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 compose d of two related domains: an N-terminal chrome domain and a C-terminal shad ow chrome domain. Present results suggest that chrome domains may function as protein interaction motifs, bringing together different proteins in mult i-protein complexes and locating them in heterochromatin, We have previousl y determined the structure of the chrome domain from the mouse HP1 beta pro tein, 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 ch rome domains moving independently of each other at the end of flexible link ers. Mapping studies, with fragments of the CAF1 and TIF1 beta proteins, sh ow that an intact, dimeric, shadow chrome domain structure is required for complex formation.