The Mre11 complex is required for ATM activation and the G 2 /M checkpoint Carson, C.T. et al The EMBO J (Vol. 22), 2003

Mre11 complex: Phenotypes AT: cerebellar degeneration, immune def, IR sensitivity, chromosomal instability, cancer predisposition Mre11: ATLD Nbs1: Nijmegen breakage syndrome Rad50: Variant of NBS

The Mre11 (MRN) complex Involved in DNA damage response, replication, meiotic recombination, checkpoint function, telomere-length regulation Role in HR, NHEJ Mre11: exo- and endonuclease Rad50: ATPase Nbs1: Interaction with histone, localization Preferentially binds DNA ends

Architecture of the complex

Mre11 Complex Dynamics Nbs1 interacts directly with  H2AX, forms nuclear foci (indep. of Mre11) Is phosphorylated by ATM Mre11 nuclear localization and focus formation requires Nbs1, but not its ATM-mediated phosphorylation  H2AX formation does not require Nbs1

ATM Central role in DNA damage response- DSB Cellular phenotype Suggested as sensor for DSB/ chromatin modification Activation by autophosphorylation (Bakkenist & Kastan, 2003)

Adenovirus system used (Stracker et al, 2002) Concatemer formation detrimental to Ad replication During infection, it degrades proteins responsible for concatemer formation Nbs1, Mre11, Rad50: (mis) localized and degraded by viral oncoproteins E4orf6/E1b55K Mutating/deleting E4 (dl1004/ E4) restores functional MRN, allows concatemerization

Fig. 1 Infection with dl1004 results in DNA damage response

Fig. 2Role of ATM and ATR

Fig. 4 E1b55K/ E4orf6 target the Mre11 complex for degradation

Fig. 5 Degradation of MRN complex prevents activation of damage response

Fig. 6 Degradation of MRN by viral protein prevents IR-response

Fig. 7 Effect of viral protein on ATM signaling is due to MRN degradation

Supplementary Fig. 3: Mre11 complementation in ATLD1 cells

Fig. 8 Human cell lines

The Model

Uziel, et al (The EMBO J, 2003): Requirement of the MRN complex for ATM activation by DNA damage Mochan, et al (Cancer Res, 2003): 53BP1 and NFD1/MDC-Nbs1 function in parallel interacting pathways activating ATM in response to DNA damage Similar suggestions….

So, is the MRN complex actually “sensing” the damage?