Heterochromatin protein 1: a pervasive controlling influence

Heterochromatin protein 1 (HP1), a component of condensed chromatin, was discovered more than 10 years ago and subsequently found to play important roles in chromosomal biology and gene silencing. Consistent with the hypothesis that post-translational modifications of histones may functionally 'mark' DNA sequences, HP1 was found to bind to 'silent' chromatin via the methylated lysine 9 (K9) residue on the histone H3 tail that protrudes from the nucleosome. The discovery of several HP1-associating proteins has given us insight into how HP1 may function. Although initially found to localise predominantly at heterochromatin, recent data suggest that HP1 also localises and dynamically participates in gene regulation in euchromatin. Moreover, the initial definition of HP1 as a gene repressor may need to be revisited, as HP1 has been shown, in some cases, to localise at transcriptionally active chromosomal sites. Here we review current knowledge on HP1 and explore possible mechanisms whereby HP1 might exert divergent effects on gene regulation.