Heterochromatin protein 1 (HP1beta), a key component of condensed DNA, is strongly implicated in gene silencing and centromeric cohesion. Heterochromatin has been considered a static structure, stabilizing crucial aspects of nuclear organization and prohibiting access to transcription factors. We demonstrate here, by fluorescence recovery after photobleaching, that a green fluorescent protein-HP1beta fusion protein is highly mobile within both the euchromatin and heterochromatin of ex vivo resting murine T cells. Moreover, T cell activation greatly increased this mobility, indicating that such a process may facilitate (hetero)chromatin remodeling and permit access of epigenetic modifiers and transcription factors to the many genes that are consequently derepressed.

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.