ASCC2

ASCC2 is a ubiquitin-binding scaffold protein that couples K63-linked polyubiquitin recognition to the recruitment of the ASCC3 helicase at sites of alkylation DNA damage, stalled ribosomes, and stalled replication forks. Its CUE domain specifically contacts both the distal and proximal ubiquitin moieties of K63-linked diubiquitin, conferring linkage selectivity; this recognition is essential for nuclear foci formation upon alkylation damage (signaled by the E3 ligase RNF113A) and for triggering ribosome-associated quality control as part of the human RQT complex (with ASCC3 and TRIP4) (PMID:29144457, PMID:34971705, PMID:32099016). Crystal structure analysis reveals that ASCC2 forms a compact unit clasped by ASCC3 helical and extended-arm segments, an interface disrupted by recurrent somatic cancer mutations, while the accessory subunit ASCC1 coordinates correct ASCC2 incorporation into the complex at damage foci (PMID:33139697, PMID:29997253). At stalled replication forks, ASCC2 recruitment depends on PCNA-K164 polyubiquitylation by SHPRH/HLTF/RFWD3, and downstream ASCC3 helicase activity promotes fork reversal, SMARCAL1 recruitment, and ATR checkpoint activation (PMID:41785087).

1. 2017 High

   The first mechanistic role for ASCC2 was established: its CUE domain selectively recognizes K63-linked polyubiquitin generated by RNF113A, and this recognition is required for ASCC complex recruitment to alkylation damage sites, resolving how the repair machinery is targeted to alkylated DNA.

   Evidence CUE domain mutagenesis, nuclear foci imaging, KO sensitivity assays with alkylating agents, epistasis with RNF113A in human cells

   PMID:29144457

   Open questions at the time

   • Structural basis for K63-linkage selectivity of the CUE domain was not yet resolved
   • Whether ASCC2 functions beyond alkylation repair was unknown
   • The direct interaction interface between ASCC2 and ASCC3 had not been defined
2. 2018 Medium

   ASCC1 was shown to regulate proper assembly of the ASCC complex at damage foci by ensuring ASCC2 co-localizes with ASCC3, establishing that an accessory subunit coordinates scaffold recruitment rather than ASCC2 acting autonomously.

   Evidence Co-IP, live-cell foci imaging, CRISPR KO with alkylation sensitivity epistasis in human cells

   PMID:29997253

   Open questions at the time

   • ASCC1 interacts with ASCC3 rather than ASCC2 directly; the mechanism by which ASCC1 promotes ASCC2 recruitment is unclear
   • Single-lab study without independent replication
3. 2019 Medium

   Genome-wide genetic screening extended ASCC2's function beyond DNA repair to ribosome stalling, showing that ASCC2 and ASCC3 protect cells from toxic ribosome-stalling compounds and operate in the same genetic pathway downstream of HBS1L.

   Evidence CRISPRi screen, genetic epistasis, cell growth assays in human cells

   PMID:30875366

   Open questions at the time

   • Whether ASCC2's ubiquitin-binding activity was required for ribosome quality control was not tested
   • Biochemical mechanism of ribosome splitting was not addressed
4. 2020 High

   Two advances converged: the crystal structure of the ASCC2–ASCC3 complex revealed the molecular interface (disrupted by cancer mutations), and ASCC2 was formally defined as a subunit of the human RQT complex whose ubiquitin-binding activity triggers ribosome-associated quality control upon ribosome stalling.

   Evidence Crystal structure with quantitative binding assays and cancer mutation mapping (ASCC2–ASCC3); Co-IP, dominant-negative ubiquitin-binding mutants, RQC reporter assays (hRQT complex)

   PMID:32099016 PMID:33139697

   Open questions at the time

   • The ubiquitin signal on stalled ribosomes recognized by ASCC2 was not identified
   • No cryo-EM structure of the full hRQT-ribosome complex
   • The relevance of cancer mutations to tumor biology in vivo was not tested
5. 2021 High

   The structural basis for K63-linkage specificity was resolved: unique contacts between the ASCC2 CUE domain α1 helix N-terminus and the proximal ubiquitin of K63-diubiquitin confer selectivity, and mutation of these residues ablates damage-induced recruitment.

   Evidence Crystal/NMR structure of CUE domain–K63-diubiquitin complex, in vitro binding assays, mutagenesis with cellular recruitment readouts

   PMID:34971705

   Open questions at the time

   • How CUE domain engagement with polyubiquitin is coordinated with ASCC3 binding was not determined
   • No structural view of full-length ASCC2 in the context of the complete ASCC complex
6. 2026 High

   ASCC2's ubiquitin-reading function was extended to replication stress: ASCC2 recruitment to stalled forks requires PCNA-K164 polyubiquitylation by SHPRH/HLTF/RFWD3, and ASCC3 helicase activity downstream promotes fork reversal, SMARCAL1 loading, and ATR activation.

   Evidence Co-IP, ubiquitin-binding mutant recruitment assays, in vitro fork reversal reconstitution, epistasis with PCNA ubiquitylation pathway

   PMID:41785087

   Open questions at the time

   • Whether ASCC2 directly binds polyubiquitylated PCNA or an intermediate reader is involved was not resolved
   • In vivo relevance to replication-associated genome instability or tumor suppression not tested

• Key unresolved questions include how ASCC2 discriminates among its three recruitment contexts (alkylation damage, stalled ribosomes, stalled replication forks), whether competition or regulated switching occurs, and the identity of the ubiquitylated substrate recognized by ASCC2 at stalled ribosomes.
• No structure of ASCC2 in the context of a stalled ribosome or replication fork
• The ubiquitin substrate at stalled ribosomes recognized by ASCC2 is unknown
• Mechanism by which context-specific recruitment is regulated has not been addressed