The mechanisms and regulation of the final stages of chromosome duplication are likely to be just as interesting and complex as the initiation process, but until recently almost nothing was known about termination. Just as CMG helicase assembly is the key regulated step during initiation, disassembly of the CMG helicase is likely to be the final regulated step of eukaryotic chromosome replication. In order to ensure the completion of DNA synthesis, helicase disassembly must never occur until two forks converge, but then it must always happen, leading to replisome disassembly.
We found that an E3 ubiquitin ligase drives ubiquitylation of the CMG helicase at the end of DNA replication in budding yeast and also in metazoa (Maric et al, Science, 2014; Maculins et al, Curr. Biol., 2015, Sonneville et al, Nat. Cell Biol., 2017), leading to a disassembly reaction that requires the Cdc48/p97 ‘segregase’ and its partners Ufd1-Npl4 (Maric et al, Cell Rep., 2017, Sonneville et al, Nat. Cell Biol., 2017). It’s an exciting time to work on this very new area of research, as most of the fundamental questions remain unanswered. What is the mechanism of helicase disassembly at the end of replication? What is the signal that triggers CMG ubiquitylation when two forks converge, and what prevents this signal from being generated at earlier stages of replication? Is there a universal mechanism for the termination of replication in diverse eukaryotic species, just as there seems to be for initiation? We take a variety of approaches ranging from in vitro reconstitution to in vivo studies in budding yeast, C. elegans, and more recently in mouse ES cells too.
Of particular interest, it now seems clear that metazoa have additional pathways for CMG disassembly, which are either activated in response to specific forms of DNA damage, or else provide important backup during mitosis. We aim to understand the mechanisms and regulation of these new pathways for CMG disassembly, which involve factors that are mutated in a variety of human cancers, and are likely to remain a hot area of research in the years to come.