Figure 6. Model for the collaborative activity of XPF‐ERCC1, RPA and SNM1A to unhook a crosslink.

1. When a single replication fork encounters an ICL, the nascent leading strand initially stalls 20–40 nt from the ICL (“−20” position; step a‐i). It gradually progresses to 1 nt from the ICL (“0” position; step a‐ii), and its arrival at the ICL triggers an XPF‐ERCC1‐RPA‐induced incision six nucleotides 5′ to the junction, in a duplex region (step a‐iii). SNM1A loads from these incisions and digests past the ICL, unhooking the ICL from the DNA duplex, leaving a residual single nucleotide moiety (step a‐iv), which has been demonstrated as the reaction product using mass spectrometry to characterise the reaction products of SNM1A activity in previous work (Wang et al, 2011). This enables translesion synthesis to occur and repair of the broken DNA strand via homologous recombination (step a‐v).
2. In the event of dual replication fork convergence onto an ICL, both nascent leading strands initially stall ˜20–40 nt from the ICL (step b‐i). CMG complexes from both replication forks unload from both leading strands, as previously described (Long et al, 2014; Zhang et al, 2015) which enables one nascent leading strand to gradually progresses to 1 nt from the ICL (“0” position; step b‐ii) as previously described (Raschle et al, 2008; Zhang et al, 2015). The structure that arises at this stage is inhibitory for XPF‐ERCC1. However, in the presence of RPA, XPF‐ERCC1 will be able to incise the structure (on the lagging‐strand template associated with the fork which has progressed to 0 nt) within the duplex region, 6 nt from the ICL (step b‐iv). This XPF‐ERCC1‐RPA‐induced incision enables SNM1A to load onto and digest past the ICL (step b‐v). The net result of XPF‐ERCC1‐RPA‐SNM1A is ICL unhooking, which enable the translesion (TLS) synthesis step, where the strand extended by the TLS polymerase is the nascent leading strand which remained arrested at ˜20–40 nt from the ICL on the second converged fork and did not strike the ICL (step b‐vi). Homologous recombination‐based repair of the broken chromatid completes repair and facilitates fork restart.

Data information: Black dotted arrows represent initial approach by nascent leading strands. Blue arrows represent incisions by XPF‐ERCC1; green dotted arrows represent digestion by SNM1A; maroon dotted arrows represent nascent leading strand progression.