Post-translational modification and mitochondrial relocalization of histone H3 during apoptosis induced by staurosporine

Post-translational modifications (PTMs) of histones such as phosphorylation, acetylation, and ubiquitination, collectively referred to as the "histone-code", have been known to regulate gene expression and chromatin condensation for over a decade. They are also implicated in processes such as DNA repair and apoptosis. However, the study of the phosphorylation of histones has been mainly focused on chromosome condensation and mitosis. Therefore, the phosphorylation of histones in apoptosis is not fully understood. It was recently demonstrated by Tang et al. that histones are released from nucleosome during apoptosis, an observation that is in agreement with our findings. In addition to the release of histones, the dephosphorylation of histone H3 at Thr-3 and Ser-10 was observed during apoptosis in some cancer cells. Our data suggest that the modification and release of histones could serve markers of apoptosis in human cancer cells. We also suggest that the released histones, especially H3, could be translocated to mitochondria during apoptosis.

Nucleosome spacing and chromatin higher-order folding

Packing of about two meters of the human genome DNA into chromatin occupying a several micron-sized cell nucleus requires a high degree of compaction in a manner that allows the information encoded on DNA to remain easily accessible. This packing is mediated by repeated coiling of DNA double helix around histones to form nucleosome arrays that are further folded into higher-order structures. Relatively straight DNA linkers separate the nucleosomes and the spacing between consecutive nucleosome varies between different cells and between different chromosomal loci. In a recent work¹ our group used a biochemically defined in vitro reconstituted system to explore how do various DNA linkers mediate nucleosome array packing into higher-order chromatin structures. For long nucleosome linkers (about 60 bp) we observed a more open chromatin structure and no effect of small linker length alterations (±2−4 bp) on chromatin folding. In striking contrast, for shorter linkers (20−32 bp) we found more compact packing with strong periodical dependence upon the linker DNA lengths. Our data together with high-resolution nucleosome position mapping provide evidence for the natural nucleosome repeats to support a chromatin architecture that, by default, restricts spontaneous folding of nucleosome arrays into compact chromatin fibers. We suggest that incomplete folding of the nucleosome arrays may promote global inter-array interactions that lead to chromatin condensation in metaphase chromosomes and heterochromatin.

Purification and analysis of variant and modified histones using 2D PAGE

Two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE) systems employing combinations of acetic acid/urea (AU), acetic acid/urea/Triton X-100 (AUT) and sodium dodecyl sulfate (SDS) gel formulations are uniquely effective for resolution of histone variants and their modified derivatives. Coupled with Western transfer methods using modification-specific antibodies and recent advances in mass spectrometry, 2D PAGE emerges as a versatile tool for histone purification and analysis. This chapter describes 2D PAGE gel systems appropriate for histone proteins, including detailed procedures for designing, running, and staining gels. Methods for electrophoretic transfer of histones from AUTxSDS and AUTxAU 2D gels are described and evaluated. Alternatively, methods are provided for obtaining highly purified protein samples from fixed and stained gels via electroelution of proteins from specific gel spots.

Two kinase activities are sufficient for sea urchin sperm chromatin decondensation in vitro

Decondensation of compact and inactive sperm chromatin by egg cytoplasm at fertilization is necessary to convert the male germ cell chromatin to an active somatic form. We studied decondensation of sea urchin sperm nuclei in a cell-free extract of sea urchin eggs to define conditions promoting decondensation. We find that egg cytosol specifically phosphorylates two sperm-specific (Sp) histones in vitro in the same regions as in vivo. This activity is blocked by olomoucine, an inhibitor of cdc2-like kinases, but not by chelerythrine, an inhibitor of protein kinase C (PKC). PKC phosphorylates and solubilizes the sperm nuclear lamina, one requirement for decondensation. Olomoucine, which does not inhibit lamina removal, blocks sperm nuclear decondensation in the same concentration range over which it is effective in blocking Sp histone phosphorylation. In a system free of other soluble proteins, neither PKC nor cdc2 alone elicit sperm chromatin decondensation, but the two act synergistically to decondense sperm nuclei. We conclude that two kinases activities are sufficient for sea urchin male pronuclear decondensation in vitro, a lamin kinase (PKC) and a cdc2-like Sp histone kinase.