Heterochromatin Protein 1: An Epigenetic Mechanism for Chromatin Condensation

Gene regulation within the nucleus of eukaryotes requires the segregation of certain silenced genomic regions into densely packed heterochromatin, leaving the active genes within euchromatin regions more accessible.  We have modeled the condensation of chromatin fibers through the binding of heterochromatin protein 1 (HP1) to methylated histone 3 at lysine-9 (H3K9).  HP1 binds cooperatively and condenses the chromatin through bridging interactions between nearby HP1.  We have applied our model to recent results of in vitro nucleosome array systems by Canzio et al., 2011 in Cell.  Their results demonstrate cooperative binding of HP1 and the potential of a bridging mechanism in enabling chromatin condensation.  We make further predictions for the effect of HP1 on nucleosome array condensation, and compare our resulting model with experimental measurements of HP1 concentrations and binding in vivo.