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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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Hierarchical dependence of HP1a, DDP1, HRB87F, and PEP in their assembly on RNA transcripts.Immunopatterns of each protein on polytene chromosomes of wild type larvae and larvae mutant for the genes encoding each of the other proteins. (Only the abnormal patterns are reported here; a complete version of the results is in Figure S3). In DDP1 mutants, the immunopatterns of the other proteins are abnormal. In Hrb87F mutants, the HP1a and PEP immunopatterns are abnormal while the DDP1 immunopattern is unaltered. In HP1a mutants only the PEP immunopattern is abnormal, whereas in PEP mutants the immunopatterns of all other proteins are normal (see Figure S3).
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pgen-1000670-g005: Hierarchical dependence of HP1a, DDP1, HRB87F, and PEP in their assembly on RNA transcripts.Immunopatterns of each protein on polytene chromosomes of wild type larvae and larvae mutant for the genes encoding each of the other proteins. (Only the abnormal patterns are reported here; a complete version of the results is in Figure S3). In DDP1 mutants, the immunopatterns of the other proteins are abnormal. In Hrb87F mutants, the HP1a and PEP immunopatterns are abnormal while the DDP1 immunopattern is unaltered. In HP1a mutants only the PEP immunopattern is abnormal, whereas in PEP mutants the immunopatterns of all other proteins are normal (see Figure S3).

Mentions: To analyze possible interdependencies in chromosomal localization among all these proteins we immunolocated each protein on the polytene chromosomes of larvae mutant for genes encoding each of the other proteins. From the results reported in Figure 5 and in Figure S3, it appears that the correct localization of each protein on the euchromatin depends on the presence of the others according to a hierarchical order with DDP1 on the top: DDP1>HRB87F>HP1a>PEP. We conclude that all these proteins interact for their localization in an ordered manner. Intriguingly, we have also observed that DDP1 is required for the localization of HRB87F and PEP proteins on the chromocenter while DDP1 and Hrb87F mutations partially affect the HP1a heterochromatic immunopattern. We further tested the functional interaction of HP1a with DDP1 and PEP by analyzing the expression of a subset of genes, corresponding to the HP1a target transcripts, in DDP1 and PEP mutant larvae. As reported in Figure 6, we found similar effects to those observed on the expression of same genes in HP1a mutant larvae.


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)

Hierarchical dependence of HP1a, DDP1, HRB87F, and PEP in their assembly on RNA transcripts.Immunopatterns of each protein on polytene chromosomes of wild type larvae and larvae mutant for the genes encoding each of the other proteins. (Only the abnormal patterns are reported here; a complete version of the results is in Figure S3). In DDP1 mutants, the immunopatterns of the other proteins are abnormal. In Hrb87F mutants, the HP1a and PEP immunopatterns are abnormal while the DDP1 immunopattern is unaltered. In HP1a mutants only the PEP immunopattern is abnormal, whereas in PEP mutants the immunopatterns of all other proteins are normal (see Figure S3).
© Copyright Policy
Related In: Results  -  Collection

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

pgen-1000670-g005: Hierarchical dependence of HP1a, DDP1, HRB87F, and PEP in their assembly on RNA transcripts.Immunopatterns of each protein on polytene chromosomes of wild type larvae and larvae mutant for the genes encoding each of the other proteins. (Only the abnormal patterns are reported here; a complete version of the results is in Figure S3). In DDP1 mutants, the immunopatterns of the other proteins are abnormal. In Hrb87F mutants, the HP1a and PEP immunopatterns are abnormal while the DDP1 immunopattern is unaltered. In HP1a mutants only the PEP immunopattern is abnormal, whereas in PEP mutants the immunopatterns of all other proteins are normal (see Figure S3).
Mentions: To analyze possible interdependencies in chromosomal localization among all these proteins we immunolocated each protein on the polytene chromosomes of larvae mutant for genes encoding each of the other proteins. From the results reported in Figure 5 and in Figure S3, it appears that the correct localization of each protein on the euchromatin depends on the presence of the others according to a hierarchical order with DDP1 on the top: DDP1>HRB87F>HP1a>PEP. We conclude that all these proteins interact for their localization in an ordered manner. Intriguingly, we have also observed that DDP1 is required for the localization of HRB87F and PEP proteins on the chromocenter while DDP1 and Hrb87F mutations partially affect the HP1a heterochromatic immunopattern. We further tested the functional interaction of HP1a with DDP1 and PEP by analyzing the expression of a subset of genes, corresponding to the HP1a target transcripts, in DDP1 and PEP mutant larvae. As reported in Figure 6, we found similar effects to those observed on the expression of same genes in HP1a mutant larvae.

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