Abstract
Chromatin reorganization involving histone modifications, histone variant exchange, histone eviction and nucleosome remodeling play an important role in DNA repair, apoptosis and cell cycle checkpoints. Among proteins involved in chromatin reorganization, TIP60 histone acetylase has been shown to play a role in DNA repair and apoptosis. However, how TIP60 regulates chromatin reorganization in the response to DNA damage is largely unknown. To better understand the mechanism of TIP60 complex in DNA repair, TIP60 complex was purified from chromatin soluble fraction after DNA damage. We found that the TIP60 complex is associated with histone H2AX, a variant form of H2A, after DNA damage. Furthermore, DSBs facilitate the association of TIP60 with the ubiquitin-conjugating enzyme UBC13. The TIP60-UBC13 complex regulates the acetylation and ubiquitination of H2AX following the formation of DSBs. The DSB-induced acetylation of H2AX is required for this ubiquitination and occurs independently of the phosphorylation of H2AX. We also show that damage-induced acetylation and ubiquitination provoke the eviction of H2AX from chromatin immediately after induction of DSBs. Interestingly, the eviction of H2AX depends not on phosphorylation, but rather on acetylation and ubiquitination by TIP60. We conclude that sequential acetylation and ubiquitination of H2AX by TIP60-UBC13 promotes enhanced histone H2AX dynamics, which in turn stimulates a DNA damage response. We will discuss about the role of the eviction of H2AX upon DNA damage in chromatin remodeling and checkpoint activation.
