MRN promotes dissociation of ATM dimers to ATM monomers which is accompanied by ATM trans-autophosphorylation on serine residue S1981 (Bakkenist et al. 2003, Du et al. 2014). ATM autophosphorylation at serine residues S367 and S1893 is also implicated in ATM activation (Kozlov et al. 2006). Dissociation of ATM dimers requires the ATP-dependent DNA-helicase activity of the MRN subunit RAD50 (Lee and Paull 2005). KAT5 (Tip60) mediated acetylation of ATM dimers at lysine K3016 is a prerequisite for ATM kinase activity (Sun et al. 2007). Upon the dissociation of ATM dimers induced by DNA double strand breaks (DSBs), a fraction of activated ATM is retained at DSB sites, co-localizing with the MRN complex (Andegeko et al. 2001, Uziel et al. 2003) at ionizing radiation-induced foci (IRIF). MRN facilitates the binding of a portion of ATM substrates to ATM (Lee and Paull 2004).
After the DNA double strand breaks (DSBs) are repaired, ATM is dephosphorylated by an unidentified PP2A phosphatase complex, leading to dimer reformation (Goodarzi et al. 2004).