^{Genomics and Molecular Biology}

Histone deacetylase 3 (HDAC3) and linker histone H1 are involved in both chromatin compaction and the regulation of mitotic progression. However, the mechanisms by which HDAC3 and H1 regulate mitosis as well as the factors controlling HDAC3 and H1 activity during mitosis are unclear. Furthermore, as of now, no other association between class I, II or IV HDACs and linker histones has been reported. Here, we describe a novel HDAC3-H1.3 complex containing SMRT and N-CoR and at least four other proteins which accumulated in synchronized HeLa cells in late G2 and mitosis. Nonetheless, the deacetylation activity by the HDAC3 in the complex was evident only in mitotic complexes. HDAC3 associated to H1.3 was highly phosphorylated on S424 only during mitosis. Isolation of inactive HDAC3-H1.3 complexes from late-G2 cells and phosphorylation of HDAC3 in the complexes at serine 424 by protein kinase CK2 (also known as casein kinase 2), activated the HDAC3 in vitro. In vivo, CK2a and CK2a’ double knockdown cells demonstrated a signifi cant decrease in HDAC3 S424 phosphorylation during mitosis. HDAC3 and H1.3 co-localized in between the chromosomes with polar microtubules and spindle poles during metaphase through telophase and partially co-localized with chromatin during prophase and interphase. H1 was previously reported to associate with microtubules; thus, it could potentially function in targeting HDAC3 to the microtubules. We recently demonstrated that HDAC3, H1.3 or double knockdown cells have a lower microtubule polymerization rate in mitotic cells, thus supporting the role of the activated HDAC3-H1.3 complex in regulating mitotic microtubule growth.