ISG20L2

ISG20L2 is a vertebrate-specific DEDDh-motif 3'-5' exoribonuclease of the RNase T superfamily that drives ribosome biogenesis through processing of pre-rRNA spacers (PMID:38153123, PMID:41261865). A 2.9 Å crystal structure established its αβα fold and defined the catalytic DEDDh residues (Asp183, Glu185, Asp267, His322, Asp327), with mutagenesis confirming the catalytic mechanism and mapping RNA substrate-binding residues (PMID:38153123). Functionally, it removes the ITS1 region during 18S pre-rRNA maturation (PMID:38153123) and is also required for efficient removal of the 3'ETS, ITS1 trimming after cleavage at site 2, and 3' processing of the U8 snoRNA precursor, thereby contributing to maturation of both 18S and 28S rRNA 3' ends; its loss causes accumulation of 3'-extended pre-rRNAs and disorganization of the nucleolar peripheral dense fibrillar component (PMID:41261865). Consistent with a nucleolar ribosome-biogenesis role, ISG20L2 co-immunoprecipitates with preribosomal complexes sharing components with nucleophosmin-, SBDS-, and NIP7-associated complexes, and its depletion impairs ribosome biogenesis and cell proliferation (PMID:22540864, PMID:38153123). Beyond the nucleolus, ISG20L2 preferentially degrades uridylated miRNA substrates and is induced in T cells by TCR and type I IFN stimulation, where it shapes activation markers, cytokine output, immune-synapse organization, and expression of immunoregulatory molecules (PMID:37646974). In the cytoplasm of multiple myeloma cells, ISG20L2 directly binds the proteasome inhibitor bortezomib and competes with PSMB5 for the drug, attenuating proteasome inhibition and conferring bortezomib resistance (PMID:36040812).

1. 2010 Medium

   Tested whether ISG20L2 shares the antiviral exonuclease activity of its paralog ISG20, establishing that the two proteins are functionally distinct.

   Evidence Ectopic overexpression in cell culture with an HCV replication readout

   PMID:21036379

   Open questions at the time

   • Negative result from a single overexpression assay does not exclude antiviral roles under other conditions
   • No catalytic or mechanistic characterization of ISG20L2 performed here
2. 2012 Medium

   Placed ISG20L2 physically within the ribosome biogenesis machinery, the first link to its core function.

   Evidence Co-immunoprecipitation followed by mass spectrometry of preribosomal complexes in human cells

   PMID:22540864

   Open questions at the time

   • Association is correlative; no functional mutagenesis of ISG20L2
   • Does not identify which processing step ISG20L2 catalyzes
3. 2022 High

   Revealed a non-canonical cytoplasmic function: direct drug binding that confers proteasome-inhibitor resistance, independent of any RNA role.

   Evidence Biotinylated bortezomib pull-down, surface plasmon resonance, and gain/loss-of-function in multiple myeloma cells and in vivo

   PMID:36040812

   Open questions at the time

   • Structural basis of bortezomib binding by ISG20L2 not resolved
   • Relationship between this drug-sequestering role and the enzyme's RNase activity unclear
4. 2023 Medium

   Defined a substrate preference (uridylated miRNAs) and an immune-cell role, extending ISG20L2 function beyond the nucleolus.

   Evidence siRNA knockdown and CRISPR knockout in T cells with flow cytometry, ELISA, confocal microscopy, and exonuclease substrate assays

   PMID:37646974

   Open questions at the time

   • Direct miRNA targets that mediate the T cell phenotypes not identified
   • Whether nucleolar pre-rRNA processing contributes to the immune phenotype is not addressed
5. 2024 High

   Established the catalytic mechanism and a defined pre-rRNA substrate, converting the prior associative evidence into a mechanistic identity as a DEDDh 3'-5' exonuclease.

   Evidence 2.9 Å crystal structure, active-site mutagenesis, and cellular loss-of-function and pre-rRNA processing assays

   PMID:38153123

   Open questions at the time

   • Structure of an RNA-bound complex not determined
   • Cofactors or partner proteins guiding substrate selection in vivo not defined
6. 2025 High

   Mapped the full repertoire of pre-rRNA and snoRNA processing steps ISG20L2 controls at nucleotide resolution and linked its loss to nucleolar architecture defects.

   Evidence Loss-of-function with 3'-RACE high-throughput sequencing and nucleolar morphology analysis

   PMID:41261865

   Open questions at the time

   • Order and regulation of the multiple processing steps not fully resolved
   • Functional coupling between U8 snoRNA processing and rRNA maturation by ISG20L2 not established
7. 2025 Low

   Connected ISG20L2 to a cancer-relevant transcriptional output, proposing it promotes lung adenocarcinoma via NKX2-1.

   Evidence Transient overexpression and siRNA depletion in A549 cells with proliferation, invasion, and apoptosis assays

   PMID:40950659

   Open questions at the time

   • Single transient-transfection approach in one cell line; not independently confirmed
   • Mechanism linking ISG20L2 exonuclease activity to NKX2-1 regulation is undefined

• How ISG20L2's distinct activities — nucleolar pre-rRNA processing, cytoplasmic miRNA degradation, and bortezomib sequestration — are partitioned and regulated within the same protein remains unresolved.
• No model reconciling nucleolar versus cytoplasmic localization and function
• Determinants of substrate and drug selectivity not defined