Phosphorylation of CBX2 controls its nucleosome-binding specificity.

Area of Science:

• Epigenetics
• Molecular Biology
• Gene Regulation

Background:

• Chromobox 2 (CBX2) is a key component of Polycomb Repressive Complex 1 (PRC1).
• CBX2 binds to lysine 27-methylated histone H3 (H3K27me3) via its chromodomain (CD).
• The biological significance of CBX2 phosphorylation remains largely unknown.

Purpose of the Study:

• To investigate the role of CBX2 phosphorylation in its nucleosome binding and transcriptional repression.
• To identify the specific phosphorylation sites and kinases involved in CBX2 modification.

Main Methods:

• In vivo and in vitro phosphorylation analysis of CBX2.
• Domain analysis to identify phosphorylation sites.
• Nucleosome pull-down assays to assess H3K27me3-modified nucleosome binding.
• Electrophoretic mobility-shift assays (EMSA) to evaluate DNA-binding activity.
• Analysis of transcriptional repression of the p21 gene using CBX2 mutants.

Main Results:

• CBX2 is stably phosphorylated in vivo, primarily in its serine-rich (SR) region.
• Casein kinase II (CK2) efficiently phosphorylates the SR region in vitro.
• CK2-phosphorylated CBX2 exhibits enhanced specificity for H3K27me3-modified nucleosomes.
• Phosphorylation by CK2 reduces CBX2's DNA-binding activity.
• CBX2 mutants lacking the SR region or acidic-residue (AR) cluster fail to repress p21 transcription.

Conclusions:

• CBX2 phosphorylation, particularly by CK2, is critical for its nucleosome binding specificity.
• Phosphorylation modulates CBX2's DNA-binding activity, impacting its function in gene repression.
• These findings highlight the regulatory role of CBX2 phosphorylation in epigenetic gene silencing.