Complement C4b/d Products

In eukaryotic cells, DNA is associated with histones and other proteins to form chromatin. The cell division cycle constitutes a series of processes that have evolved to create two genetically identical daughter cells from a mother cell. One of these processes is the conversion of relatively amorphous, extended interphase chromatin into condensed, highly ordered mitotic chromosomes. Proper mitotic chromosome condensation is essential for the correct segregation of sister chromatids into two daughter cells. The basic unit of chromatin is the nucleosome core particle, which consists of 140 bp DNA wrapped around an octameric core containing two each of the four conserved core histones: H2A, H2B, H3 and H4. A fifth histone, the linker histone H1, interacts with DNA of variable length, linking adjacent nucleosome cores, and further compacting the chromatin. Chromatin changes are initiated during G2 phase of the cell cycle, in preparation for cell division. The most striking morphological change is chromatin condensation, which becomes apparent during prophase and is maximal during the subsequent stages of mitosis. Histone H1 and the N-terminal tail of H3 have key roles in the folding and inter-association of the chromatin fiber. Two currently known phosphorylation sites are present in the N-terminus of H3; serine-10 and serine-28.1,2 Mitogenic stimulation, oncogenic transformation, or induction of oncogenic ras expression are accompanied with increase in serine-10 phosphorylation of the H3 N-terminal domain. Indeed, it has been shown that phosphorylated H3 is associated with c-fos and c-myc genes in stimulated cells. Phosphorylation of H3, at both serine-10 and -28, coincides with the induction of mitotic chromosome condensation. H3 phosphorylation may contribute to proto-oncogene induction by modulating chromatin structure and releasing blocks in elongation. In contrast to H1 hyperphosphorylation, site-specific phosphorylation of core histone H3 at serine-10 and -28 appears to occur exclusively during mitosis in mammalian cells. H3 dephosphorylation occurs quite rapidly after mitosis and serine-10/28 remain unphosphorylated throughout the remainder of interphase. PP1 has been identified as the H3 phosphatase. Monoclonal antibodies reacting specifically with phosphorylated histone H3, are useful tools to study molecular mechanisms associated with the G2 to M transition and chromatin condensation, and for the analysis of protein kinase(s) and phosphatase(s) involved in H3 phosphorylation or dephosphorylation. They may also be used in multiparameter analysis to relate H3 phosphorylation in individual cells to the cell