HP1: heterochromatin binding proteins working the genome

Heterochromatin Protein 1 (HP1) is a transcriptional repressor that directly binds to the methylated lysine 9 residue of histone H3 (H3K9me), which is a hallmark histone modification for transcriptionally silenced heterochromatin. Studies of homologs in different organisms have provided significant insight into the function of HP1 and the role of H3K9me. Initially discovered to be a major constituent of heterochromatin important for gene silencing, HP1 is now known to be a dynamic protein that also functions in transcriptional elongation, centromeric sister chromatid cohesion, telomere maintenance and DNA repair. Furthermore, recent studies have begun to uncover functional differences between HP1 variants and their H3K9me-independent mode of action. As our understanding of HP1 expands, however, conflicting data has also been reported that requires further reconciliation. Here we focus on some of the recent findings and controversies concerning HP1 functions in mammalian cells in comparison to studies in other organisms.