Biology Reference
In-Depth Information
inactivating mutations in Tip60's chromodomain inhibited both the activation
of Tip60's acetyltransferase activity by DNA damage and the subsequent
acetylation and activation of ATM's kinase activity.
51
Further, when global
H3K9me3 levels were reduced, either by overexpression of KDM4D, a
H3K9me3 demethylase,
112
or by genetic inactivation of the major H3K9
methyltransferases Suv39h1 and Suv39h2,
113
Tip60 activation following DNA
damage was significantly decreased.
51
The recruitment of Tip60 to DSBs
therefore leads to productive interactions between Tip60's chromodomain
and methylated lysine residues on histones at DSBs, and this interaction
activates Tip60's HAT activity. The chromodomain of Tip60 therefore functions
as the sensor for activation of Tip60 by DNA damage. Further, this work
demonstrated that H3K9me3 plays an essential role in the DDR. This conclu-
sion is supported by the observation that cells with reduced levels of H3K9
methylation have defective activation of ATM kinase activity and increased
genomic instability.
51,113
Overall, this indicates that recruitment of ATM and
Tip60 to the chromatin at DSBs promotes interaction between Tip60's chro-
modomain and H3K9me3 on adjacent histones. This leads to activation of
Tip60's acetyltransferase activity, acetylation of ATM, and activation of ATM's
kinase activity.
E. Interaction Between ATM Tip60 and the
MRN Complex
Although these results demonstrate a key role for acetylation of the PRD
domain of ATM by Tip60 in activating ATM's kinase activity, it is critical to
incorporate the published data on the role of the MRN complex into this
model. As discussed earlier, both cell-based
40-43
and biochemical sys-
tems
37,38,49
support the hypothesis that the MRN complex makes a crucial
contribution to activating ATM's kinase activity. The MRN complex physically
interacts with ATM,
43,48
and this interaction is sufficient (in reconstituted
systems) for activation of ATM's kinase activity.
40-43
In addition, ATM and
Tip60 can be co-immunoprecipitated from cells, and this interaction requires
the FATC domain of ATM.
60,90
However, despite extensive
in vitro
biochemical
analysis, we have been unable to demonstrate direct binding of Tip60 to ATM
(BD Price, unpublished data). This implies the presence of additional proteins
connecting Tip60 and the FATC domain of ATM. Several studies have indicat-
ed that Tip60 can bind to the MRN complex,
114,115
implying that the MRN
complex is a potential candidate for bringing ATM and Tip60 together. This
raises the possibility that, because both ATM and Tip60 interact with MRN,
they are co-recruited to DSBs through the ability of MRN to bind to DNA
ends. This is supported by the observation that inactivation of the MRN
complex led to defective recruitment of Tip60 to DSBs, and significant delays
