Post translational modification of residues on histone protein tails are known to significantly affect gene expression. In particular, the methylation of lysine residues at position 9 on histone H3 proteins (H3K9) has proven to be a hallmark indicator of inactive genes. To stop the unwanted expression of certain genes in cells it is important to maintain this gene silencing by protecting particular methylated H3K9 sites from demethylation. Hetrachromatin protein 1 (HP1) provides this protection by recognising and binding to trimethylated H3K9 (H3K9me3), denying demethylases access to these sites. Without HP1, H3K9me3 can be quicly demethylated and then acetylated, leading to gene activation. The model provided shows a structural breakdown of the functional region of HP1 known as the ‘chromodomain’. Select residues in the chromodomain are able to recognise an important motif around the H3K9 residue and enable the protective, high affinity binding of its trimethylated state.
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