HP1α is not necessary for the structural maintenance of centromeric heterochromatin

Heterochromatin protein 1 (HP1) was discovered as a protein essential for maintaining the silent transcriptional status of genes located within or close to centromeric regions of Drosophila chromosomes. Mammals express three variants of HP1; of these, HP1α is a direct homolog of Drosophila HP1. The prevailing view states that HP1 is a structural component of heterochromatin and is essential for compact DNA packaging. HP1 contains a chromodomain that binds to di- and- tri-methylated lysine 9 of histone H3. Additionally, it contains a chromoshadow domain that allows HP1 to dimerize and interact with other proteins. HP1 is thought to form "bridges" between neighboring rows of nucleosomes in heterochromatin. In mammalian cells, a significant portion of HP1α is located in the centromeric regions of chromosomes. In this study, we show that the majority of HP1α is removed from centromeres upon heat shock. This occurs without a loss of H3K9 trimethylation and does not correlate with a decompaction of centromeres. Furthermore, HP1α is not degraded and remains bound to chromatin. Therefore, it is likely that HP1α is simply redistributed to euchromatic regions. We propose that this redistribution is essential for reversal of the transcriptional status of euchromatic and heterochromatic compartments.