Biology Reference
In-Depth Information
lysine residue, at amino acid 3016 near the c-terminal kinase domain, was
identified as the site on ATM that was acetylated by Tip60.
32,60,90
Mutation of
this lysine blocked the activation of ATM's kinase activity by DNA damage,
indicating that Tip60-dependent acetylation of ATM is a key step in the
activation of ATM's kinase activity. The ATM acetylation site is highly conserved
among higher eukaryotes,
32
whereas the autophosphorylation sites are
not,
29,33,34
suggesting that acetylation is an evolutionarily conserved event in
ATM regulation.
The positioning of the Tip60 acetylation site adjacent to the kinase domain
has provided powerful insights into how acetylation promotes activation of the
ATM kinase. ATM is a member of the PIK family, which includes the ATM,
DNA-PKcs, and Atr and Tor protein kinases, each of which shares a common c-
terminal structure.
91,92
PIK proteins have an unusual c-terminal structure, in
which the kinase domain is sandwiched between two additional domains,
termed the FAT and FATC domains. The FAT-kinase-FATC structure is only
found in PIK proteins, and the FAT and FATC domains are not found
independent of each other in other protein families. The Tip60 acetylation
site at lysine 3016 of ATM is located in the region of the protein which links the
kinase domain to the FATC domain. Further, Tip60 binds tightly to the highly
conserved FATC domain at the extreme C-terminal of ATM.
60,90
The close
proximity between the Tip60-binding site (FATC domain: amino acids 3029-
3056) and the acetylation site (lysine 3016) indicates tight coupling between
these two events.
32,60,90
This led to the proposal that acetylation of ATM
between the kinase domain and FATC domain may activate ATM's kinase
activity by altering the conformation of the ATM protein. Studies on other
PIK proteins provide support for this idea. The region between the kinase
domain and the FATC domain of PIK proteins has been named the PIK
regulatory domain (PRD; reviewed in Refs.
16,93,94
). This region is weakly
conserved between PIK family members, such as ATM, Atr, and DNA-PKcs;
however, the PRD plays a critical role in regulating the kinase activity of
individual PIK protein family members. For example, binding of TopBP1
(a regulator of Atr function) to the PRD of Atr activates Atr's kinase activity.
93,94
The PRD of DNA-PKcs is required for activation of its kinase activity by DNA
damage,
95,96
and the PRD of mTor is essential for its
in vivo
function.
97
Further, Akt phosphorylates the PRD of Tor1, although how this phosphory-
lation impacts Tor1 function is not yet known.
98
The PRD of PIK proteins
therefore plays a key role in regulating the kinase activity of PIK proteins. For
ATM, the acetylation of lysine 3016 in the PRD of ATM may directly regulate
the kinase activity of the adjacent kinase domain. This may occur through an
alteration in the conformation of the kinase-PRD-FATC domain of ATM,
potentially increasing substrate access to the kinase domain, regulating ATM's
intrinsic kinase activity, or altering the interaction between ATM and its
