Differential cooperation between heterochromatin protein HP1 isoforms and MyoD in myoblasts

Mechanisms of transcriptional repression are important during cell differentiation. Mammalian heterochromatin protein 1 isoforms HP1 alpha, HP1 beta, and HP1 gamma play important roles in the regulation of chromatin structure and function. We explored the possibility of different roles for the three HP1 isoforms in an integrated system, skeletalmuscleterminaldifferentiation. Inthissystem, terminal differentiation is initiated by the transcription factor MyoD, whose target genes remain mainly silent until myoblasts are induced to differentiate. Here we show that HP1 beta and HP1 gamma isoforms, but not HP1 alpha, interact withMyoDin myoblasts. This interaction is direct, as shown using recombinant proteins in vitro. A gene reporter assay revealed that HP1 alpha and HP1 beta, but not HP1 gamma, inhibit MyoD transcriptional activity, suggesting a model in which MyoD could serve as a bridge between nucleosomes and chromatinbinding proteins such as HDACs and HP1. Chromatin immunoprecipitationassaysshowapreferentialrecruitmentofHP1proteins on MyoD target genes in proliferating myoblasts. Finally, modulation ofHP1protein level impairsMyoDtarget gene expression and muscle terminal differentiation. Together, our data show a nonconventional interaction between HP1 and a tissue- specific transcription factor, MyoD. In addition, they strongly suggest that HP1 isoforms play important roles during muscle terminal differentiation in an isoform- dependent manner.