The chromatin organizer modifier domain (chromodomain) is present in proteins that contribute to chromatin organization and mediates their binding to methylated histone H3. Despite a high level of sequence conservation, individual chromodomains manifest substantial differences in binding preference for methylated forms of histone H3, suggesting that posttranslational modification of the chromodomain might be an important determinant of binding specificity. We now show that mouse Cbx2 (also known as M33), a homolog of Drosophila Polycomb protein, is highly phosphorylated in some cell lines. A low-mobility band of Cbx2 observed on SDS-polyacrylamide gel electrophoresis was thus converted to a higher-mobility band by treatment with alkaline phosphatase. Mass spectrometric analysis revealed serine-42, a conserved amino acid in the chromodomain, as a phosphorylation site of Cbx2. Phosphorylation of the chromodomain of Cbx2 on this residue in vitro resulted in a reduced level of binding to an H3 peptide containing trimethylated lysine-9 as well as an increase in the extent of binding to an H3 peptide containing trimethylated lysine-27, suggesting that such phosphorylation changes the binding specificity of Cbx2 for modified histone H3. Phosphorylation of the chromodomain of Cbx2 may therefore serve as a molecular switch that affects the reading of the histone modification code and thereby controls epigenetic cellular memory. (C) 2010 Elsevier Inc. All rights reserved.