Histone H1 interphase phosphorylation pattern becomes largely established during G1/S transition in proliferating cells

Histone H1 is an important constituent of chromatin, and is believed to be involved in regulation of chromatin structure. During the cell cycle, chromatin becomes locally decondensed in S phase, highly condensed during metaphase and again decondensed before re-entry into G1. This has been connected to increasing phosphorylation of H1 histones during the cell cycle. However, many of these experiments have been performed in non-human and human cancer cell lines, and by the use of cell synchronization techniques and cell cycle-arresting drugs. In this study, we have investigated the H1 subtype composition and phosphorylation pattern in the cell cycle. Exponentially growing normal human activated T cells and Jurkat lymphoblastoid cells were sorted by fluorescence activated cell sorting into G1, S and G2/M populations, without the use of cell cycle arresting drugs. We found that the H1.5 protein level increased after T-cell activation. Our data indicate that serine phosphorylation of H1 subtypes occurred to a large extent in late G1 phase or early S, while some additional serine phosphorylation took place during S, G2 and M phases. Furthermore, our data suggest that the newly synthesized H1 molecules during S phase also achieve a similar phosphorylation pattern as the previous ones. Jurkat cells showed 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. In conclusion, our data is consistent with a model where a major part of interphase H1 serine phosphorylation takes place within a narrow time window during the G1/Stransition. This implies that H1 serine phosphorylation may be coupled to changes in chromatin structure necessary for DNA replication.