A Nonessential HP1-Like Protein Affects Starvation-Induced Assembly of Condensed Chromatin and Gene Expression in Macronuclei of <i>Tetrahymena thermophila</i>

Huang, Hui; Smothers, James; Wiley, Emily A.; Allis, C. David

doi:10.1128/mcb.19.5.3624

Cited by 21 publications

(26 citation statements)

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“…Macronuclear chromatin normally condenses upon starvation, coincident with decreased global gene expression and cellular metabolic activity. These events require linker histone H1 for the regulation of specific genes and an HP1-like protein (Hhp1p) (25,26,46,47). Previously, cells at least fivefold reduced in Thd1p protein levels ("⌬THD1" cells) were characterized during logarithmic growth in nutrient-rich medium (66).…”

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confidence: 99%

“…These chromatin changes coincide with cell cycle arrest and decreased transcription of many genes (49), characteristics that are reversed upon refeeding. At the molecular level, starvation-induced chromatin condensation and gene regulation is dependent on the dephosphorylation of histone H1 and the presence of the heterochromatin protein Hhp1, an HP1-like protein that is enriched in chromatin bodies (25,46).…”

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Class I Histone Deacetylase Thd1p Promotes Global Chromatin Condensation in Tetrahymena thermophila

et al. 2007

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Class I histone deacetylases (HDACs) regulate DNA-templated processes such as transcription. They act both at specific loci and more generally across global chromatin, contributing to acetylation patterns that may underlie large-scale chromatin dynamics. Although hypoacetylation is correlated with highly condensed chromatin, little is known about the contribution of individual HDACs to chromatin condensation mechanisms. Using the ciliated protozoan Tetrahymena thermophila, we investigated the role of a specific class I HDAC, ⌻hd1p, in the reversible condensation of global chromatin. In this system, the normal physiological response to cell starvation includes the widespread condensation of the macronuclear chromatin and general repression of gene transcription. We show that the chromatin in Thd1p-deficient cells failed to condense during starvation. The condensation failure correlated with aberrant hyperphosphorylation of histone H1 and the overexpression of CDC2, encoding the major histone H1 kinase. Changes in the rate of acetate turnover on core histones and in the distribution of acetylated lysines 9 and 23/27 on histone H3 isoforms that were found to correlate with normal chromatin condensation were absent from Thd1p mutant cells. These results point to a role for a class I HDAC in the formation of reversible higher-order chromatin structures and global genome compaction through mechanisms involving the regulation of H1 phosphorylation and core histone acetylation/ deacetylation kinetics.

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Class I Histone Deacetylase Thd1p Promotes Global Chromatin Condensation in Tetrahymena thermophila

et al. 2007

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“…Also, branch lengths are similar when either HP1b or HP1c is traced to a common node with known heterochromatin-specific HP1 homologs (HP1␣ and HP1␤) or homologs that are implicated in euchromatin localization (HP1␥; Tetrahymena and Planococcus homologs). Ciliate HP1 localizes to discrete chromatin compartments of nuclei that excise most heterochromatic sequences during development, suggesting that their localization is not entirely exclusive to a heterochromatin compartment (19). The only characterized mealybug HP1 homolog (Pchet 1) localizes to both heterochromatin and euchromatin, indicating that the protein has less specificity for heterochromatin (13).…”

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confidence: 99%

The Hinge and Chromo Shadow Domain Impart Distinct Targeting of HP1-Like Proteins

Smothers

Henikoff

2001

Molecular and Cellular Biology

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Drosophila heterochromatin-associated protein 1 (HP1) is an abundant component of heterochromatin, a highly condensed compartment of the nucleus that comprises a major fraction of complex genomes. Some organisms have been shown to harbor multiple HP1-like proteins, each exhibiting spatially distinct localization patterns within interphase nuclei. We have characterized the subnuclear localization patterns of two newly discovered Drosophila HP1-like proteins (HP1b and HP1c), comparing them with that of the originally described fly HP1 protein (here designated HP1a). While HP1a targets heterochromatin, HP1b localizes to both heterochromatin and euchromatin and HP1c is restricted exclusively to euchromatin. All HP1-like proteins contain an amino-terminal chromo domain, a connecting hinge, and a carboxyl-terminal chromo shadow domain. We expressed truncated and chimeric HP1 proteins in vivo to determine which of these segments might be responsible for heterochromatin-specific and euchromatin-specific localization. Both the HP1a hinge and chromo shadow domain independently target heterochromatin, while the HP1c chromo shadow domain is implicated solely in euchromatin localization. Comparative sequence analyses of HP1 homologs reveal a conserved sequence block within the hinge that contains an invariant sequence (KRK) and a nuclear localization motif. This block is not conserved in the HP1c hinge, possibly accounting for its failure to function as an independent targeting segment. We conclude that sequence variations within the hinge and shadow account for HP1 targeting distinctions. We propose that these targeting features allow different HP1 complexes to be distinctly sequestered in organisms that harbor multiple HP1-like proteins.

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“…Each cell has a transcriptionally active, highly acetylated macronucleus and a transcriptionally inert micronucleus containing unacetylated, highly condensed chromatin for most of the cell cycle (1,9,51). Within the macronucleus are bodies of highly condensed chromatin whose size and number are affected by mutations in chromatin-associated proteins such as histone H1 and Hhp1 (23,46). Moreover, due to polyploidy of the macronucleus, it is possible to create partial deletion mutants in which expression of essential genes is only reduced instead of eliminated (16).…”

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Class I Histone Deacetylase Thd1p Affects Nuclear Integrity in Tetrahymena thermophila

et al. 2005

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Class I histone deacetylases (HDACs) participate in the regulation of DNA-templated processes such as transcription and replication. Members of this class can act locally at specific sites, or they can act more globally, contributing to a baseline acetylation state, both of which actions may be important for genome maintenance and organization. We previously identified a macronuclear-specific class I HDAC in Tetrahymena thermophila called Thd1p, which is expressed early in the development of the macronucleus when it initially becomes transcriptionally active. To test the idea that Thd1p is important for global chromatin integrity in an active macronucleus, Tetrahymena cells reduced in expression of Thd1p were generated. We observed phenotypes that indicated loss of chromatin integrity in the mutant cells, including DNA fragmentation and extrusion of chromatin from the macronucleus, variable macronuclear size and shape, enlarged nucleoli, and reduced phosphorylation of histone H1 from bulk chromatin. Macronuclei in mutant cells also contained more DNA. This observation suggests a role for Thd1p in the control of nuclear DNA content, a previously undescribed role for class I HDACs. Together, these phenotypes implicate Thd1p in the maintenance of macronuclear integrity in multiple ways, probably through site-specific changes in histone acetylation since no change in the acetylation levels of bulk histones was detected in mutant cells.

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A Nonessential HP1-Like Protein Affects Starvation-Induced Assembly of Condensed Chromatin and Gene Expression in Macronuclei of Tetrahymena thermophila

Cited by 21 publications

References 61 publications

Class I Histone Deacetylase Thd1p Promotes Global Chromatin Condensation in Tetrahymena thermophila

Class I Histone Deacetylase Thd1p Promotes Global Chromatin Condensation in Tetrahymena thermophila

The Hinge and Chromo Shadow Domain Impart Distinct Targeting of HP1-Like Proteins

Class I Histone Deacetylase Thd1p Affects Nuclear Integrity in Tetrahymena thermophila

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