Abasic (AP) sites are one of the most frequently occurring types of DNA damage. They lead to DNA strand breaks, interstrand DNA crosslinks, and block transcription and replication. Mutagenicity of AP sites arises from translesion synthesis (TLS) by error-prone bypass polymerases. Recently, a new cellular response to AP sites was discovered, in which the protein HMCES (5-hydroxymethlycytosine (5hmC) binding, embryonic stem cell-specific) forms a stable, covalent DNA-protein crosslink (DPC) to AP sites at stalled replication forks. The stability of the HMCES-DPC prevents strand cleavage by endonucleases and mutagenic bypass by TLS polymerases. Crosslinking is carried out by a unique SRAP (SOS Response Associated Peptidase) domain conserved across all domains of life. Here, we review the collection of recently reported SRAP crystal structures from human HMCES and E. coli YedK, which provide a unified basis for SRAP specificity and a putative chemical mechanism of AP site crosslinking. We discuss the structural and chemical basis for the stability of the SRAP DPC and how it differs from covalent protein-DNA intermediates in DNA lyase catalysis of strand scission.

脱碱基 (AP) 位点是最常见的 DNA 损伤类型之一。它们导致 DNA 链断裂、链间 DNA 交联，并阻止转录和复制。AP 位点的致突变性源于易错旁路聚合酶的跨损伤合成 (TLS)。最近，发现了一种对 AP 位点的新细胞反应，其中蛋白质 HMCES（5-羟甲基胞嘧啶 (5hmC) 结合，胚胎干细胞特异性）在复制停滞时与 AP 位点形成稳定的共价 DNA-蛋白质交联 (DPC)叉子。HMCES-DPC 的稳定性可防止内切核酸酶和 TLS 聚合酶的诱变旁路切割链。交联是由一个独特的 SRAP（SOS 响应相关肽酶）结构域进行的，该结构域在生命的所有领域都保守。这里，大肠杆菌YedK，为 SRAP 特异性和 AP 位点交联的假定化学机制提供了统一的基础。我们讨论了 SRAP DPC 稳定性的结构和化学基础，以及它在链断裂的 DNA 裂解酶催化中与共价蛋白质-DNA 中间体有何不同。