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Summary: To ensure timely duplication of the entire eukaryotic genome, thousands of replication machineries (replisomes) act on genomic DNA at any time during S phase. In the final stages of this process, replisomes are unloaded from chromatin. Unloading is driven by polyubiquitylation of MCM7, a subunit of the terminated replicative helicase, and processed by p97/VCP segregase. Most of our knowledge of replication termination comes from model organisms, and little is known about how this process is executed and regulated in human somatic cells. Here we show that replisome disassembly in this system requires CUL2LRR1-driven MCM7 ubiquitylation, p97, and UBXN7 for unloading and provide evidence for “backup” mitotic replisome disassembly, demonstrating conservation of such mechanisms. Finally, we find that small-molecule inhibitors against Cullin ubiquitin ligases (CULi) and p97 (p97i) affect replisome unloading but also lead to induction of replication stress in cells, which limits their usefulness to specifically target replisome disassembly processes.

Summary: The cohesin- and condensin-related SMC5/6 complex has largely been studied in the context of DNA repair. Nevertheless, SMC5/6 has an undefined essential function even in the absence of cellular stress. Through the use of an auxin-inducible degradation system for rapidly depleting subunits of the SMC5/6 complex, we show that SMC5/6 is essential for viability in cancer-derived and normal human cells. Impairment of SMC5/6 function is associated with spontaneous induction of DNA damage, p53 activation, cell-cycle arrest, and senescence, as well as an increased frequency of various mitotic chromosome segregation abnormalities. However, we show that this chromosome missegregation is apparent only when SMC5/6 function is impaired during the preceding S and G2 phases. In contrast, degradation of SMC5/6 immediately prior to mitotic entry has little or no impact on the fidelity of chromosome segregation, highlighting the importance of the complex during interphase in order to ensure faithful sister chromatid disjunction. : Venegas et al. employ an auxin-inducible degron system for different subunits of the SMC5/6 complex to interrogate the short- and long-term effects of SMC5/6 impairment in human cells. Degradation of SMC5/6’s subunits at different cell cycle stages places the essential role of the complex during S phase.

Studying the role of essential proteins is dependent upon a method for rapid inactivation, in order to study the immediate phenotypic consequences. Auxin-inducible degron (AID) technology allows rapid depletion of proteins in animal cells and fungi, but its application to human cells has been limited by the difficulties of tagging endogenous proteins. We have developed a simple and scalable CRISPR/Cas-based method to tag endogenous proteins in human HCT116 and mouse embryonic stem (ES) cells by using donor constructs that harbor synthetic short homology arms. Using a combination of AID tagging with CRISPR/Cas, we have generated conditional alleles of essential nuclear and cytoplasmic proteins in HCT116 cells, which can then be depleted very rapidly after the addition of auxin to the culture medium. This approach should greatly facilitate the functional analysis of essential proteins, particularly those of previously unknown function.