COMMENTARY p31comet and TRIP13 recycle Rev7 to regulate DNA repair COMMENTARY Kevin D. Corbetta,b,c,1 Proteins of the HORMA domain family, named for its of Rev7’s roles in two important DNA repair pathways: three founding members Hop1, Rev7, and Mad2, play translesion DNA synthesis and DNA double-strand key roles in a broad range of eukaryotic signaling break (DSB) repair. Rev7 was first characterized as a pathways, from chromosome segregation and meiotic subunit of DNA polymerase ζ, which aids replication recombination, to DNA repair, to the initiation of of DNA past lesions that stall replicative polymerases autophagy (1). The HORMA domain nucleates assem- (23, 24). A groundbreaking recent structure of the po- bly of multiprotein signaling complexes by wrapping lymerase ζ holoenzyme shows that a dimer of Rev7 its C-terminal “safety belt” region entirely around a coordinates assembly of this complex by binding short, 6- to 10-amino acid “closure motif” in a binding two closure motifs (also called Rev7-binding motifs partner, resulting in a highly stable complex (Fig. 1A). or RBMs) in the catalytic Rev3 subunit, and mediating While the mechanisms governing assembly of interactions with the accessory subunits Pol31 and HORMA protein signaling complexes vary, many Pol32 (25). At the same time, Rev7 binds a second HORMA proteins share a common disassembly path- DNA polymerase, Rev1, which, in turn, recruits addi- way involving two proteins, p31comet and the AAA+ tional Y-family DNA polymerases like Polη,Polι,and ATPase TRIP13. p31comet, itself a diverged HORMA Polκ (26). In this manner, Rev7 functionally links the ac- protein, specifically binds HORMA proteins in their tivity of Y-family “inserter” polymerases that insert a “closed” partner-bound conformation, and recruits base directly opposite a DNA lesion with that of the them to TRIP13 (2–4). TRIP13 partially unfolds the “extender” polymerase Rev3 that synthesizes past the HORMA domain, releasing the bound closure motif lesion for eventual handoff to a replicative polymerase. and converting the substrate HORMA protein to an In vertebrates, Rev7 plays a second key role in inactive “open” conformation poised for rebinding DNA repair through the newly discovered Shieldin (5–10). This HORMA recycling pathway was first de- complex, which regulates DSB repair pathway choice scribed for the spindle assembly checkpoint protein (27). Cells possess two major pathways for DSB repair: Mad2 (11–13), and has since been extended to in- the homologous recombination (HR) pathway that clude the meiotic recombination factor Hop1 and its uses an unbroken DNA template to mediate error- relatives, collectively termed meiotic HORMADs free repair, and the more error-prone nonhomologous (14–20). A key question has been whether TRIP13 end-joining (NHEJ) pathway. HR and NHEJ are tightly and p31comet regulate other HORMA protein families, regulated during the cell cycle, with NHEJ dominant including the DNA repair factor Rev7 and the auto- in G1, and HR dominant in S and G2 when a sister phagy regulators Atg13 and Atg101. In two recent chromosome generated by DNA replication is avail- manuscripts, Clairmont et al. (21) and Sarangi et al. able as a repair template. A key factor in repair path- (22) provide convincing evidence that TRIP13 and way choice is the Shieldin complex, which comprises p31comet regulate Rev7 function in two DNA repair Rev7, SHLD1, SHLD2, and SHLD3. A recent structure pathways, and that overexpression of TRIP13 and/or of the Rev7−SHLD2−SHLD3 subcomplex revealed p31comet in cancer causes resistance to a widely used that a dimer of Rev7 nucleates Shieldin assembly class of anticancer drugs known as Poly-ADP ribose through a complex set of interactions, including bind- polymerase (PARP) inhibitors. ing to a closure motif in SHLD3 (28). In the cell, Shiel- Rev7 has long been under-studied compared to din is recruited to DSB sites by 53BP1 and Rif1, where Mad2 and the meiotic HORMADs, but recent years it inhibits DNA end resection to suppress HR and pro- have witnessed a renaissance in our understanding mote NHEJ (Fig. 1B) (27). aDepartment of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA 92093; bDepartment of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093; and cSan Diego Branch, Ludwig Institute for Cancer Research, La Jolla, CA 92093 Author contributions: K.D.C. wrote the paper. The author declares no competing interest. Published under the PNAS license. See companion article, “p31comet promotes homologous recombination by inactivating REV7 through the TRIP13 ATPase,” 10.1073/pnas. 2008830117. 1Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.2020103117 PNAS Latest Articles | 1of3 Downloaded by guest on September 30, 2021 Fig. 1. Regulation of HORMA proteins by p31comet and TRIP13. (A) HORMA-mediated signaling is activated when an open HORMA domain (light blue) binds a closure motif (yellow) in a binding partner and converts to the closed conformation (dark blue). To inactivate signaling, p31comet binds a HORMA-closure motif complex and recruits it to TRIP13 for disassembly. (B) Healthy cells maintain a balance between two DNA DSB repair pathways, HR and NHEJ. When recruited to DNA ends by 53BP1/Rif1, Rev7-Shieldin inhibits DNA end resection to suppress HR. In − − HR-deficient BRCA1 / cancers, PARP inhibition leads to increased DNA breaks and cell death. Overexpression of TRIP13 and/or p31comet in these cells reactivates HR to promote PARP inhibitor resistance, cell survival, and proliferation. −/− Given its central roles in two major DNA repair pathways, one Clairmont et al. (21) and Sarangi et al. (22) find that BRCA1 cells important question is whether Rev7, like the related Mad2 and overexpressing TRIP13 or p31comet become resistant to the PARP meiotic HORMADs, is regulated by the TRIP13/p31comet HORMA inhibitor Olaparib, and show that this resistance arises through reac- recycling pathway. In two recent studies, Clairmont et al. (21) and tivation of HR-mediated DNA repair due to lowered Shieldin com- Sarangi et al. (22) provide convincing evidence that TRIP13 and plex levels (Fig. 1B). Thus, inhibition of TRIP13/p31comet-mediated p31comet regulate Rev7 through conformational recycling and Shieldin disassembly may represent a promising treatment strat- Rev7-closure motif complex disassembly. The two studies show egy for patients with BRCA1-deficient cancers that develop resis- comet that TRIP13 and p31 physically interact with Rev7 and medi- tance to PARP inhibitors. ate disassembly of Shieldin and polymerase ζ, both in vitro and in Many questions remain with respect to the roles of TRIP13 comet cells. Overexpression of TRIP13 or p31 reduces association of and p31comet in healthy cells and in disease. First, while Clairmont Rev7 with SHLD3 and, in turn, increases DNA end resection at et al. (21) show that Rev7 can adopt two folded states in solution, comet DSBs and promotes their repair by HR. TRIP13/p31 overex- how these two states correspond to Mad2’s well-defined open pression also reduces the association of Rev7 with Rev3 and im- and closed conformations requires additional exploration. An- ’ pairs cells ability to repair DNA lesions caused by ultraviolet other question is whether and how Rev7 recycling is regulated radiation and the interstrand crosslinker mitomycin C, demon- through the cell cycle. Two studies have reported that p31comet strating a defect in translesion DNA synthesis. These data firmly is phosphorylated during mitosis to suppress disassembly of mi- comet ’ establish a role for TRIP13 and p31 in regulating Rev7 s func- totic checkpoint complexes (30, 31); whether this or other regula- tion in two important DNA repair pathways. comet tory mechanisms applies to Rev7 regulation is unknown. Finally, A direct role for TRIP13 and p31 in Rev7 regulation can comet whether TRIP13 and p31 regulate autophagy through Atg13 also explain these proteins’ contributions to cancer. TRIP13 is and Atg101, or other signaling pathways through as yet undiscov- known to be overexpressed in many human cancers, and Sarangi ered HORMA proteins, remains mostly unexplored. While many et al. (22) now find that p31comet overexpression is also common in questions remain, the studies by Clairmont et al. (21) and Sarangi cancer. Moreover, high TRIP13 and/or p31comet levels correlate et al. (22) provide significant insight into HORMA recycling by with poor prognosis across many cancer types (21, 22). While in- TRIP13 and p31comet, and clearly define a key connection between creased HORMA recycling activity in cancerous cells likely impacts these proteins and human disease. many HORMA-dependent signaling pathways, Clairmont et al. (21) and Sarangi et al. (22) demonstrate that Shieldin-regulated DSB repair is of particular clinical relevance. Many cancers lose the Acknowledgments ability to repair DNA breaks by HR through mutation or loss of I thank members of the Corbett laboratory and A. Desai for critical reading and BRCA1, and these cancers are sensitive to inhibition of PARP (29). helpful suggestions, and the NIH (Grant R01 GM104141) for support. 1 S. C. 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