Silent decision: HP1 protein escorts heterochromatic RNAs to their destiny

Abstract

EMBO J advance online publication June 07 2012; doi:; DOI: 10.1016/j.molcel.2012.05.009

Heterochromatin is classically perceived to be refractory to transcription because of its compact structure. However, Keller et al (2012) now demonstrated that heterochromatic transcripts can accumulate even when heterochromatin is normally packaged. By tracking down the fate of these heterochromatic RNAs, they revealed a new post-transcriptional mechanism of silencing in heterochromatin that involves the dynamic turnover of HP1^Swi6 between its free, chromatin-bound and RNA-bound forms. The latter form escorts heterochromatic RNA to degradation.

Figure 1

HP1^Swi6 undergoes rapid turnover between its (A) free, (B) H3K9me-bound and (C) heterochromatic RNA-bound forms. The major structural component of heterochromatin, H3K9me-bound HP1^Swi6 (B) exchanges dynamically with its free ensemble (A). Contrary to the classical view, RNA polymerase can get access to heterochromatin, but the transcripts are captured by HP1^Swi6 (C) and escorted to Cid14-mediated RNA degradation. RNA competes with H3K9me for binding with HP1^Swi6 and causes structural change to HP1^Swi6. Thus, both heterochromatin and HP1^Swi6–RNA association contributes to the tight repression of genes within heterochromatin.