MITOSIS-SPECIFIC PHOSPHORYLATION OF HISTONE H3 INITIATES PRIMARILY WITHIN PERICENTROMERIC HETEROCHROMATIN DURING G2 AND SPREADS IN AN ORDERED FASHION COINCIDENT WITH MITOTIC CHROMOSOME CONDENSATION

We have generated and characterized a novel site-specific antibody hig hly specific for the phosphorylated form of the amino-terminus of hist one H3 (Ser10). In this study, we used this antibody to examine in det ail the relationship between H3 phosphorylation and mitotic chromosome condensation in mammalian cells. Our results extend previous biochemi cal studies by demonstrating that mitotic phosphorylation of H3 initia tes nonrandomly in pericentromeric heterochromatin in late G2 interpha se cells. Following initiation, H3 phosphorylation appears to spread t hroughout the condensing chromatin and is complete in most cell lines just prior to the formation of prophase chromosomes, in which a phosph orylated, but nonmitotic, chromosomal organization is observed. In gen eral, there is a precise spatial and temporal correlation between H3 p hosphorylation and initial stages of chromatin condensation. Dephospho rylation of H3 begins in anaphase and is complete immediately prior to detectable chromosome decondensation in telophase cells. We propose t hat the singular phosphorylation of the amino-terminus of histone H3 m ay be involved in facilitating two key functions during mitosis: (1) r egulate protein protein interactions to promote binding of trans-actin g factors that ''drive'' chromatin condensation as cells enter M-phase and (2) coordinate chromatin decondensation associated with M-phase.