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.