Phosphorylation-induced rearrangement of the histone H3 NH2-terminal domain during mitotic chromosome condensation

The NH2-terminal domain (N-tail) of histone H3 has been implicated in chrom atin compaction and its phosphorylation at Ser10 is tightly correlated with mitotic chromosome condensation. We have developed one mAb that specifical ly recognizes histone H3 N-tails phosphorylated at Ser10 (H3P Ab) and anoth er that recognizes phosphorylated and unphosphorylated H3 N-tails equally w ell (H3 Ab). Immunocytochemistry with the H3P Ab shows that Ser10 phosphory lation begins in early prophase, peaks before metaphase, and decreases duri ng anaphase and telophase, Unexpectedly, the H3 Ab shows stronger immunoflu orescence in mitosis than interphase, indicating that the H3 N-tail is more accessible in condensed mitotic chromatin than in decondensed interphase c hromatin. In vivo ultraviolet laser cross-linking indicates that the H3 N-t ail is bound to DNA in interphase cells and that binding is reduced in mito tic cells. Treatment of mitotic cells with the protein kinase inhibitor sta urosporine causes histone H3 dephosphorylation and chromosome decondensatio n. It also decreases the accessibility of the H3 N-tail to H3 Ab and increa ses the binding of the N-tail to DNA. These results indicate that a phospho rylation-dependent weakening of the association between the H3 N-tail and D NA plays a role in mitotic chromosome condensation.