossref PubMed Scopus (157) Google Scholar). A COP1 fragment composed of the Ring finger and Helix domains, the allele COP1–4, can satisfy the need for the latter, light-independent, functions of COP1, but for repression of light-inducible gene expression, the full COP1 protein is required (4McNellis T.W. von Arnim A.G. Araki T. Komeda Y. Miséra S. Deng X.-W. Plant Cell. 1994; 6: 487-500Crossref PubMed Scopus (304) Google Scholar).The regulatory function of COP1 appears to be mediated by interactions with other nuclear proteins, including the COP1-interactive protein-7 (CIP7), a likely transcriptional activator (6Yamamoto Y.Y. Matsui M. Ang L.-H. Deng X.-W. Plant Cell. 1998; 10: 1083-1094Crossref PubMed Scopus (112) Google Scholar), and the basic leucine zipper protein HY5 (7Ang L-H. Chattopadhyay S. Wei N. Oyama T. Okada K. Batschauer A. Deng X.-W. Mol. Cell. 1998; 1: 213-222Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar). Given that hy5 mutants display reduced responsiveness to light and that HY5 can bind to light-regulatory promoter elements, COP1 may repress transcription by interfering with light-regulated transcriptional activation (8Chattopadhyay S. Ang L.-H. Puente P. Deng X.-W. Wei N. Plant Cell. 1998; 10: 673-683Crossref PubMed Scopus (333) Google Scholar). Light signals may regulate the activity of COP1 at least in part by modulating the nuclear level of the COP1 protein. A fusion protein between COP1 and β-glucuronidase as a reporter accumulates in the nucleus of Arabidopsis seedling stem (hypocotyl) cells in darkness, yet it is excluded from the nucleus under light conditions (9von Arnim A.G. Deng X.-W. Cell. 1994; 79: 1035-1045Abstract Full Text PDF PubMed Scopus (362) Google Scholar, 10von Arnim A.G. Osterlund M.T. Kwok S.F. Deng X.-W. Plant Physiol. 1997; 114: 779-788Crossref PubMed Scopus (107) Google Scholar). The redistribution of the β-glucuronidase-COP1 protein by light is mediated by multiple photoreceptors of the phytochrome and cryptochrome families (11Osterlund M.T. Deng X.-W. Plant J. 1998; 16: 201-208Crossref PubMed Google Scholar). Cytoplasmic localization of COP1 is mediated by a cytoplasmic localization signal (CLS),1 which counteracts a classical bipartite nuclear localization signal (NLS), located in the central core domain, in a light-dependent manner (12Stacey M.G. Hicks S.N. von Arnim A.G. Plant Cell. 1999; 11: 349-364Crossref PubMed Scopus (99) Google Scholar).The COP1 protein displays a characteristic localization to discrete subnuclear sites under a variety of experimental conditions. Immunofluorescence labeling has highlighted COP1 in discrete subnuclear regions in wild-type Arabidopsis cells. Moreover, both β-glucuronidase-COP1 and green fluorescent protein (GFP)-COP1 fusion proteins accumulate in subnuclear foci when expressed in transgenic Arabidopsis or in transiently transformed onion epidermal cells (13von Arnim A.G. Deng X.-W. Stacey M.G. Gene. 1998; 221: 35-43Crossref PubMed Scopus (201) Google Scholar, 7Ang L-H. Chattopadhyay S. Wei N. Oyama T. Okada K. Batschauer A. Deng X.-W. Mol. Cell. 1998; 1: 213-222Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar). Like the nucleus of animal cells (14Lamond, A. I., and Earnshaw, W. C. (1998) 280, 547–553Google Scholar, 15Strouboulis J. Wolffe A.P. J. Cell Sci. 1996; 109: 1991-2000Crossref PubMed Google Scholar), the plant cell nucleus is a highly structured organelle (16Shaw P.J. Essays Biochem. 1996; 31: 77-89PubMed Google Scholar). For example, telomeres appear to be located preferentially at the nuclear periphery (17Rawlins D.J. Highett M.I. Shaw P.J. Chromosoma (Berl.). 1991; 100: 424-431Crossref Scopus (79) Google Scholar). Ribosomal gene transcription and ribosome preassembly are sequestered into nucleoli, organelles that are further subdivided into domains (18Beven A.F. Lee R. Razaz M. Leader D.J. Brown J.W.S. Shaw P.J. J. Cell Sci. 1996; 109: 1241-1251Crossref PubMed Google Scholar, 19Shaw, P. J., Beven, A. F., Wells, B., Highett, M. I., and Jordan, E. G. (1996) 181, 178–185Google Scholar). Certain splicing components are concentrated in subnuclear granules or speckles, some of which are immunologically related to coiled bodies (20Beven A.F. Simpson G.G. Brown J.W.S. Shaw P.J. J. Cell Sci. 1995; 108: 509-518Crossref PubMed Google Scholar, 21Boudonck K. Dolan L. Shaw P.J. J. Cell Sci. 1998; 18: 1687-3694Google Scholar, 22Glyn M.C.P. Leitch A.R. Plant J. 1995; 8: 531-540Crossref PubMed Scopus (19) Google Scholar, 23Lamond A.I. Carmo-Fonseca M. Trends Cell Biol. 1993; 3: 198-201Abstract Full Text PDF PubMed Scopus (159) Google Scholar). In animal cell nuclei, numerous proteins are distributed in diverse “micro-punctate” patterns, which may be functionally relevant in the regulation of gene expression (e.g. Refs. 24Brett D. Whitehouse S. Antonson P. Shipley J. Cooper C. Goodwin G. Hum. Mol. Genet. 1997; 6: 1559-1564Crossref PubMed Scopus (131) Google Scholar, 25Kelley R.L. Solovyeva I. Lyman L.M. Richman R. Solovyev V. Kuroda M.I. Cell. 1995; 81: 867-877Abstract Full Text PDF PubMed Scopus (252) Google Scholar). However, the subnuclear compartmentalization of the plant nucleus and its role in gene expression are comparatively poorly understood.We reasoned that a mutational analysis of the structural requirements in COP1 for localization to nuclear foci may shed light on the biological role of the foci for COP1 function and on the cooperation between the cytoplasmic, nuclear, and subnuclear targeting signals within COP1. Using primarily fusion proteins between COP1 mutants and green fluorescent protein, we found that a short subfragment of the COP1 coiled-coil domain confers localization to foci on a heterologous protein, that a domain responsible for the formation of cytoplasmic inclusion bodies can be separated from the subnuclear localization signal, and that three phenotypically lethal mutations in the WD-40 domain of COP1 interfere with the subnuclear targeting of GFP-COP1. Our data represent the first mutational analysis of the subnuclear targeting of a plant protein.DISCUSSIONA variety of nuclear factors can dynamically localize to discrete subnuclear domains or foci rather than being randomly dispersed throughout the nucleus (reviewed in Refs. 14Lamond, A. I., and Earnshaw, W. C. (1998) 280, 547–553Google Scholar and 15Strouboulis J. Wolffe A.P. J. Cell Sci. 1996; 109: 1991-2000Crossref PubMed Google Scholar). These factors include catalytic protein complexes involved in replication (33Hozak P. Hassan A.B. Jackson D.A. Cook P.R. Cell. 1993; 73: 361-373Abstract Full Text PDF PubMed Scopus (391) Google Scholar), transcription (e.g. ribosomal RNA transcription by polymerase I (16Shaw P.J. Essays Biochem. 1996; 31: 77-89PubMed Google Scholar)), and splicing (34Huang S. Spector D.L. Genes Dev. 1991; 5: 2288-2302Crossref PubMed Scopus (186) Google Scholar), as well as regulatory proteins that control these processes, for instance Drosophila Polycomb (35Messmer S. Franke A. Paro R. Genes Dev. 1992; 6: 1241-1254Crossref PubMed Scopus (197) Google Scholar) and MSL-2 (25Kelley R.L. Solovyeva I. Lyman L.M. Richman R. Solovyev V. Kuroda M.I. Cell. 1995; 81: 867-877Abstract Full Text PDF PubMed Scopus (252) Google Scholar). In plants, pioneering studies have confirmed the general notion derived from studies in animals that the plant nucleus possesses a well defined architecture (17Rawlins D.J. Highett M.I. Shaw P.J. Chromosoma (Berl.). 1991; 100: 424-431Crossref Scopus (79) Google Scholar, 19Shaw, P. J., Beven, A. F., Wells, B., Highett, M. I., and Jordan, E. G. (1996) 181, 178–185Google Scholar, 20Beven A.F. Simpson G.G. Brown J.W.S. Shaw P.J. J. Cell Sci. 1995; 108: 509-518Crossref PubMed Google Scholar). However, few regulatory proteins have been examined closely for their subnuclear distribution. The COP1 protein, a fundamental plant nuclear regulator encoded by a member of the pleiotropic COP/DET/FUS genes, exhibits a characteristic localization to discrete nuclear foci besides a faint diffuse distribution (7Ang L-H. Chattopadhyay S. Wei N. Oyama T. Okada K. Batschauer A. Deng X.-W. Mol. Cell. 1998; 1: 213-222Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar, 12Stacey M.G. Hicks S.N. von Arnim A.G. Plant Cell. 1999; 11: 349-364Crossref PubMed Scopus (99) Google Scholar, 13von Arnim A.G. Deng X.-W. Stacey M.G. Gene. 1998; 221: 35-43Crossref PubMed Scopus (201) Google Scholar). In transient co-expression assays, native COP1 is able to redistribute the basic leucine zipper protein HY5 into nuclear foci, suggesting that the COP1 protein in the foci must be in an at least partly native configuration (7Ang L-H. Chattopadhyay S. Wei N. Oyama T. Okada K. Batschauer A. Deng X.-W. Mol. Cell. 1998; 1: 213-222Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar). Here, we have begun to delineate the structural motifs that mediate the distribution of COP1 between a soluble form and a form associated with nuclear foci.Two distinct structural elements in COP1 are important for localization to nuclear foci, namely the WD-40 domain and the Helix domain. The WD-40 domain may play an indirect role, because it did not mediate foci formation alone. However, a loss or a reduction in foci formation was observed for three GFP-COP1 fusions mutated in the WD-40 domain, COP1–8, COP1–9, and COP1–11. The COP1–9 allele has a G524Q missense mutation in a residue conserved among three plant COP1 homologs from Arabidopsis, pea (Zhao et al. (36Zhao L. Wang C. Zhu Y. Zhao J. Wu X. Biochim. Biophys. Acta. 1998; 1395: 326-328Crossref PubMed Scopus (10) Google Scholar)), and tomato. COP1–8 is an exon-skipping mutant, and COP1–11 has a premature stop codon. All three are loss-of-function alleles (4McNellis T.W. von Arnim A.G. Araki T. Komeda Y. Miséra S. Deng X.-W. Plant Cell. 1994; 6: 487-500Crossref PubMed Scopus (304) Google Scholar). Neither mutant disrupted the localization of wild-type GFP-COP1 in our transient assay, consistent with the recessive nature of the mutations.Other than the WD-40 domain, the Helix domain was both necessary and sufficient for foci formation, as indicated first by the complete disappearance of foci upon its deletion. Second, the Helix domain, together with the central core domain containing the NLS, formed the minimum COP1 fragment that showed foci. In addition, a 58-amino acid fragment within the Helix, residues 120–177, conferred localization to foci onto the heterologous nuclear-targeted NIa protein. The NIa protein from tobacco etch virus was chosen because it contains a strong and context-independent NLS (27Carrington J.C. Freed D.D. Leinicke A.J. Plant Cell. 1991; 3: 953-962PubMed Google Scholar). The GFP-NIa fusion, unlike GFP-COP1, was dispersed throughout the nucleoplasm, with some preferential association with nucleoli (37Baunoch D. Das P. Hari V. J. Ultrastruct. and Mol. Struct. Res. 1988; 99: 203-212Crossref Scopus (18) Google Scholar), which may be mediated by an RNA binding activity of the NIa protein (38Daros J.-A. Carrington J.C. Virology. 1997; 237: 327-336Crossref PubMed Scopus (47) Google Scholar). The 58-residue fragment, which we refer to as a SNLS (Fig. 7), also prevented the characteristic nucleolar enrichment of NIa. Like the GFP-COP1 foci, the foci formed by the GFP-SNLS-NIa fusion were rounded, approximately 1 μm in diameter, and evenly distributed throughout the nucleoplasm, indicating that both are equivalent structures. Moreover, co-expression of the SNLS-NIa fusion with COP1 clearly showed that all the COP1 foci contained the SNLS-NIa protein.Few proteins have been examined closely for the targeting signals that actively confer localization to subnuclear sites. In the Drosophila Polycomb protein, the chromodomain is able to confer a speckled localization onto β-galactosidase (35Messmer S. Franke A. Paro R. Genes Dev. 1992; 6: 1241-1254Crossref PubMed Scopus (197) Google Scholar). In the human ALL-1 protein, two distinct motifs, both with similarity to Drosophila trithorax, were able to localize covalently linked cytoplasmic pyruvate kinase to nuclear foci (39Yano T. Nakamura T. Blechmann J. Sorrio C. Dang C.V. Geiger B. Canaani E. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 7286-7291Crossref PubMed Scopus (85) Google Scholar). In the mammalian protein SP100, targeting to nuclear bodies containing the promyelocytic leukelia protein PML requires a domain thought to include a helical motif (40Sternsdorf T. Jensen K. Reich B. Will H. J. Biol. Chem. 1999; 274: 12555-12566Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar). The SNLS of COP1 does not show obvious sequence similarity with any of these proteins nor with any other known proteins apart from COP1 orthologs. However, physiologically, COP1 functions as a repressor of gene expression, and the localization to nuclear foci may be instrumental in regulating the access of COP1 to its target sites.The SNLS between residues 120 and 177 represents a portion of the COP1 CLS (residues 67–177; Ref. 12Stacey M.G. Hicks S.N. von Arnim A.G. Plant Cell. 1999; 11: 349-364Crossref PubMed Scopus (99) Google Scholar; Fig. 7), as well as of a COP1 fragment mediating dimerization (residues 105–211 (41Torii K.U. McNellis T.W. Deng X.-W. EMBO J. 1998; 17: 5577-5587Crossref PubMed Scopus (113) Google Scholar)). The Helix domain (coiled-coil: 125–220) also mediates interactions with proteins other than COP1, namely the predominantly cytoplasmic CIP1 protein (42Matsui M. Stoop C.D. von Arnim A.G. Wei N. Deng X.-W. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 4239-4243Crossref PubMed Scopus (65) Google Scholar) and the nuclear CIP7 (6Yamamoto Y.Y. Matsui M. Ang L.-H. Deng X.-W. Plant Cell. 1998; 10: 1083-1094Crossref PubMed Scopus (112) Google Scholar), neither of which, however, is known to localize to nuclear foci. It is possible that the SNLS interacts with yet other nuclear proteins to target COP1 to the foci. In addition, COP1-COP1 multimerization may play a role, given that COP1(1–282) dimerizes strongly in the yeast two-hybrid assay (32McNellis T.W. Torii K.U. Deng X.-W. Plant Cell. 1996; 8: 1491-1503Crossref PubMed Scopus (64) Google Scholar, 40Sternsdorf T. Jensen K. Reich B. Will H. J. Biol. Chem. 1999; 274: 12555-12566Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar) and the SNLS motif retained approximately 25% of two-hybrid activity (data not shown). It is intriguing that the SNLS is part of a larger motif that mediates the cytoplasmic localization of COP1, and the CLS is itself under regulation by other COP1 domains that inhibit CLS activity in darkness (12Stacey M.G. Hicks S.N. von Arnim A.G. Plant Cell. 1999; 11: 349-364Crossref PubMed Scopus (99) Google Scholar). Maybe, in the context of wild-type COP1, a factor that inhibits localization to nuclear foci is required to activate the CLS and vice versa.Currently, our data do not allow us to distinguish whether the foci represent a site of active COP1 protein or a dispensable storage site for inactive COP1. However, the observed correlation between loss-of-function and loss of nuclear foci among three mutations in the WD-40 domain is certainly consistent with a functional role of the foci. Perturbations in the subnuclear localization of specific proteins have been implicated in the pathogenesis of human disease (43Kastner P. Perez A. Lutz Y. Rochette-Egli C. Gaub M.-P. Durand B. Lanotte M. Berger R. Chambon P. EMBO J. 1992; 11: 629-642Crossref PubMed Scopus (418) Google Scholar). For example, the Wilms tumor 1 (WT1) gene product is distributed between a diffuse phase and a punctate phase, the latter containing splicing factors involved in RNA maturation (44Larsson S.H. Charlieu J.P. Miyagawa K. Engelkamp D. Rassoulzadegan M. Ross A. Cuzin F. van Heyningen V. Hastie N.D. Cell. 1995; 81: 391-401Abstract Full Text PDF PubMed Scopus (439) Google Scholar). Loss of DNA binding of WT1 was associated with accumulation in the punctate form (28Englert C. Vidal M. Maheshwaran S. Ge Y. Ezzell R.M. Isselbacher K.J. Haber D.A. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 11960-11964Crossref PubMed Scopus (127) Google Scholar, 44Larsson S.H. Charlieu J.P. Miyagawa K. Engelkamp D. Rassoulzadegan M. Ross A. Cuzin F. van Heyningen V. Hastie N.D. Cell. 1995; 81: 391-401Abstract Full Text PDF PubMed Scopus (439) Google Scholar). Our data on the subnuclear localization of COP1 suggest that subnuclear partitioning of regulatory proteins is not confined to animal cells but also occurs in plants . Recently, a GFP fusion of the phytochrome B photoreceptor was shown to localize to nuclear foci highly reminiscent of those seen for COP1 (29Yamaguchi R. Nakamura M. Mochizuki N. Kay S.A. Nagatani A. J. Cell Biol. 1999; 145: 437-445Crossref PubMed Scopus (294) Google Scholar), an intriguing result given that COP1 nuclear localization is negatively regulated by phytochrome B (11Osterlund M.T. Deng X.-W. Plant J. 1998; 16: 201-208Crossref PubMed Google Scholar). Additional examples of subnuclear localization patterns in plant cells are likely to be discovered. Their biochemical characterization with respect to the foci formed by COP1 and by foci seen in animal cells may shed light on the structural basis of gene regulation. In Arabidopsis thaliana, the constitutive photomorphogenesis 1 (COP1) protein mediates diverse developmental adaptations in response to environmental light signals. When grown under light conditions, Arabidopsis seedlings follow a developmental pathway known as photomorphogenesis, during which aerial portions of the seedling are prepared for photoautotrophic metabolism. When germinating in darkness, however, seedlings use their seed storage reserves to follow an alternative pathway, termed etiolation, in apparent adaptation for rapid growth toward a light source (1von Arnim A.G. Deng X.-W. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 215-243Crossref PubMed Scopus (255) Google Scholar). Loss of function mutants in the COP1 gene cause constitutive photomorphogenesis, implicating COP1 as a repressor of photomorphogenesis or an activator of etiolation. In cop1 mutants, photomorphogenesis in darkness is mediated at least in part by the transcriptional derepression of light-inducible nuclear genes, indicating that COP1 functions, directly or indirectly, as a transcriptional repressor (1von Arnim A.G. Deng X.-W. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 215-243Crossref PubMed Scopus (255) Google Scholar, 2Deng X.-W. Matsui M. Wei N. Wagner D. Chu A.M. Feldmann K.A. Quail P.H. Cell. 1992; 71: 791-801Abstract Full Text PDF PubMed Scopus (469) Google Scholar, 3Chattopadhyay S. Puente P. Deng X.-W. Wei N. Plant J. 1998; 15: 69-77Crossref PubMed Scopus (77) Google Scholar). The COP1 protein contains an amino-terminal zinc binding Ring finger domain (Ring), a coiled-coil domain (Helix), a central core domain, and a carboxyl-terminal domain composed of WD-40 repeats (2Deng X.-W. Matsui M. Wei N. Wagner D. Chu A.M. Feldmann K.A. Quail P.H. Cell. 1992; 71: 791-801Abstract Full Text PDF PubMed Scopus (469) Google Scholar, 4McNellis T.W. von Arnim A.G. Araki T. Komeda Y. Miséra S. Deng X.-W. Plant Cell. 1994; 6: 487-500Crossref PubMed Scopus (304) Google Scholar). COP1 protein is expressed under both light and dark conditions, and severe cop1 mutants show a seedling-lethal phenotype even under light conditions, suggesting that COP1, besides regulating photomorphogenesis, plays a second fundamental role during late embryogenesis, seedling, and vegetative development (5Miséra S. Müller A.J. Weiland-Heidecker U. Jürgens G. Mol. Gen. Genet. 1994; 244: 242-252Crossref PubMed Scopus (157) Google Scholar). A COP1 fragment composed of the Ring finger and Helix domains, the allele COP1–4, can satisfy the need for the latter, light-independent, functions of COP1, but for repression of light-inducible gene expression, the full COP1 protein is required (4McNellis T.W. von Arnim A.G. Araki T. Komeda Y. Miséra S. Deng X.-W. Plant Cell. 1994; 6: 487-500Crossref PubMed Scopus (304) Google Scholar). The regulatory function of COP1 appears to be mediated by interactions with other nuclear proteins, including the COP1-interactive protein-7 (CIP7), a likely transcriptional activator (6Yamamoto Y.Y. Matsui M. Ang L.-H. Deng X.-W. Plant Cell. 1998; 10: 1083-1094Crossref PubMed Scopus (112) Google Scholar), and the basic leucine zipper protein HY5 (7Ang L-H. Chattopadhyay S. Wei N. Oyama T. Okada K. Batschauer A. Deng X.-W. Mol. Cell. 1998; 1: 213-222Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar). Given that hy5 mutants display reduced responsiveness to light and that HY5 can bind to light-regulatory promoter elements, COP1 may repress transcription by interfering with light-regulated transcriptional activation (8Chattopadhyay S. Ang L.-H. Puente P. Deng X.-W. Wei N. Plant Cell. 1998; 10: 673-683Crossref PubMed Scopus (333) Google Scholar). Light signals may regulate the activity of COP1 at least in part by modulating the nuclear level of the COP1 protein. A fusion protein between COP1 and β-glucuronidase as a reporter accumulates in the nucleus of Arabidopsis seedling stem (hypocotyl) cells in darkness, yet it is excluded from the nucleus under light conditions (9von Arnim A.G. Deng X.-W. Cell. 1994; 79: 1035-1045Abstract Full Text PDF PubMed Scopus (362) Google Scholar, 10von Arnim A.G. Osterlund M.T. Kwok S.F. Deng X.-W. Plant Physiol. 1997; 114: 779-788Crossref PubMed Scopus (107) Google Scholar). The redistribution of the β-glucuronidase-COP1 protein by light is mediated by multiple photoreceptors of the phytochrome and cryptochrome families (11Osterlund M.T. Deng X.-W. Plant J. 1998; 16: 201-208Crossref PubMed Google Scholar). Cytoplasmic localization of COP1 is mediated by a cytoplasmic localization signal (CLS),1 which counteracts a classical bipartite nuclear localization signal (NLS), located in the central core domain, in a light-dependent manner (12Stacey M.G. Hicks S.N. von Arnim A.G. Plant Cell. 1999; 11: 349-364Crossref PubMed Scopus (99) Google Scholar). The COP1 protein displays a characteristic localization to discrete subnuclear sites under a variety of experimental conditions. Immunofluorescence labeling has highlighted COP1 in discrete subnuclear regions in wild-type Arabidopsis cells. Moreover, both β-glucuronidase-COP1 and green fluorescent protein (GFP)-COP1 fusion proteins accumulate in subnuclear foci when expressed in transgenic Arabidopsis or in transiently transformed onion epidermal cells (13von Arnim A.G. Deng X.-W. Stacey M.G. Gene. 1998; 221: 35-43Crossref PubMed Scopus (201) Google Scholar, 7Ang L-H. Chattopadhyay S. Wei N. Oyama T. Okada K. Batschauer A. Deng X.-W. Mol. Cell. 1998; 1: 213-222Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar). Like the nucleus of animal cells (14Lamond, A. I., and Earnshaw, W. C. (1998) 280, 547–553Google Scholar, 15Strouboulis J. Wolffe A.P. J. Cell Sci. 1996; 109: 1991-2000Crossref PubMed Google Scholar), the plant cell nucleus is a highly structured organelle (16Shaw P.J. Essays Biochem. 1996; 31: 77-89PubMed Google Scholar). For example, telomeres appear to be located preferentially at the nuclear periphery (17Rawlins D.J. Highett M.I. Shaw P.J. Chromosoma (Berl.). 1991; 100: 424-431Crossref Scopus (79) Google Scholar). Ribosomal gene transcription and ribosome preassembly are sequestered into nucleoli, organelles that are further subdivided into domains (18Beven A.F. Lee R. Razaz M. Leader D.J. Brown J.W.S. Shaw P.J. J. Cell Sci. 1996; 109: 1241-1251Crossref PubMed Google Scholar, 19Shaw, P. J., Beven, A. F., Wells, B., Highett, M. I., and Jordan, E. G. (1996) 181, 178–185Google Scholar). Certain splicing components are concentrated in subnuclear granules or speckles, some of which are immunologically related to coiled bodies (20Beven A.F. Simpson G.G. Brown J.W.S. Shaw P.J. J. Cell Sci. 1995; 108: 509-518Crossref PubMed Google Scholar, 21Boudonck K. Dolan L. Shaw P.J. J. Cell Sci. 1998; 18: 1687-3694Google Scholar, 22Glyn M.C.P. Leitch A.R. Plant J. 1995; 8: 531-540Crossref PubMed Scopus (19) Google Scholar, 23Lamond A.I. Carmo-Fonseca M. Trends Cell Biol. 1993; 3: 198-201Abstract Full Text PDF PubMed Scopus (159) Google Scholar). In animal cell nuclei, numerous proteins are distributed in diverse “micro-punctate” patterns, which may be functionally relevant in the regulation of gene expression (e.g. Refs. 24Brett D. Whitehouse S. Antonson P. Shipley J. Cooper C. Goodwin G. Hum. Mol. Genet. 1997; 6: 1559-1564Crossref PubMed Scopus (131) Google Scholar, 25Kelley R.L. Solovyeva I. Lyman L.M. Richman R. Solovyev V. Kuroda M.I. Cell. 1995; 81: 867-877Abstract Full Text PDF PubMed Scopus (252) Google Scholar). However, the subnuclear compartmentalization of the plant nucleus and its role in gene expression are comparatively poorly understood. We reasoned that a mutational analysis of the structural requirements in COP1 for localization to nuclear foci may shed light on the biological role of the foci for COP1 function and on the cooperation between the cytoplasmic, nuclear, and subnuclear targeting signals within COP1. Using primarily fusion proteins between COP1 mutants and green fluorescent protein, we found that a short subfragment of the COP1 coiled-coi" @default.
• W2000863102 created "2016-06-24" @default.
• W2000863102 creator A5041760433 @default.
• W2000863102 creator A5083629998 @default.
• W2000863102 date "1999-09-01" @default.
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