The structure of mouse HP1 suggests a unique mode of single peptide recognition by the shadow chrome domain dimer

The heterochromatin protein 1 (HP1) family of proteins is involved in gene silencing via the formation of heterochromatic structures. They are composed of two related domains: an N-terminal chrome domain and a C-terminal shadow chrome domain. Present results suggest that chrome domains may function as protein interaction motifs, bringing together different proteins in multi-protein complexes and locating them in heterochromatin, We have previously determined the structure of the chrome domain from the mouse HP1 beta protein, MOD1, We show here that, in contrast to the chrome domain, the shadow chrome domain is a homodimer. The intact HP1 beta protein is also dimeric, where the interaction is mediated by the shadow chrome domain, with the chrome domains moving independently of each other at the end of flexible linkers. Mapping studies, with fragments of the CAF1 and TIF1 beta proteins, show that an intact, dimeric, shadow chrome domain structure is required for complex formation.