Cell cycle dependent association and activation of a histone H1.3-HDAC3 complex

Eukaryotic DNA is organized with nucleoproteins into a dynamic structure named chromatin. Chromatin compaction and decompaction plays an important role in modulation of various DNA-dependent functions in the cell. Histone deacetylases (HDACs), which remove acetyl groups from core histone proteins, regulate the compaction of the chromatin fiber, thus modulating gene transcription levels. Chromatin binding proteins, such as linker histone H1, are also known to cause chromatin compaction. Histone H1 and HDAC3 were independently reported to be involved in the regulation of mitosis. The hypothesis of this study was that histone H1 and HDAC3 could form a complex that will be involved in chromatin condensation and mitosis regulation. Co-immunoprecipitation assay results demonstrated the formation of stable complexes between HDAC3 and histone H1.3 in HeLaS3 human cervical carcinoma epithelial cells. Although the amount of complex increased during the late-G2 and mitosis phases of the cell cycle, the complex was activated for chromatin deacetylation only in mitosis. Studies in vitro demonstrated that HDAC3 was highly phosphorylated on Ser-424 in the mitotic complex but not the G 2 complex. Further studies also showed that HDAC3 from late-G2 complex can be activated in vitro by phosphorylating on Ser-424 with casein kinase II (CK2). Based on the immunocytochemistry and confocal imaging studies, the HDAC3-H1.3 complexes co-localized with mitotic polar microtubules. The complex included also silencing mediator of retinoic acid receptor and thyroid hormone receptor (SMRT), glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and annexin I. GAPDH, annexin I and histone H1 are known to bind tubulins and thus increases confidence that they may tether HDAC3 to the polar spindle mitotic microtubules. This report is the first, to the best of our knowledge, in which histone H1 and a class I HDAC are shown to form a complex. This is also the first record of a co-localization of histone H1 with microtubules in an animal kingdom.