E3 ubiquitin-protein ligase required for the repair of DNA interstrand cross-links (ICL) in response to DNA damage. Plays a key role in RPA-mediated DNA damage signaling and repair. Acts by mediating ubiquitination of the RPA complex (RPA1, RPA2 and RPA3 subunits) and RAD51 at stalled replication forks, leading to remove them from DNA damage sites and promote homologous recombination. Also mediates the ubiquitination of p53/TP53 in the late response to DNA damage, and acts as a positive regulator of p53/TP53 stability, thereby regulating the G1/S DNA damage checkpoint. May act by catalyzing the formation of short polyubiquitin chains on p53/TP53 that are not targeted to the proteasome. In response to ionizing radiation, interacts with MDM2 and enhances p53/TP53 ubiquitination, possibly by restricting MDM2 from extending polyubiquitin chains on ubiquitinated p53/TP53. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with MDM2 and p53/TP53. Binds to the RPA complex via direct interaction with RPA2. Interacts with RAD51. In undamaged cells, found both in the cytoplasm and in the nucleus, partially associated with PML nuclear bodies. In response to replication block, such as that caused by hydroxyurea treatment, or to DNA damage caused by ionizing radiations or doxorubicin, recruited to the nucleus, to stalled replication forks or to sites of DNA repair. This recruitment depends upon RPA2. The coiled coil domain may be involved in RPA2-binding. Phosphorylated at Ser-46 and Ser-63 upon DNA damage by ATM or ATR. ATM phosphorylation occurs at early times upon DNA damage, while ATR is the major kinase at later times. Phosphorylation by ATM and ATR is required to stabilize p53/TP53. Part of the phosphorylation depends upon RPA2 presence.