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Heterochromatin protein 1 (HP1a) positively regulates euchromatic gene expression through RNA transcript association and interaction with hnRNPs in Drosophila.

Piacentini L, Fanti L, Negri R, Del Vescovo V, Fatica A, Altieri F, Pimpinelli S - PLoS Genet. (2009)

Bottom Line: To test this suggestion, we performed an extensive screening by RIP-chip assay (RNA-immunoprecipitation on microarrays), and we found that HP1a is associated with transcripts of more than one hundred euchromatic genes.Surprisingly, we found that all these hnRNP proteins also bind heterochromatin and are dominant suppressors of position effect variegation.This suggests that, in general, similar epigenetic mechanisms have a significant role on both RNA and heterochromatin metabolisms.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Genetica e Biologia Molecolare, Università La Sapienza, Istituto Pasteur, Fondazione Cenci Bolognetti, Roma, Italy.

ABSTRACT
Heterochromatin Protein 1 (HP1a) is a well-known conserved protein involved in heterochromatin formation and gene silencing in different species including humans. A general model has been proposed for heterochromatin formation and epigenetic gene silencing in different species that implies an essential role for HP1a. According to the model, histone methyltransferase enzymes (HMTases) methylate the histone H3 at lysine 9 (H3K9me), creating selective binding sites for itself and the chromodomain of HP1a. This complex is thought to form a higher order chromatin state that represses gene activity. It has also been found that HP1a plays a role in telomere capping. Surprisingly, recent studies have shown that HP1a is present at many euchromatic sites along polytene chromosomes of Drosophila melanogaster, including the developmental and heat-shock-induced puffs, and that this protein can be removed from these sites by in vivo RNase treatment, thus suggesting an association of HP1a with the transcripts of many active genes. To test this suggestion, we performed an extensive screening by RIP-chip assay (RNA-immunoprecipitation on microarrays), and we found that HP1a is associated with transcripts of more than one hundred euchromatic genes. An expression analysis in HP1a mutants shows that HP1a is required for positive regulation of these genes. Cytogenetic and molecular assays show that HP1a also interacts with the well known proteins DDP1, HRB87F, and PEP, which belong to different classes of heterogeneous nuclear ribonucleoproteins (hnRNPs) involved in RNA processing. Surprisingly, we found that all these hnRNP proteins also bind heterochromatin and are dominant suppressors of position effect variegation. Together, our data show novel and unexpected functions for HP1a and hnRNPs proteins. All these proteins are in fact involved both in RNA transcript processing and in heterochromatin formation. This suggests that, in general, similar epigenetic mechanisms have a significant role on both RNA and heterochromatin metabolisms.

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Related in: MedlinePlus

Modifications of PEP immunopattern on polytene chromosomes after RNase treatment.(A) PEP immunopattern on wild type polytene chromosomes. (B) PEP immunopattern on wild type polytene chromosomes after RNase treatment. Several immunosignals along euchromatin are removed while other immunosignals (arrows), including those on the chromocenter (arrowheads), become punctuated. Chr = chromocenter.
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pgen-1000670-g010: Modifications of PEP immunopattern on polytene chromosomes after RNase treatment.(A) PEP immunopattern on wild type polytene chromosomes. (B) PEP immunopattern on wild type polytene chromosomes after RNase treatment. Several immunosignals along euchromatin are removed while other immunosignals (arrows), including those on the chromocenter (arrowheads), become punctuated. Chr = chromocenter.

Mentions: Though the immunofluorescence patterns of the hnRNP proteins are somewhat different, they seem to share common sites in the euchromatin of polytene chromosomes, and they are also present at the heterochromatic chromocenter. RNase treatment removes or modifies the euchromatic immunosignals of all these proteins, but it does not completely remove the heterochromatic immunosignals. For example, PEP is not completely removed from either the euchromatic sites or from the chromocenter, but the immunostaining loses its homogeneity and become punctuated (Figure 10). This change could be explained by the fact that PEP binds both DNA and RNA and that the RNase treatment removes only the protein bound to transcripts. On the other hand, the same RNase treatment seems to almost completely remove the euchromatic immunosignals and partially remove the heterochromatic immunosignals produced by the antibody against HRB87F (data not shown). In this case, it seems that the euchromatic localization of HRB87F depends exclusively on RNA, while its localization in different part of the heterochromatin depends on RNA or DNA.


Heterochromatin protein 1 (HP1a) positively regulates euchromatic gene expression through RNA transcript association and interaction with hnRNPs in Drosophila.

Piacentini L, Fanti L, Negri R, Del Vescovo V, Fatica A, Altieri F, Pimpinelli S - PLoS Genet. (2009)

Modifications of PEP immunopattern on polytene chromosomes after RNase treatment.(A) PEP immunopattern on wild type polytene chromosomes. (B) PEP immunopattern on wild type polytene chromosomes after RNase treatment. Several immunosignals along euchromatin are removed while other immunosignals (arrows), including those on the chromocenter (arrowheads), become punctuated. Chr = chromocenter.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2743825&req=5

pgen-1000670-g010: Modifications of PEP immunopattern on polytene chromosomes after RNase treatment.(A) PEP immunopattern on wild type polytene chromosomes. (B) PEP immunopattern on wild type polytene chromosomes after RNase treatment. Several immunosignals along euchromatin are removed while other immunosignals (arrows), including those on the chromocenter (arrowheads), become punctuated. Chr = chromocenter.
Mentions: Though the immunofluorescence patterns of the hnRNP proteins are somewhat different, they seem to share common sites in the euchromatin of polytene chromosomes, and they are also present at the heterochromatic chromocenter. RNase treatment removes or modifies the euchromatic immunosignals of all these proteins, but it does not completely remove the heterochromatic immunosignals. For example, PEP is not completely removed from either the euchromatic sites or from the chromocenter, but the immunostaining loses its homogeneity and become punctuated (Figure 10). This change could be explained by the fact that PEP binds both DNA and RNA and that the RNase treatment removes only the protein bound to transcripts. On the other hand, the same RNase treatment seems to almost completely remove the euchromatic immunosignals and partially remove the heterochromatic immunosignals produced by the antibody against HRB87F (data not shown). In this case, it seems that the euchromatic localization of HRB87F depends exclusively on RNA, while its localization in different part of the heterochromatin depends on RNA or DNA.

Bottom Line: To test this suggestion, we performed an extensive screening by RIP-chip assay (RNA-immunoprecipitation on microarrays), and we found that HP1a is associated with transcripts of more than one hundred euchromatic genes.Surprisingly, we found that all these hnRNP proteins also bind heterochromatin and are dominant suppressors of position effect variegation.This suggests that, in general, similar epigenetic mechanisms have a significant role on both RNA and heterochromatin metabolisms.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Genetica e Biologia Molecolare, Università La Sapienza, Istituto Pasteur, Fondazione Cenci Bolognetti, Roma, Italy.

ABSTRACT
Heterochromatin Protein 1 (HP1a) is a well-known conserved protein involved in heterochromatin formation and gene silencing in different species including humans. A general model has been proposed for heterochromatin formation and epigenetic gene silencing in different species that implies an essential role for HP1a. According to the model, histone methyltransferase enzymes (HMTases) methylate the histone H3 at lysine 9 (H3K9me), creating selective binding sites for itself and the chromodomain of HP1a. This complex is thought to form a higher order chromatin state that represses gene activity. It has also been found that HP1a plays a role in telomere capping. Surprisingly, recent studies have shown that HP1a is present at many euchromatic sites along polytene chromosomes of Drosophila melanogaster, including the developmental and heat-shock-induced puffs, and that this protein can be removed from these sites by in vivo RNase treatment, thus suggesting an association of HP1a with the transcripts of many active genes. To test this suggestion, we performed an extensive screening by RIP-chip assay (RNA-immunoprecipitation on microarrays), and we found that HP1a is associated with transcripts of more than one hundred euchromatic genes. An expression analysis in HP1a mutants shows that HP1a is required for positive regulation of these genes. Cytogenetic and molecular assays show that HP1a also interacts with the well known proteins DDP1, HRB87F, and PEP, which belong to different classes of heterogeneous nuclear ribonucleoproteins (hnRNPs) involved in RNA processing. Surprisingly, we found that all these hnRNP proteins also bind heterochromatin and are dominant suppressors of position effect variegation. Together, our data show novel and unexpected functions for HP1a and hnRNPs proteins. All these proteins are in fact involved both in RNA transcript processing and in heterochromatin formation. This suggests that, in general, similar epigenetic mechanisms have a significant role on both RNA and heterochromatin metabolisms.

Show MeSH
Related in: MedlinePlus