A non‐proteolytic release mechanism for HMCES‐DNA‐protein crosslinks

The conserved protein HMCES crosslinks to abasic (AP) sites in ssDNA to prevent strand scission and the formation of toxic dsDNA breaks during replication. Here, we report a non‐proteolytic release mechanism for HMCES‐DNA‐protein crosslinks (DPCs), which is regulated by DNA context. In ssDNA and at ssDNA‐dsDNA junctions, HMCES‐DPCs are stable, which efficiently protects AP sites against spontaneous incisions or cleavage by APE1 endonuclease. In contrast, HMCES‐DPCs are released in dsDNA, allowing APE1 to initiate downstream repair. Mechanistically, we show that release is governed by two components. First, a conserved glutamate residue, within HMCES' active site, catalyses reversal of the crosslink. Second, affinity to the underlying DNA structure determines whether HMCES re‐crosslinks or dissociates. Our study reveals that the protective role of HMCES‐DPCs involves their controlled release upon bypass by replication forks, which restricts DPC formation to a necessary minimum.