LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
provide a means to regulate rejoining by modulating the interactions among Xrcc4, DNA
and DNA ligase IV. Both Ku subunits and DNA-PKcs are essential for repair by NHEJ
although defects in DNA-PKcs generally confer a milder phenotype than defects in Ku.
DNA-PK activity is undetectable in Ku-defective cell lines, indicating that DNA binding
by the Ku70:80 heterodimer is essential for its activation. The carboxy-terminal portion
of Ku80 is also important for DNA-PK function and distinct regions are responsible for
Ku70 interaction and DNA-PKcs activation. Deletion of the carboxy-terminal region of
Chinese hamster Ku80 imparts a phenotype similar to DNA-PK deficiency even when
high levels of DNA-PKcs are present[126]. The requirement for this carboxy-terminal
region in kinase activation is consistent with the absence of an analogous Ku80 carboxyterminal
tail in S. cerevisiae which also lack a homologous DNA-PKcs protein. The
molecular architecture of DNA-PKcs suggests a structural role for the protein in the
rejoining process. It has a potential DNA-binding groove and an enclosed cavity with
three apertures through which single-stranded DNA could pass [68]. These findings are
consistent with DNA-PKcs mediating the alignment of short stretches of single stranded
DNA prior to ligation. This would be in addition to any role of DNA-PK in signaling.
Repair by NHEJ is completed by the DNA ligase IV/Xrcc4 complex. Mice with targeted
inactivation of either component exhibit embryonic lethality [127-129]. Consistent with
their joint participation in the NHEJ pathway, the sensitivity of DNA ligase IV or Xrcc4
knockout mouse cells to ionizing radiation, and their defects in V(D)J recombination
mimic those of Ku null mice. As Ku and DNA-PKcs deficient mice are viable, however,
there appears to be an additional requirement for DNA ligase IV and Xrcc4 during
embryonic development. Reduced NHEJ activity may confer radiosensitivity in the
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