The p97 segregase cofactor Ubxn7 facilitates replisome disassembly during S-phase

Complex cellular processes are driven by the regulated assembly and disassembly of large multiprotein complexes. While we are beginning to understand the molecular mechanism for assembly of the eukaryotic DNA replication machinery (replisome), we still know relatively little about the regulation of its disassembly at replication termination. Recently, the first elements of this process have emerged, revealing that the replicative helicase, at the heart of the replisome, is polyubiquitylated prior to unloading and that this unloading requires p97 segregase activity. Two different E3 ubiquitin ligases have now been shown to ubiquitylate the helicase under different conditions: Cul2(Lrr1) and TRAIP. Here, using Xenopus laevis egg extract cell-free system and biochemical approaches, we have found two p97 cofactors, Ubxn7 and Faf1, which can interact with p97 during replisome disassembly during S-phase. We show only Ubxn7, however, facilitates efficient replisome disassembly. Ubxn7 delivers this role through its interaction via independent domains with both Cul2(Lrr1 )and p97 to allow coupling between Mcm7 ubiq-uitylation and its removal from chromatin. Our data therefore characterize Ubxn7 as the first substrate-specific p97 cofactor regulating replisome disassembly in vertebrates and a rationale for the efficacy of the Cul2(Lrr1) replisome unloading pathway in unperturbed S-phase.

Automated summary

Complex cellular processes depend on the assembly and disassembly of multiprotein complexes. While the molecular mechanism for assembly of the replisome is understood, little is known about its disassembly at replication termination. In this study, the authors identified the p97 cofactors Ubxn7 and Faf1 as regulators of replisome disassembly. They found that Ubxn7 interacts with both Cul2 (Lrr1) and p97 to facilitate efficient replisome disassembly. This study provides new insights into the regulation of replisome disassembly in vertebrates.