G2 histone methylation is required for the proper segregation of chromosomes

Trimethylation of lysine 9 on histone H3 (H3K9me3) is known both to be necessary for proper chromosome segregation and to increase in late G2. We investigated the role of late G2 methylation, specifically in mitotic progression, by inhibiting methylation for 2 hours prior to mitosis using the general methylation inhibitor adenosine dialdehyde (AdOx). AdOx inhibits all methylation events within the cell but, by shortening the treatment length to 2 hours and studying mitotic cells, the only methylation events that are affected are those that occur in late G2. We discovered that methylation events in this time period are crucial for proper mitosis. Mis-segregation of chromosomes is observed with AdOx treatment. Through studies of histone modifications, we have found that inhibiting late G2 methylation affects trimethylation of H3K9 and H4K20. The mitotic checkpoint is active and many kinetochore proteins localize properly, however, pericentric chromatin in these cells is found to be less compact (dense). The reduced integrity of pericentric heterochromatin might be responsible for a noted loss of tension at the centromere in AdOx-treated cells and activation of the spindle assembly checkpoint. We postulate that late G2 methylation is necessary for proper pericentric heterochromatin formation. The results suggest that a reduction in heterochromatin integrity might interfere both with microtubule attachment to chromosomes and with the proper sensing of tension from correct microtubule-kinetochore connections, either of which will result in activation of the mitotic checkpoint.