LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
Perhaps one of the most important recent developments in the field has been
insight into the mechanism of ATM activation in response to DNA damage. ATM is a
predominantly nuclear protein, the levels of which do not change when cells are exposed
to IR [73-76]. However, the protein kinase activity of ATM, as determined using
immunoprecipitation (IP) kinase assays, increases two- to three-fold following cellular
exposure to IR [75] or the radiomimetic neocarzinostatin (NCS) [73]. IR also induces
increased incorporation of phosphate into ATM [65, 77]. DNA damage-induced
phosphate incorporation was not observed in cells expressing kinase-dead ATM,
suggesting that IR induces autophosphorylation of ATM. Indeed, serine 1981, an SQ site
located in the amino-terminal region of the FAT domain, is rapidly phosphorylated in
cells that have been exposed to even very low doses of IR [65]. ATM containing a serine
to alanine mutation at amino acid 1981 failed to support IR-induced phosphorylation of
p53 or cell cycle arrest, suggesting that serine 1981 phosphorylation is critical for the
function of ATM [65]. trans-Phosphorylation and dissociation of ATM from an inactive
dimeric (or higher order) form to an active monomeric form has, thus, emerged as one of
the earliest events in the activation of ATM [65]. While phosphorylation of serine 1981 is
certainly critical to the activation of ATM, it is also possible that ATM undergoes
autophosphorylation at other sites important for the DNA damage response [77].
Although these elegant experiments place ATM autophosphorylation as an
important upstream event in the activation pathway, they do not address whether ATM is
the primary sensor of DNA damage. If it were a direct sensor of DNA damage, ATM
would be expected to interact directly with the damaged DNA. Addition of dsDNA does
not stimulate ATM protein kinase activity in IP kinase assays [73], although addition of
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