LRR1

LRR1 is a substrate recognition subunit of the CRL2 (CUL2–Elongin B/C) E3 ubiquitin ligase that drives replisome disassembly at replication termination by ubiquitylating the CMG helicase subunit MCM7, enabling p97/CDC-48-dependent extraction of the CMG complex and associated replisome components from chromatin (PMID:28235849, PMID:28368371, PMID:33590678). CRL2LRR1 is recruited to chromatin specifically at terminating replisomes during S-phase, and its loss causes persistent chromatin-bound CMG, progressive slowing of DNA replication through sequestration of rate-limiting replisome factors, ATR-dependent G2/M checkpoint activation, and lethality in human cells (PMID:34037657). The deubiquitylase USP37 antagonizes CRL2LRR1 at active forks to protect replisomes from premature disassembly during replication stress, while a TRAIP-dependent mitotic pathway provides a backup mechanism for CMG removal when CRL2LRR1-mediated S-phase unloading fails (PMID:40411782, PMID:33590678). In C. elegans, CRL2LRR1 additionally targets the CDK inhibitor CKI-1 (p21) for degradation to promote G1/S progression in the germline, and in human cells cytoplasmic p21 degradation by CRL2LRR1 regulates the Rho/ROCK/LIMK–cofilin pathway and actin cytoskeleton dynamics (PMID:21074724).

1. 2001 Low

   An initial yeast two-hybrid screen suggested LRR1 could interact with the 4-1BB receptor cytoplasmic domain and negatively regulate NF-κB and JNK1 signaling, providing the first functional hint for LRR1 but without loss-of-function or endogenous validation.

   Evidence Yeast two-hybrid and overexpression reporter/kinase assays in human cells

   PMID:11804328

   Open questions at the time

   • No reciprocal Co-IP or endogenous interaction confirmed
   • No loss-of-function validation
   • Physiological relevance of 4-1BB interaction unresolved
2. 2010 High

   CRL2LRR1 was established as a bona fide E3 ligase targeting the CDK inhibitor CKI-1/p21 for degradation, linking LRR1 to cell cycle control in the C. elegans germline and to cytoplasmic p21-dependent actin cytoskeleton regulation in human cells.

   Evidence In vivo ubiquitylation assays, Co-IP, C. elegans genetics, RNAi, actin/motility assays in human cells

   PMID:21074724

   Open questions at the time

   • Whether p21 targeting is direct or through an adaptor was not structurally resolved
   • Relative importance of cytoplasmic vs. nuclear p21 degradation unclear
3. 2013 Medium

   CRL2LRR1 was shown to counteract the ATR checkpoint and to degrade the HORMA domain protein HTP-3, expanding its substrate repertoire beyond CDK inhibitors and linking it to the mitosis-to-meiosis transition in C. elegans germ cells.

   Evidence Temperature-sensitive cul-2 allele, genetic epistasis, protein stability assays in C. elegans germline

   PMID:23555289

   Open questions at the time

   • HTP-3 degradation not demonstrated biochemically with purified components
   • Whether HTP-3 targeting is conserved in vertebrates is unknown
4. 2017 High

   Two independent studies identified CRL2LRR1 as the E3 ligase that ubiquitylates the CMG helicase subunit MCM7 at replication termination, establishing the central mechanism of p97-dependent replisome disassembly and revealing that CRL2LRR1 chromatin recruitment is temporally restricted to converging forks.

   Evidence Proteomic screens, depletion/add-back, ubiquitylation and chromatin fractionation assays in Xenopus egg extracts; Co-IP, epistasis, and synthetic lethality in C. elegans

   PMID:28235849 PMID:28368371

   Open questions at the time

   • Structural basis for LRR1 recognition of MCM7 not determined
   • Signal that recruits CRL2LRR1 specifically to terminated replisomes not identified
5. 2021 High

   CUL2LRR1 was validated as the S-phase CMG disassembly ligase in mammalian cells, while TRAIP was identified as the mitotic backup pathway, and loss of LRR1 was shown to be lethal due to progressive replisome sequestration and ATR checkpoint activation.

   Evidence Auxin-degron depletion in mouse ES cells; CRISPR knockout in human cells with replication rate measurements, checkpoint assays, and cell viability

   PMID:33590678 PMID:34037657

   Open questions at the time

   • How CRL2LRR1 and TRAIP are switched between S-phase and mitosis is mechanistically unresolved
   • Whether LRR1 loss contributes to human disease is unknown
6. 2025 High

   USP37 was identified as the deubiquitylase that directly opposes CRL2LRR1-mediated CMG ubiquitylation at active replication forks, revealing a regulatory circuit that protects elongating replisomes from premature disassembly during replication stress.

   Evidence Structure-guided PH domain mutagenesis of USP37, Co-IP with CDC45, genetic epistasis between Usp37 and CUL2LRR1 depletion, replication stress sensitivity assays

   PMID:40411782

   Open questions at the time

   • Whether additional DUBs also protect CMG from CRL2LRR1 is untested
   • Quantitative balance between USP37 and CRL2LRR1 activity at individual forks not measured

• The molecular signal that triggers CRL2LRR1 recruitment specifically to converging/terminated replisomes — and how this recruitment is prevented at active forks — remains the central unresolved question in the field.
• No structural model of the LRR1–MCM7 interface exists
• Post-translational modifications or conformational changes that license CRL2LRR1 binding at termination are unknown
• Whether CRL2LRR1 has additional replication-associated substrates beyond MCM7 in vertebrates is uncharacterized