ISG20L2

ISG20L2 is a vertebrate-specific nucleolar 3'-to-5' exoribonuclease of the DEDDh/RNase T superfamily that plays essential roles in ribosome biogenesis and immune regulation. Its crystal structure reveals a DEDDh active site (Asp183, Glu185, Asp267, His322, Asp327) with an αβα fold; loss-of-function studies demonstrate that it processes multiple pre-rRNA intermediates—removing the 3' external transcribed spacer (3'ETS), trimming ITS1 after endonucleolytic cleavage during 18S maturation, contributing to ITS2/5.8S 3'-end maturation, and processing the U8 snoRNA precursor—and its inactivation impairs large ribosomal subunit formation, disorganizes the nucleolar dense fibrillar component, and reduces cell proliferation (PMID:38153123, PMID:41261865). Outside the nucleolus, ISG20L2 is upregulated by TCR and type I interferon stimulation in T lymphocytes, where it preferentially degrades uridylated miRNA substrates and regulates immune synapse formation and expression of immunomodulatory receptors including PD-1, CTLA-4, and NKG2D (PMID:37646974). ISG20L2 also directly binds bortezomib and competes with the proteasome subunit PSMB5, conferring proteasome-inhibitor resistance in multiple myeloma cells (PMID:36040812).

1. 2010 Medium

   Establishing functional divergence from ISG20: despite sequence homology, ISG20L2 lacks the antiviral exonuclease activity of ISG20, delimiting it as functionally distinct from the broader ISG20 family.

   Evidence Ectopic overexpression in cell culture with HCV replication readout

   PMID:21036379

   Open questions at the time

   • Single virus system (HCV); antiviral activity against other viral classes not tested
   • Endogenous substrates of ISG20L2 remained unidentified
2. 2012 Medium

   Placing ISG20L2 in the ribosome biogenesis pathway: proteomic identification of ISG20L2 in preribosomal complexes alongside nucleophosmin, SBDS, and NIP7 provided the first evidence linking this nuclease to ribosome maturation.

   Evidence FLAG-tagged co-immunoprecipitation followed by mass spectrometry

   PMID:22540864

   Open questions at the time

   • No reciprocal immunoprecipitation or direct functional validation of ISG20L2 within the complex
   • Specific pre-rRNA substrate(s) processed by ISG20L2 remained unknown
3. 2022 High

   Revealing an unexpected non-catalytic function: ISG20L2 directly binds the proteasome inhibitor bortezomib and competes with PSMB5, establishing a drug-sequestration mechanism of resistance in myeloma.

   Evidence Surface plasmon resonance binding, biotinylated bortezomib pull-down, gain- and loss-of-function studies in vitro and in vivo

   PMID:36040812

   Open questions at the time

   • Whether bortezomib binding is mediated by the DEDDh active site or a distinct pocket is unresolved
   • Relevance to resistance against other proteasome inhibitors not established
4. 2023 High

   Defining ISG20L2's catalytic activity and immune function: ISG20L2 was shown to be a bona fide 3'-to-5' exoribonuclease with preferential activity on uridylated miRNAs, and its knockout in T cells disrupted immune synapse formation and expression of multiple checkpoint molecules, establishing it as an immunoregulatory enzyme.

   Evidence ISG20L2 knockout/silencing in T cells, biochemical substrate affinity assays, flow cytometry, immune synapse imaging

   PMID:37646974

   Open questions at the time

   • Specific miRNA targets mediating immune phenotypes not individually validated
   • Whether exonuclease catalytic activity is required for all observed immune phenotypes not dissected by catalytic-dead mutants
5. 2024 High

   Structural and mechanistic basis of rRNA processing: the 2.9 Å crystal structure of ISG20L2's nuclease domain revealed a canonical DEDDh active site, and mutagenesis coupled with pre-rRNA processing assays demonstrated its role in ITS1 removal during 18S maturation, directly linking nuclease activity to ribosome biogenesis and cell proliferation.

   Evidence X-ray crystallography at 2.9 Å, active-site and RNA-binding residue mutagenesis, pre-rRNA processing and proliferation assays

   PMID:38153123

   Open questions at the time

   • No co-crystal structure with RNA substrate; mode of substrate recognition remains modeled
   • Relative contributions of ITS1 vs. ITS2/5.8S processing to proliferation phenotype not separated
6. 2025 High

   Broadening the substrate repertoire: nucleotide-resolution sequencing after ISG20L2 inactivation revealed it is critical for 3'ETS removal from pre-rRNA and for U8 snoRNA precursor trimming, and that its loss disorganizes the nucleolar dense fibrillar component, establishing ISG20L2 as a multi-substrate rRNA/snoRNA processing enzyme essential for large subunit biogenesis.

   Evidence Loss-of-function with 3'-RACE high-throughput sequencing and nucleolar compartment imaging

   PMID:41261865

   Open questions at the time

   • Whether ISG20L2 acts alone or requires cofactors for 3'ETS and U8 processing not determined
   • Structural basis for substrate selectivity among its multiple RNA targets remains unresolved
7. 2025 Medium

   Connecting ISG20L2 to cancer cell phenotypes: ISG20L2 promotes lung adenocarcinoma proliferation and invasion through modulation of NKX2-1 expression, extending its functional relevance beyond ribosome biogenesis.

   Evidence Overexpression/depletion in A549 cells with proliferation and invasion assays

   PMID:40950659

   Open questions at the time

   • Mechanism by which ISG20L2 regulates NKX2-1 expression (direct RNA processing vs. indirect) not established
   • Single cell line; generalizability to other lung cancer models untested

• Key open questions include how ISG20L2 achieves substrate selectivity among its diverse RNA targets (pre-rRNA, snoRNA, uridylated miRNAs), whether its catalytic activity is required for bortezomib sequestration, and what cofactors or complexes recruit it to specific processing steps.
• No co-crystal structure with any RNA substrate
• Cofactors and protein partners that direct ISG20L2 to specific substrates are uncharacterized
• Whether catalytic-dead mutants retain the bortezomib-binding and immune-regulatory functions is untested