Distinct Cytoplasmic and Nuclear Fractions of Drosophila Heterochromatin Protein 1: Their Phosphorylation Levels and Associations with Origin Recognition Complex Proteins |
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Authors: | Da Wei Huang Laura Fanti Daniel T.S. Pak Michael R. Botchan Sergio Pimpinelli Rebecca Kellum |
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Affiliation: | *Department of Biology, McGill University, Montreal, Quebec H3A 1B1, Canada; ‡Istituto di Genetica, Università di Bari, 00176 Bari, Italy; §Istituto Pasteur, Fondazione Cenci Bolognetti, Dipartimento di Genetica e Biologia Molecolare, Università di Roma La Sapienza, 00185 Rome, Italy; ‖ Department of Molecular and Cell Biology, University of California, Berkeley, California 94720 |
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Abstract: | The distinct structural properties of heterochromatin accommodate a diverse group of vital chromosome functions, yet we have only rudimentary molecular details of its structure. A powerful tool in the analyses of its structure in Drosophila has been a group of mutations that reverse the repressive effect of heterochromatin on the expression of a gene placed next to it ectopically. Several genes from this group are known to encode proteins enriched in heterochromatin. The best characterized of these is the heterochromatin-associated protein, HP1. HP1 has no known DNA-binding activity, hence its incorporation into heterochromatin is likely to be dependent upon other proteins. To examine HP1 interacting proteins, we isolated three distinct oligomeric species of HP1 from the cytoplasm of early Drosophila embryos and analyzed their compositions. The two larger oligomers share two properties with the fraction of HP1 that is most tightly associated with the chromatin of interphase nuclei: an underphosphorylated HP1 isoform profile and an association with subunits of the origin recognition complex (ORC). We also found that HP1 localization into heterochromatin is disrupted in mutants for the ORC2 subunit. These findings support a role for the ORC-containing oligomers in localizing HP1 into Drosophila heterochromatin that is strikingly similar to the role of ORC in recruiting the Sir1 protein to silencing nucleation sites in Saccharomyces cerevisiae. |
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Keywords: | HP1 ORC SIR1 heterochromatin assembly silencing |
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