Histone H1 interphase phosphorylation becomes largely established in G1 or early S phase and differs in G1 between T-lymphoblastoid cells and normal T cells

We found that the relative amount of H1.5 protein increased significantly after T-cell activation. Serine phosphorylation of H1 subtypes occurred to a large extent in late G1 or early S phase in both activated T cells and Jurkat cells. Furthermore, our data confirm that the H1 molecules newly synthesized during S phase achieve a similar phosphorylation pattern to the previous ones. Jurkat cells had more extended H1.5 phosphorylation in G1 compared with T cells, a difference that can be explained by faster cell growth and/or the presence of enhanced H1 kinase activity in G1 in Jurkat cells.Our data are consistent with a model in which a major part of interphase H1 phosphorylation takes place in G1 or early S phase. This implies that H1 serine phosphorylation may be coupled to changes in chromatin structure necessary for DNA replication. In addition, the increased H1 phosphorylation of malignant cells in G1 may be affecting the G1/S transition control and enabling facilitated S-phase entry as a result of relaxed chromatin condensation. Furthermore, increased H1.5 expression may be coupled to the proliferative capacity of growth-stimulated T cells.Cell division is a complex process, in which correct passage through the cell cycle is essential for cell survival and correct transmission of genetic information to the daughter cells. During the cell cycle, the cell nucleus undergoes dramatic structural changes. DNA, which is compacted into chromatin by various proteins, is locally decondensed in S phase, but condenses in prophase. In metaphase, highly condensed chromosomes are visible, which start to segregate during anaphase. Segregation is completed during telophase, and two daughter cells are produced. Before re-entry into G1, the chromatin again becomes dispersed.In the nucleosome, the basic unit of chromatin, approximately 146 bp of DNA are wrapped 1.65 turns around an octamer consisting of two copies of each core histone: H2A, H2B, H3 and H4 [1]. A fifth histone, his