A Transient Rise in Free Mg 2+ Ions Released from ATP-Mg Hydrolysis Contributes to Mitotic Chromosome Condensation

For cell division, negatively charged chromatin, in which nucleosome fibers (10 nm fibers) are irregularly folded [1-5], must be condensed into chromosomes and segregated. While condensin and other proteins are critical for organizing chromatin into the appropriate chromosome shape [6-17], free divalent cations such as Mg and Ca , which condense chromatin or chromosomes in vitro [18-28], have long been considered important, especially for local condensation, because the nucleosome fiber has a net negative charge and is by itself stretched like "beads on a string" by electrostatic repulsion. For further folding, other positively charged factors are required to decrease the charge and repulsion [29]. However, technical limitations to measure intracellular free divalent cations, but not total cations [30], especially Mg , have prevented us from elucidating their function. Here, we developed a Förster resonance energy transfer (FRET)-based Mg indicator that monitors free Mg dynamics throughout the cell cycle. By combining this indicator with Ca [31] and adenosine triphosphate (ATP) [32] indicators, we demonstrate that the levels of free Mg , but not Ca , increase during mitosis. The Mg increase is coupled with a decrease in ATP, which is normally bound to Mg in the cell [33]. ATP inhibited Mg -dependent chromatin condensation in vitro. Chelating Mg induced mitotic cell arrest and chromosome decondensation, while ATP reduction had the opposite effect. Our results suggest that ATP-bound Mg is released by ATP hydrolysis and contributes to mitotic chromosome condensation with increased rigidity, suggesting a novel regulatory mechanism for higher-order chromatin organization by the intracellular Mg -ATP balance.