Biology Reference
In-Depth Information
a trimeric DNA-binding complex, which is rapidly recruited to DSBs.
44
The
mre11 protein contains both a DNA-binding and a nuclease domain.
45
Rad50
is a SWI/SNF-related ATPase and functions as a structural component to hold
the two ends of the DSB together.
44
Finally, nbs1 functions as a regulatory
subunit of MRN, which also undergoes regulatory phosphorylation by ATM.
39
Mutations in either mre11 (AT-like disease (ATLD)
46
) or nbs1 (Nijmegen
break syndrome (NBS)
47
) have clinical defects similar to
A-T
, including neu-
rological defects, increased cancer incidence, and increased radiosensitivity.
Further, ATM interacts with the c-terminal of nbs1,
43,48
and mutation or
deletion of either mre11 or nbs1in
ATLD
and
NBS
cells significantly reduces
the activation of ATM's kinase activity by DNA damage
in vivo.
40-43
Taken
together, this work indicates that the MRN complex plays a key role in activa-
tion of ATM's kinase activity. This idea was further strengthened by biochemical
studies, which showed that purified MRN was sufficient to activate ATM's
kinase activity
in vitro.
37,38,49
The gene inactivation and biochemical studies
therefore indicate that MRN is upstream of ATM. This led to the idea that
MRN was rapidly recruited to DSBs through the ability of the mre11 subunit to
bind to the broken ends of the DNA. Subsequently, interaction between ATM
and the nbs1 subunit of the MRN complex activated ATM's kinase activity,
allowing ATM to phosphorylate its target proteins. Importantly, both mre11
and nbs1 are phosphorylated by ATM,
50
underscoring the complexity of the
regulatory interactions in the DDR response, in which upstream regulators
such as MRN are also targets for phosphorylation by downstream effectors such
as ATM.
D. Other Contributors to ATM Activation
In addition to ATM, there is evidence that other factors contribute to the
activation of ATM's kinase activity. For example, inactivation of the MRN com-
plex does not completely abolish ATM activation in cells. Loss of MRN func-
tion is associated with a significant delay in ATM activation at low levels of DNA
damage
40-43,51
; however, at high levels of damage loss of MRN function has
limited impact on the activation of ATM's kinase activity.
52-54
This type of
observation indicates that, while MRN is critical for ATM activation at low
levels of DNA damage, high levels of DSBs may bypass the need for MRN to
activate ATM's kinase activity. This implies that,
in vivo
, factors in addition to
MRN are involved in modulating ATM activation at DSBs. For example, the
ATM-interacting protein ATMIN
55,56
is important for ATM activation through
a pathway that is independent of the nbs1 protein. Other studies have shown
that chromatin structure adjacent to the site of DNA damage can have a
profound impact on both ATM activation and DNA repair itself. For example,
ATM can be activated by agents that directly alter chromatin condensation.
29
Molecular-imaging studies indicate that tethering of DNA repair proteins to
