Cells must elicit an effective DNA damage response in order to maintain genetic integrity and to survive. One of the major DNA damage repair pathways in humans is homologous recombination (HR). One of the most important players in this repair pathway is the MRN complex (made up of Mre11, Rad50 and Nbs1). MRN eliminates the most damaging types of breaks in which both strands of the DNA double helix are severed. If left unrepaired, DNA double strand breaks (DSBs) are lethal or can lead to chromosomal instability and/or malignant transformation. We discovered a novel human single stranded DNA (ssDNA) binding protein (hSSB1) from the conserved oligonucleotide-binding (OB) domain family and have recently solved its structure in complex with ssDNA (1). hSSB1 controls the activation and activity of the DSB signalling kinase, ATM, in response to DSBs (2). Fluorescence microscopy and pull-down experiments reveal that hSSB1 rapidly binds at sites of DSBs and is required for the efficient recruitment and activity of the MRN complex at these sites (3,4). We show that hSSB1 physically interacts with the Nbs1 component of the MRN complex via its flexible C-terminal extension. Using structural and biophysical approaches we show that the OB domain of hSSB1 can bind short stretches of ssDNA, mimicking overhangs that are common at DSBs. Taken together, these data indicate that hSSB1 links DSBs to the recruitment of the MRN repair complex.
(1) Touma et al., Cubeddu & Gamsjaeger (2016) Nucleic Acids Research 44: 7963-7973.
(2) Richard, Bolderson & Cubeddu et al., (2008) Nature 453: 677-681
(3) Richard et al., (2011) Nucleic Acids Research 39:1692-1702
(4) Richard & Cubeddu et al., (2011) Nucleic Acids Research 39: 3643-3651.