N-terminal phosphorylation of HP1α increases its nucleosome-binding specificity

N-terminal phosphorylation of HP1α increases its nucleosome-binding specificity

Nishibuchi G., Machida S., Osakabe A., Murakoshi H., Hiragami-Hamada K., Nakagawa R., Fischle W., Nishimura Y., Kurumizaka H., Tagami H. and Nakayama J. (2014) N-terminal phosphorylation of HP1α increases its nucleosome-binding specificity. Nucleic Acids Research 42:12498-12511​​
Nishibuchi G., Machida S., Osakabe A., Murakoshi H., Hiragami-Hamada K., Nakagawa R., Fischle W., Nishimura Y., Kurumizaka H., Tagami H., Nakayama J.
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2014
Heterochromatin protein 1 (HP1) is an evolutionarily conserved chromosomal protein that binds to lysine 9-methylated histone H3 (H3K9me), a hallmark of heterochromatin. Although HP1 phosphorylation has been described in several organisms, the biological implications of this modification remain largely elusive. Here we show that HP1's phosphorylation has a critical effect on its nucleosome binding properties. By in vitro phosphorylationassays and conventional chromatography, we demonstrated that casein kinase II (CK2) is the kinase primarily responsible for phosphorylating the N-terminus of human HP1α. Pull-down assays using in vitro-reconstituted nucleosomes showed that unmodified HP1α bound H3K9-methylated and H3K9-unmethylated nucleosomes with comparable affinity, whereas CK2-phosphorylated HP1α showed a high specificity for H3K9me3-modified nucleosomes. Electrophoretic mobility shift assays showed that CK2-mediated phosphorylation diminished HP1α's intrinsic DNA binding, which contributed to its H3K9me-independent nucleosome binding. CK2-mediated phosphorylation had a similar effect on the nucleosome-binding specificityof fly HP1a and S. pombe Swi6. These results suggested that HP1 phosphorylation has an evolutionarily conserved role in HP1's recognition of H3K9me-marked nucleosomes.​