Covalent DNA-protein crosslinks (DPCs) are toxic DNA lesions that interfere with essential chromatin transactions, such as replication and transcription. Spartan (SPRTN) is a DNA-dependent metalloprotease which cleaves DPCs and plays a key role in maintaining genomic integrity []. SPRTN consists of an N-terminal SprT domain (responsible for the proteolytic cleavage of DPCs) followed by a basic region (BR, a DNA-binding element), both contributing to DNA binding. The C-terminal motifs and domains interact with proliferating cell nuclear antigen (PCNA) and ubiquitin, which collectively recruit SPRTN and the associated ATPase p97 or valosin-containing protein (p97 or VCP) to sites of DNA damage. The structure of the human SPRTN revealed a Zn2-binding sub-domain (ZBD) in SprTdomain that shields its active site located in the metalloprotease sub-domain (MPD). The ZBD contains an ssDNA-binding site, with a DNA-base-binding pocket formed by aromatic residues and is thought to contribute to the ssDNA specificity of SPRTN, restricts the access of globular substrates, and positions DPCs, which may need to be partially unfolded, for optimal cleavage []. The proteolytic activity of SPRTN is regulated by various mechanisms. To prevent the recruitment of SPRTN to chromatin, SPRTN undergoes monoubiquitylation, which can then be deubiquitylated by an unknown ubiquitin protease triggered by DPC induction. Once SPRTN is recruited to chromatin, DNA binding stimulates its protease activity. It has been shown that SPRTN is uniquely activated by single-stranded DNA (ssDNA). Moreover, SPRTN can degrade itself, which may switch off its proteolytic function when repair is complete [].SPRTN is also an activator of CHK1 (checkpoint kinase 1) during normal DNA replication by mediating proteolytic cleavage of CHK1, thereby promoting CHK1 removal from chromatin and subsequent activation. CHK1 phosphorylate SPRTN at the C-terminal regulatory domain and induces SPRTN recruitment to chromatin promoting DNA replication fork progression and DPC repair [].In humans mutations of SPRTN are linked to human Ruijs-Aalfs syndrome (RJALS), a syndrome characterised by is characterised by genomic instability, premature aging, and hepatocellular carcinoma []. In mice, loss of SPRTN is embryonically lethal, and conditional inactivation of SPRTN in murine embryonic fibroblasts (MEFs) blocks cell proliferation [].
VCPIP1, also known as VCIP135, is a deubiquitinating enzyme that functions in p97/p47-mediated Golgi reassembly []. It hydrolyzes 'Lys-11'- and 'Lys-48'-linked polyubiquitin chains []. It has also been found to dictate the duration of botulinum neurotoxin type A intoxication []. VCPIP1 can also deubiquitinate SPRTN (a metalloprotease that cleaves DNA-protein crosslinks) and promotes its chromatin relocalization []. VCPIP1 is a member of peptidase family C64.
