During normal development the spread of heterochromatin provides a mechanism to heritably silence large regions of the genome. This allows selective transcription of a subset of genes required for determining and maintaining cell identity. Defects in heterochromatin assembly are associated with various cancers including breast cancer metastasis and acute myeloid leukemias. The most conserved form of heterochromatin is characterized by methylation of lysine 9 on histone H3 (H3K9me3). Key to both the assembly and the spreading process is the heterochromatin protein 1 (HP1), which binds the H3K9me3 mark via its chromodomain (CD). This interaction forms a versatile platform for recruitment of diverse chromatin modifying proteins. However, the molecular basis of how the HP1-H3K9me3 chromatin complex achieves these diverse interactions is poorly understood. Our work on the major S. pombe HP1 protein, Swi6, suggests that Swi6 switches between an auto-inhibited state and a spreading competent state upon binding methylated chromatin. Cryo-EM allows visualization of the spreading competent state of Swi6 on a methylated nucleosome. Our data suggest testable hypothesis for how specific post-translation modifications on HP1 can regulate its association with methylated chromatin and its interaction with different heterochromatin proteins.

Citation Format: Daniele Canzio, Moafu Liang, Nariman Naber, Adam Larson, Diana Marina, Jennifer Garcia, Hiten Madhani, Roger Cooke, Peter Schuck, Yifan Cheng, Geeta J. Narlikar. Functional and structural studies of HP1 heterochromatin. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr IA23.