LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
1.3.1.1 DNA-PK: a DNA damage sensor for DNA non-homologous end-joining
DNA-PK is a DSB repair enzyme composed of a catalytic subunit, DNA-PKcs that is
part of the PIKK family [14, 15] and a targeting subunit, the Ku70–Ku80 heterodimer
[16, 17]. DNA-PK was discovered more than a decade ago and has since been studied
extensively at a biochemical level. DNA-PK is characterized by its DNA activated
serine/threonine-directed protein kinase activity and can be considered as a paradigm for
the biochemical functions of PIKK family kinases [15]. DNA-PKcs possesses an
intrinsic, albeit weak, DNA-end binding activity that is greatly stimulated and stabilized
by the Ku70–Ku80 heterodimer [17, 18]. Whether DNA-PK signals DSBs to the
checkpoint machinery [19, 20] remains controversial, but the emerging consensus is that
it does not [21-23]. DNA-PK may, however, be involved in signaling DNA damage to
the apoptosis machinery [24]. Genetically, the main role of DNA-PK seems to be in
promoting the rejoining of DSBs by NHEJ. Thus, cells and animals deficient in DNA-PK
are defective in DSB repair and in V(D)J recombination, a process by which antibody
and T-cell receptor diversity is generated, and are consequently radiosensitive and
immunodeficient [15]. The kinase activity of DNA-PK is required for these processes,
indicating that reversible protein phosphorylation is important and that DNA-PK plays a
catalytic role [25]. However, the proteins that have to be phosphorylated by DNA-PK for
it to exert its effects are unknown.
1.3.1.2 ATR is the main sensor of stalled replication forks
The ATR kinase appears to be the master activator of the replication stress response [26,
27]. ATR is a member of a superfamily of serine-threonine kinases, which include ATM,
DNA-PK, mTOR, TRRAP and SMG-1 [28, 29]. ATR is an essential kinase [30, 31] that
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