Biology Reference
In-Depth Information
B. Activation of the ATM Kinase by DSBs
A crucial issue is to understand how the introduction of a DSB into the
chromatin is converted into a signal which activates the kinase activity of the
ATM protein. A key step in elucidating ATM activation was the observation that
ATM undergoes a rapid (within 1 min) autophosphorylation on serine 1981
following DNA damage.
29
Further, using an antibody which specifically detects
ATM autophosphorylated on serine 1981, it was clearly shown that phosphory-
lated ATM accumulated at DSBs within the first minute postirradiation.
29
Subsequently, several additional autophosphorylation sites were identified on
ATM.
30,31
This led to the proposal that autophosphorylation of ATM was
required to convert inactive ATM dimers to active ATM monomers,
29
a mech-
anism supported by the observation that mutation of ATM autophosphorylation
sites blocks ATM activation
29,30
and dimer-monomer transition.
32
This sug-
gested a model in which ATMwas recruited toDSBs as an inactive dimer. ATM's
kinase activity was then activated, leading to autophosphorylation of ATM at
multiple sites, and the conversion of ATM from an inactive dimer to an active
monomer.
29
However, when this was tested in mouse models in which one or
more of the ATM autophosphorylation sites were mutated, no significant defect
in either ATM activation or ATM function at the whole organism level was
seen.
33,34
This discrepancy could have several explanations. One possibility is
that the murine system used to express the non-phosphorylatable ATM con-
struct in mice has influenced the outcome.
33,34
A second possibility is that there
are differences in ATM function and activation between mice and humans. For
example, mice in which ATM is deleted replicate both the increased radiosen-
sitivity and increased tumor formation seen in patients with
A-T.
35,36
However,
the
ATM
/
mice do not exhibit the pronounced cerebellar degeneration or
other significant neurological defects which are the hallmark of the human
disease. In addition, studies on human ATM using biochemical approaches
have shown that ATM's kinase activity can be activated,
in vitro
, in the absence
of detectable autophosphorylation of ATM.
37,38
Thus, although it is clear that
ATM is recruited to DSBs and that this leads to activation of ATM's kinase
activity, how autophosphorylation and dimer-monomer conversion contribute to
forming the fully active ATM kinase remains unclear. The differences between
murine and human ATM, and between cell-based and biochemical systems
suggest that complex layers of ATM regulation contribute to ATM activation
under different cellular conditions in different cell types.
C. ATM and the mre11-rad50-nbs1 Complex
In addition to examining how ATM's kinase activity is upregulated by DNA
damage, several groups have clearly demonstrated that the mre11-rad50-nbs1
(MRN) complex is required for activation of ATM's kinase activity.
39-43
MRN is
