Automethylation of G9a and its implication in wider substrate specificity and HP1 binding

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Abstract
Methylation of lysine residues on histones partici- pates in transcriptional gene regulation. Lysine 9 methylation of histone H3 is a transcriptional repres- sion signal, mediated by a family of SET domain containing AdoMet-dependent enzymes. G9a meth- yltransferase is a euchromatic histone H3 lysine 9 methyltransferase. Here, G9a is shown to methylate other cellular proteins, apart from histone H3, including automethylation of K239 residue. Auto- methylation of G9a did not impair or activate the en- zymatic activity in vitro. The automethylation motif of G9a flanking target K239 (ARKT) has similarity with histone H3 lysine 9 regions (ARKS), and is identical to amino acids residues in EuHMT (ARKT) and mAM (ARKT). Under steady-state kinetic assay conditions, full-length G9a methylates peptides representing ARKS/T motif of H3, G9a, mAM and EuHMT effi- ciently. Automethylation of G9a at ARKT motif creates a binding site for HP1 class of protein and mutation of lysine in the motif impairs this binding. In COS-7 cells GFP fusion of the wild-type G9a co-localized with HP1a and HP1c isoforms whereas the G9a mutant with K239A displayed poor co-localization. Thus, apart from transcriptional repression andregulatoryrolesoflysine methylation, the non-histone protein methylation may create bindingsites forcellular protein-proteininteractions.