Phosphorylation states of different histone 1 subtypes and their relationship to chromatin functions during the HeLa S-3 cell cycle.

The histone 1 (H1) fraction of HeLa S-3 cells contains two principal subtypes, H1A (Mr approximately 21 000) and H1B (Mr approximately 22 000). In G1 cells, the H1 molecules are distributed among several phosphorylation states, most H1A molecules containing 0 or 1 phosphate groups and most H1B molecules containing 0, 1, 2, or 3 phosphate groups. Both subtypes undergo a general increase in phosphorylation levels of approximately 1 P/mol during the S phase and a further increase or 3--4 P/mol during mitosis. These two increases affect most of the H1 molecules and thus reflect phosphorylations occurring widely throughout the chromatin, presumably in association with replication and mitotic chromosome condensation. During all these periods, multiple phosphorylation levels of H1 molecules persist, as does the phosphorylation differential between H1A and H1B. Thus, there appear to be phosphorylation states that only some of the H1 molecules occupy, a fact that may be related to the conformational diversity in interphase and mitotic chromatin. The existence of differences between H1A and H1B phosphorylation states throughout the cell cycle, and within a single cell type, is in accord with the hypothesis that the H1 subtypes are functionally distinct, such that subtype-specific phosphorylations contribute to the control of chromatin organization.