HP1 Complexes and Heterochromatin Assembly
Since its discovery almost two decades ago, h eterochromatin protein 1 (HP1) has emerged as a major player in the transcriptional regulation of both heterochromatic and euchromatic genes as well as the mechanics of chromosome segregation and the functiona
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HPI Complexes and Heterochromatin Assembly R.KELLUM
Heterochromatin Defined
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Heterochromatin Protein I HPI Homologs . HPI Phosphoisoforms ..
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Heterochromatin Assembly . Yeast Model for Heterochromatin Assembly . Parallels to Metazoan Heterochromatin Assembly Histone Bind ing . Cooperative Self-Association
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HPI-Interacting Proteins HPI Target ing TIFI Proteins SPIOO AFIO ATRX . ORC/HOAP . Su(var)3-7 . HPI and Nuclear Architecture LBR . INCENP . Cohesin . Ku70 Heterochromatin Maintenance
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Insights to Heterochromatin Function
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References
R. KELLUM School of Biological Sciences, 101 T. H. Morgan Building, University of Kentucky, Lexington, KY 40506-0225 , USA e-mail: [email protected]
J.L. Workman (ed.), Protein Complexes that Modify Chromatin © Springer- Verlag Berlin Heidelberg 2003
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R .KELLUM
Abstract. Since its discovery almost two decades ago, heterochromatin llrotein 1 (HPl) has emerged as a major player in the transcriptional regulation of both heterochromatic and euchromatic genes as well as the mechanics of chromosome segregation and the functional and structural organization of the interphase nucleus. Recent years have brought the identification of a myriad of HP1-interacting proteins. Each of these is discussed in relationship to its role in heterochromatin assembly and HP1 function. The breadth of functions represented by HP1-interacting proteins testifies to its pivotal role in the daily operations of the nucleus. 1
Heterochromatin Defined Heterochromatin was first recognized in the polytene nuclei of Drosophila salivary glands as chromosome regions that fail to decondense during telophase and aggregate into a dense mass of deeply staining chromatin along the nuclear envelope of the interphase nucleus (HEITZ 1928). Later studies showed these regions to also have different functional properties from the bulk of the genome. They are both transcriptionally inert and late replicating relative to decondensed euchromatin (LIMA DE FARIA and JAWORSKI 1968; RIS and KORENBERG 1979). The enrichment of heterochromatin with repetitive non-coding DNA sequence partIy accounts for its reduced transcriptional activity (KIT 1961; RAI 1970; PARDUE and GALL 1970). However, the gene silencing properties of heterochromatin were also underscored by early genetic studies showing euchromatic genes to be subjected to mosaic repression when placed next to a block of centric heterochromatin by a chromosome rearrangement (MULLER 1930). The mosaic pattern of repression is characteristic of heterochromatin-induced silencing and is thought to reflect a mechanism for maintaining the repressed state through cell division. Modern molecular tools have now shown this heterochromatininduced silencing to involve an altered, more highly ordered nucleosomal array over the silenced gene (WALLRATH and ELGIN 1995). The molecular de
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