Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005737 cytoplasm	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetically inferred cytoplasmic localization. USP21 is experimentally documented in the cytoplasm and acts on cytoplasmic substrates (e.g. 40S ribosomal proteins), so this is well supported. Reason: Cytoplasmic localization is corroborated by direct experimental evidence (EXP, PMID:21888622) and HPA cytosol IDA, and is where USP21 deubiquitinates ribosomal substrates. Supporting Evidence: file:human/USP21/USP21-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm
GO:0004843 cysteine-type deubiquitinase activity	IEA GO_REF:0000120	ACCEPT	Summary: Electronic (InterPro/EC-based) assignment of the core DUB activity. This is the central, experimentally proven molecular function of USP21. Reason: The cysteine-type deubiquitinase activity is directly demonstrated in human cells (IDA, PMID:10799498; PMID:32011234) with Cys-221 as the catalytic nucleophile; the IEA assignment is correct and corroborated by experiment. Supporting Evidence: PMID:10799498 USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
GO:0005634 nucleus	IEA GO_REF:0000044	ACCEPT	Summary: Automated subcellular-location assignment of nuclear localization, consistent with USP21's documented nuclear pool and nuclear substrates (BAZ2A/TIP5, histone H2A). Reason: Nuclear localization is experimentally supported (EXP, PMID:21888622) and consistent with nucleoplasm IDA; USP21 shuttles via a CRM1-dependent NES. Supporting Evidence: file:human/USP21/USP21-uniprot.txt Nucleus
GO:0005737 cytoplasm	IEA GO_REF:0000044	ACCEPT	Summary: Automated subcellular-location assignment of cytoplasmic localization, redundant with but consistent with the IBA and experimental cytoplasm annotations. Reason: Correct compartment; agrees with stronger experimental (EXP) and IBA evidence for the same location. Supporting Evidence: file:human/USP21/USP21-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm
GO:0008234 cysteine-type peptidase activity	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: Family-level (ARBA) assignment of cysteine-type peptidase activity, a parent of the more specific deubiquitinase activity that USP21 actually performs. Reason: Correct but less precise than GO:0004843 cysteine-type deubiquitinase activity, which is the experimentally established function. Retained as a true but non-core (general) molecular-function annotation. Supporting Evidence: file:human/USP21/USP21-uniprot.txt Belongs to the peptidase C19 family. USP21 subfamily.
GO:0016579 protein deubiquitination	IEA GO_REF:0000002	ACCEPT	Summary: InterPro-based assignment of the protein deubiquitination process, which is the direct biological outcome of USP21's DUB activity. Beyond the cache-verified substrates (40S ribosomal proteins, BAZ2A/TIP5), the Falcon deep-research report compiles a broad reported substrate repertoire (e.g. RIG-I, MEK2, Nanog, histone H2A, MARK3, TCF7) processed via multiple ubiquitin chain types, all of which fall under this general process term. Reason: Protein deubiquitination is directly demonstrated for multiple human substrates (40S ribosomal proteins, BAZ2A/TIP5); this process annotation is well supported. The Falcon report (UNVERIFIED LLM synthesis; primary substrate papers not in cache) further supports breadth of substrate processing and chain-type promiscuity but does not alter this annotation. Supporting Evidence: PMID:32011234 cells lacking USP21 or OTUD3 have altered RQC activity and delayed eS10 deubiquitylation file:human/USP21/USP21-deep-research-falcon.md The enzyme exhibits promiscuous activity toward multiple ubiquitin chain types, including K6-, K11-, K29-, K48-, K63-linked, and linear ubiquitin conjugates
GO:0045893 positive regulation of DNA-templated transcription	IEA GO_REF:0000108	KEEP AS NON CORE	Summary: Automated inference (from the transcription coactivator activity term) that USP21 positively regulates transcription. This derives ultimately from the mouse "by similarity" histone H2A deubiquitination / coactivator role rather than direct human experimental evidence. Reason: A plausible downstream process inherited from the ortholog-transferred coactivator role (histone H2A deubiquitination relieving repression), but not directly demonstrated for human USP21 and peripheral to its core DUB function. USP21 acts as both a coactivator (H2A) and a repressor (rDNA via BAZ2A), so a single positive-regulation term is an oversimplification. The Falcon report adds (UNVERIFIED) that USP21 removes the repressive H2AK119ub mark with specificity for nucleosomal substrate, consistent with a transcriptional-activation role, though the primary paper is not cached. Supporting Evidence: file:human/USP21/USP21-uniprot.txt resulting in regulation of transcriptional initiation file:human/USP21/USP21-deep-research-falcon.md USP21 removes monoubiquitin from histone H2A at lysine 119 (H2AK119ub) in nucleosomal contexts, with strong specificity for nucleosomal rather than free histone substrates
GO:0005515 protein binding	IPI PMID:19615732 Defining the human deubiquitinating enzyme interaction lands...	KEEP AS NON CORE	Summary: High-throughput DUB interaction-landscape screen (Sowa et al.) capturing a USP21-UCHL1 (P09936) interaction. The bare protein binding term is uninformative and the partner does not define a specific USP21 function. Reason: Records a real physical interaction but bare protein binding (GO:0005515) is uninformative per curation guidelines; not a core function. The UCHL1 partner is an unrelated DUB from a global screen. Supporting Evidence: file:human/USP21/USP21-goa.tsv PMID:19615732 UniProtKB:P09936
GO:0005515 protein binding	IPI PMID:25416956 A proteome-scale map of the human interactome network.	KEEP AS NON CORE	Summary: Proteome-scale yeast two-hybrid interactome (HuRI) capturing a USP21-KRT40 (Q6A162) interaction. Bare protein binding from a global screen; uninformative. Reason: Single high-throughput Y2H interaction with a keratin (KRT40) that has no established functional relationship to USP21; bare protein binding is uninformative and not part of the core function. Supporting Evidence: file:human/USP21/USP21-goa.tsv PMID:25416956 UniProtKB:Q6A162
GO:0005515 protein binding	IPI PMID:32296183 A reference map of the human binary protein interactome.	KEEP AS NON CORE	Summary: Binary reference interactome (HuRI) capturing several USP21 interactions (EFEMP2, CPSF6, ADAMTSL4, KCTD9, HOXC10). These are bare protein binding hits from a systematic screen. Reason: Records real binary interactions but bare protein binding (GO:0005515) is uninformative; the partners are from a global screen and do not individually define a specific USP21 molecular function. Not core. Supporting Evidence: file:human/USP21/USP21-goa.tsv PMID:32296183 UniProtKB:O95967
GO:0005515 protein binding	IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ...	KEEP AS NON CORE	Summary: Neurodegeneration interactome screen capturing a USP21-UCHL1 (P09936) interaction. Bare protein binding from a high-throughput map. Reason: Records a real interaction but bare protein binding (GO:0005515) is uninformative and the partner does not define a specific USP21 function; not core. Supporting Evidence: file:human/USP21/USP21-goa.tsv PMID:32814053 UniProtKB:P09936
GO:0016579 protein deubiquitination	TAS Reactome:R-HSA-5688426	ACCEPT	Summary: Reactome (TAS) curation of USP21 protein deubiquitination, here in the context of ubiquitin-specific processing protease pathways. Consistent with its core process. Reason: Protein deubiquitination is the genuine process carried out by USP21 and is directly supported by experimental evidence elsewhere; the curated TAS term is appropriate. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0016579 protein deubiquitination
GO:0004843 cysteine-type deubiquitinase activity	TAS Reactome:R-HSA-5690157	ACCEPT	Summary: Reactome (TAS) annotation of the core DUB activity. Correct and experimentally corroborated. Reason: Reflects USP21's experimentally proven cysteine-type deubiquitinase activity; curated TAS evidence is consistent with direct IDA evidence. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0004843 cysteine-type deubiquitinase activity
GO:0004843 cysteine-type deubiquitinase activity	TAS Reactome:R-HSA-5690159	ACCEPT	Summary: Reactome (TAS) annotation of the core cysteine-type deubiquitinase activity. Reason: Correct core molecular function, corroborated by direct experimental evidence. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0004843 cysteine-type deubiquitinase activity
GO:0004843 cysteine-type deubiquitinase activity	TAS Reactome:R-HSA-6783177	ACCEPT	Summary: Reactome (TAS) annotation of the core cysteine-type deubiquitinase activity. Reason: Correct core molecular function, corroborated by direct experimental evidence. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0004843 cysteine-type deubiquitinase activity
GO:0005654 nucleoplasm	IDA GO_REF:0000052	ACCEPT	Summary: Direct immunofluorescence (HPA) evidence for nucleoplasmic localization, consistent with USP21's nuclear substrates (BAZ2A/TIP5, histone H2A). Reason: IDA-supported nuclear localization agrees with the documented nuclear pool of USP21 and its nuclear functions. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0005654 nucleoplasm cellular_component
GO:0005829 cytosol	IDA GO_REF:0000052	ACCEPT	Summary: Direct immunofluorescence (HPA) evidence for cytosolic localization, consistent with USP21's cytoplasmic role in ribosome quality control. Reason: IDA-supported cytosolic localization agrees with the documented cytoplasmic site of action. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0005829 cytosol cellular_component
GO:0005654 nucleoplasm	TAS Reactome:R-HSA-5690157	ACCEPT	Summary: Reactome (TAS) nucleoplasm localization, consistent with the experimentally supported nuclear pool. Reason: Agrees with HPA IDA nucleoplasm and EXP nucleus evidence. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0005654 nucleoplasm cellular_component
GO:0005654 nucleoplasm	TAS Reactome:R-HSA-6783177	ACCEPT	Summary: Reactome (TAS) nucleoplasm localization, redundant with the other nucleoplasm annotations and consistent with experimental evidence. Reason: Agrees with HPA IDA nucleoplasm and EXP nucleus evidence. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0005654 nucleoplasm cellular_component
GO:0005829 cytosol	TAS Reactome:R-HSA-5690159	ACCEPT	Summary: Reactome (TAS) cytosol localization, consistent with the experimentally supported cytoplasmic pool. Reason: Agrees with HPA IDA cytosol and EXP cytoplasm evidence. Supporting Evidence: file:human/USP21/USP21-goa.tsv GO:0005829 cytosol cellular_component
GO:0005634 nucleus	EXP PMID:21888622 A global survey of CRM1-dependent nuclear export sequences i...	ACCEPT	Summary: Experimental (EXP) evidence for nuclear localization from a study of CRM1-dependent nuclear export signals across the human DUB family; USP21 has a functional NES. Reason: Direct experimental evidence places USP21 in the nucleus with a CRM1-dependent export signal, consistent with its nuclear substrates. Supporting Evidence: file:human/USP21/USP21-uniprot.txt Nucleus {ECO:0000269\|PubMed:21888622}
GO:0005737 cytoplasm	EXP PMID:21888622 A global survey of CRM1-dependent nuclear export sequences i...	ACCEPT	Summary: Experimental (EXP) evidence for cytoplasmic localization, consistent with USP21's CRM1-dependent nuclear export and cytoplasmic substrates. The Falcon deep-research report notes a more specific cytoplasmic distribution from a systematic DUB-localization survey (Urbe et al. 2012), in which USP21 uniquely associates with microtubules and centrosomes. Reason: Direct experimental evidence supports cytoplasmic localization; consistent with IBA, IEA and HPA IDA cytosol. A more granular cytoplasmic localization (microtubule/centrosome) is reported in the Falcon synthesis but the primary paper is not in the cache, so no more-specific CC term is asserted here. Supporting Evidence: file:human/USP21/USP21-uniprot.txt Cytoplasm {ECO:0000269\|PubMed:21888622} file:human/USP21/USP21-deep-research-falcon.md USP21 is the only deubiquitinase in a systematic survey of 66 mammalian DUBs to display clear association with both microtubules and centrosomes
GO:0004843 cysteine-type deubiquitinase activity	IDA PMID:32011234 Distinct regulatory ribosomal ubiquitylation events are reve...	ACCEPT	Summary: Direct (IDA) demonstration that USP21 is a cysteine-type deubiquitinase acting on ubiquitylated 40S ribosomal proteins, with catalytic Cys-221 required. Reason: Strong direct human experimental evidence for the core DUB activity; USP21 antagonizes ZNF598-mediated 40S ubiquitylation. This is a defining piece of evidence for the core molecular function. Supporting Evidence: PMID:32011234 we identify OTUD3 and USP21 as deubiquitylating enzymes that antagonize ZNF598-mediated 40S ubiquitylation and can limit RQC activation
GO:0016579 protein deubiquitination	IDA PMID:32011234 Distinct regulatory ribosomal ubiquitylation events are reve...	ACCEPT	Summary: Direct (IDA) evidence that USP21 carries out protein deubiquitination of 40S ribosomal proteins (eS10/RPS10, uS10/RPS20) within the RQC pathway, acting as a negative regulator of RQC. Reason: Directly demonstrated process; cells lacking USP21 show delayed eS10 deubiquitylation and altered RQC activity. This is a core, well-supported biological process for USP21. Supporting Evidence: PMID:32011234 cells lacking USP21 or OTUD3 have altered RQC activity and delayed eS10 deubiquitylation
GO:0045815 transcription initiation-coupled chromatin remodeling	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: Sequence-similarity (ISS, from mouse Q9QZL6) transfer of a chromatin-remodeling / transcription-initiation role based on histone H2A deubiquitination. Not directly demonstrated for human USP21. Reason: A plausible role inherited by ortholog transfer from mouse (histone H2A deubiquitination relieving H3K4 methylation repression), but not directly shown in human and secondary to USP21's core DUB function across multiple substrates. Supporting Evidence: file:human/USP21/USP21-uniprot.txt Deubiquitination of histone H2A releaves the repression of di- and trimethylation of histone H3 at 'Lys-4'
GO:0004843 cysteine-type deubiquitinase activity	ISS GO_REF:0000024	ACCEPT	Summary: Sequence-similarity (ISS, from mouse Q9QZL6) assignment of the core DUB activity, which is independently proven by direct human experimental evidence. Reason: The cysteine-type deubiquitinase activity is the core molecular function and is directly demonstrated in human (IDA); the ISS transfer is concordant. Supporting Evidence: PMID:10799498 USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
GO:0004843 cysteine-type deubiquitinase activity	IMP PMID:26100909 BEND3 represses rDNA transcription by stabilizing a NoRC com...	ACCEPT	Summary: Mutational/functional (IMP) evidence supporting USP21 deubiquitinase activity in the context of deubiquitinating and stabilizing the NoRC component BAZ2A/TIP5. Reason: USP21 interacts with and deubiquitinates Tip5/BAZ2A, stabilizing it; this supports the core DUB activity acting on a defined nuclear substrate. Supporting Evidence: PMID:26100909 USP21 can interact with and deubiquitinate Tip5, thereby stabilizing the total levels of Tip5.
GO:0005515 protein binding	IPI PMID:26100909 BEND3 represses rDNA transcription by stabilizing a NoRC com...	KEEP AS NON CORE	Summary: IPI interactions (WITH BEND3/Q5T5X7 and BAZ2A/Q9UIF9) underlying the rDNA-silencing study. Unlike generic screen hits, these partners are functionally meaningful (BEND3 regulates USP21 stability; BAZ2A/TIP5 is a USP21 substrate), but the term itself is uninformative bare protein binding. Reason: Records biologically meaningful interactions (BEND3 interactor and BAZ2A substrate), but bare protein binding (GO:0005515) is uninformative and the relevant biology is better captured by the deubiquitination process annotation; not core. Supporting Evidence: file:human/USP21/USP21-goa.tsv PMID:26100909 UniProtKB:Q9UIF9
GO:0008234 cysteine-type peptidase activity	IMP PMID:10799498 Identification of a novel isopeptidase with dual specificity...	KEEP AS NON CORE	Summary: Mutational (IMP) evidence (Cys-221 required) for cysteine-type peptidase activity, a parent of the specific deubiquitinase activity USP21 performs. Reason: Correct but less precise than GO:0004843; the same Cys-221 mutagenesis supports the more specific deubiquitinase activity term, which should be the core MF. Supporting Evidence: file:human/USP21/USP21-uniprot.txt C->A,S: Abolishes ubiquitin thioesterase activity.
GO:0004843 cysteine-type deubiquitinase activity	IDA PMID:10799498 Identification of a novel isopeptidase with dual specificity...	ACCEPT	Summary: Original direct (IDA) demonstration that USP21 removes ubiquitin from ubiquitinated proteins in cells. Defines the core molecular function. Reason: First direct evidence of USP21's cysteine-type deubiquitinase activity; this is the gene's core molecular function. Supporting Evidence: PMID:10799498 USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
GO:0019784 deNEDDylase activity	IDA PMID:10799498 Identification of a novel isopeptidase with dual specificity...	ACCEPT	Summary: Direct (IDA) evidence that USP21 also removes NEDD8 from NEDD8 conjugates (but not SUMO/Sentrin-1), giving it dual ubiquitin/NEDD8 isopeptidase specificity. Reason: A genuine, experimentally demonstrated secondary catalytic activity; USP21 was the first USP shown to have dual ubiquitin/NEDD8 specificity. Supporting Evidence: PMID:10799498 USP21 is capable of removing NEDD8 from NEDD8 conjugates but has no effect on Sentrin-1 conjugates.
GO:0003713 transcription coactivator activity	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: Sequence-similarity (ISS, from mouse Q9QZL6) assignment of transcription coactivator activity, based on histone H2A deubiquitination relieving repression. Not a direct catalytic/binding activity and not demonstrated in human. Reason: Inherited by ortholog transfer; reflects an indirect (via H2A deubiquitination) coactivator role rather than USP21's core enzymatic function. USP21 can also repress transcription (rDNA via BAZ2A), so coactivator is context-specific. Supporting Evidence: file:human/USP21/USP21-uniprot.txt thereby acting as a coactivator
GO:0008234 cysteine-type peptidase activity	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: Sequence-similarity (ISS, from mouse Q9QZL6) assignment of the general cysteine-type peptidase activity, a parent of the specific deubiquitinase activity. Reason: Correct but less precise than GO:0004843 cysteine-type deubiquitinase activity; retained as a general non-core MF annotation. Supporting Evidence: file:human/USP21/USP21-uniprot.txt Belongs to the peptidase C19 family. USP21 subfamily.

Core Functions

Cysteine-type deubiquitinating enzyme that hydrolyzes isopeptide/peptide bonds at the C-terminal Gly of ubiquitin to remove ubiquitin from conjugated substrates, using a Cys-221 nucleophile within its USP domain.

Molecular Function:

cysteine-type deubiquitinase activity

Cellular Locations:

cytosol nucleoplasm

Supporting Evidence:

PMID:10799498
USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
PMID:32011234
we identify OTUD3 and USP21 as deubiquitylating enzymes that antagonize ZNF598-mediated 40S ubiquitylation and can limit RQC activation

Removes the ubiquitin-like modifier NEDD8 from NEDD8-conjugated proteins (deNEDDylase activity), a genuine secondary isopeptidase specificity distinct from SUMO; USP21 was the first USP shown to act on both ubiquitin and NEDD8.

Molecular Function:

deNEDDylase activity

Supporting Evidence:

PMID:10799498
USP21 is capable of removing NEDD8 from NEDD8 conjugates but has no effect on Sentrin-1 conjugates.

Through its deubiquitinase activity USP21 carries out protein deubiquitination of specific substrates, including 40S ribosomal proteins (eS10/RPS10, uS10/RPS20) to negatively regulate ribosome-associated quality control, and BAZ2A/TIP5 to stabilize the NoRC complex and promote rDNA silencing.

Molecular Function:

cysteine-type deubiquitinase activity

Directly Involved In:

protein deubiquitination

Cellular Locations:

cytosol nucleoplasm

Supporting Evidence:

PMID:32011234
cells lacking USP21 or OTUD3 have altered RQC activity and delayed eS10 deubiquitylation
PMID:26100909
USP21 can interact with and deubiquitinate Tip5, thereby stabilizing the total levels of Tip5.

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000024

Manual transfer of experimentally-verified manual GO annotation data to orthologs using Ensembl Compara

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping

GO_REF:0000052

Gene Ontology annotation based on curation of immunofluorescence data

GO_REF:0000108

Automatic assignment of GO terms using logical inference, based on inter-ontology links

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:10799498

Identification of a novel isopeptidase with dual specificity for ubiquitin- and NEDD8-conjugated proteins.

USP21 is a cysteine-protease isopeptidase that removes ubiquitin from ubiquitinated proteins and removes NEDD8 from NEDD8 conjugates, but has no effect on Sentrin-1/SUMO conjugates (dual ubiquitin/NEDD8 specificity).
Catalytic Cys-221 is required; its mutation abolishes ubiquitin thioesterase activity.

PMID:19615732

Defining the human deubiquitinating enzyme interaction landscape.

PMID:21888622

A global survey of CRM1-dependent nuclear export sequences in the human deubiquitinase family.

USP21 contains a functional CRM1-dependent nuclear export signal and localizes to both nucleus and cytoplasm.

PMID:25416956

A proteome-scale map of the human interactome network.

PMID:26100909

BEND3 represses rDNA transcription by stabilizing a NoRC component via USP21 deubiquitinase.

USP21 interacts with and deubiquitinates BAZ2A/TIP5 (a NoRC component), stabilizing it; SUMOylated BEND3 stabilizes USP21, and together they promote rDNA silencing.

PMID:32011234

Distinct regulatory ribosomal ubiquitylation events are reversible and hierarchically organized.

USP21 (with OTUD3) is a deubiquitylating enzyme that antagonizes ZNF598-mediated 40S ribosomal ubiquitylation (on eS10/RPS10 and uS10/RPS20) and limits ribosome-associated quality control (RQC) activation.
Cells lacking USP21 have altered RQC activity and delayed eS10 deubiquitylation, establishing a functional role for USP21 within the RQC pathway.

PMID:32296183

A reference map of the human binary protein interactome.

PMID:32814053

Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.

Reactome:R-HSA-5688426

Ub-specific processing proteases (protein deubiquitination)

Reactome:R-HSA-5690157

Ub-specific processing proteases (USP21 deubiquitinase activity)

Reactome:R-HSA-5690159

Ub-specific processing proteases (USP21 deubiquitinase activity)

Reactome:R-HSA-6783177

Ub-specific processing proteases (USP21 deubiquitinase activity)

file:human/USP21/USP21-deep-research-falcon.md

Falcon deep research report for USP21

USP21 is a broad-specificity DUB that processes multiple polyubiquitin chain types (K6, K11, K29, K48, K63, and linear) and cross-reacts with ISG15, with substrate-stabilizing deubiquitination reported for RIG-I (K63), MEK2 (K48), Nanog (K48), histone H2A (H2AK119ub), MARK3, TCF7, GATA3 and RIPK1.

Suggested Experiments

Experiment: Catalytically-dead (C221A) versus wild-type USP21 rescue in USP21-knockout cells combined with diGly-ubiquitin proteomics to define the direct, catalysis-dependent deubiquitinated substrate repertoire genome-wide.

Experiment: Quantitative kinetic assays comparing USP21 activity on ubiquitin- versus NEDD8-isopeptide substrates (and on different polyubiquitin linkage types) to characterize its dual specificity and linkage preference.

Experiment: Time-resolved measurement of eS10/uS10 (RPS10/RPS20) ubiquitylation and ribosome stalling / RQC reporter readouts upon USP21 depletion or overexpression to quantify its role as a negative regulator of RQC.

Deep Research

Falcon

(USP21-deep-research-falcon.md)

Comprehensive Research Report: USP21 (Ubiquitin Carboxyl-Terminal Hydrolase 21) Falcon Edison Scientific Literature 14 citations 2 artifacts 2026-06-20T06:54:07.502484

Edison artifact artifact-00 (text/markdown)

Edison artifact artifact-01 (text/markdown)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Comprehensive Research Report: USP21 (Ubiquitin Carboxyl-Terminal Hydrolase 21)

Gene and Protein Identity

USP21 (Gene Symbol: USP21; UniProt: Q9UK80) encodes ubiquitin carboxyl-terminal hydrolase 21, also known as deubiquitinating enzyme 21, ubiquitin thioesterase 21, or ubiquitin-specific-processing protease 21 (liu2016usp21deubiquitylatesnanog pages 1-2, fan2014usp21negativelyregulates pages 1-2, li2018thedeubiquitinaseusp21 pages 1-2). This enzyme belongs to the peptidase C19 family, USP21 subfamily, and functions as a cysteine protease with the EC number 3.4.19.12 (liu2016usp21deubiquitylatesnanog pages 1-2, fan2014usp21negativelyregulates pages 1-2). USP21 is a member of the ubiquitin-specific protease (USP) family, the largest family of deubiquitinating enzymes (DUBs) in mammals, comprising approximately 56-70 members (li2018thedeubiquitinaseusp21 pages 1-2).

Primary Catalytic Function and Substrate Specificity

Enzymatic Activity

USP21 is a highly active deubiquitinating enzyme that catalyzes the removal of ubiquitin from ubiquitin-conjugated protein substrates (liu2016usp21deubiquitylatesnanog pages 1-2, fan2014usp21negativelyregulates pages 1-2, ye2011polyubiquitinbindingand pages 1-2). The enzyme exhibits promiscuous activity toward multiple ubiquitin chain types, including K6-, K11-, K29-, K48-, K63-linked, and linear ubiquitin conjugates (ye2011polyubiquitinbindingand pages 1-2, pannu2015ubiquitinspecificprotease pages 1-2). Biochemical analyses demonstrate that USP21 efficiently processes both K48-linked polyubiquitin chains (which typically target proteins for proteasomal degradation) and K63-linked chains (which generally serve regulatory signaling functions) (liu2016usp21deubiquitylatesnanog pages 1-2, fan2014usp21negativelyregulates pages 1-2, ye2011polyubiquitinbindingand pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6, li2018thedeubiquitinaseusp21 pages 4-5).

Importantly, USP21 shows cross-reactivity with the ubiquitin-like modifier ISG15, though with reduced activity compared to ubiquitin, but it is inactive against NEDD8 conjugates (ye2011polyubiquitinbindingand pages 1-2, ye2011polyubiquitinbindingand pages 6-7). Structural studies reveal that USP21 interacts with ubiquitin through a conserved USP domain core and possesses a secondary ubiquitin-binding surface (S2 site) that facilitates polyubiquitin chain recognition (ye2011polyubiquitinbindingand pages 1-2, ye2011polyubiquitinbindingand pages 6-7). The enzyme's specificity for ubiquitin versus NEDD8 is mediated by critical interactions between ubiquitin Arg72 and USP21 Glu304; mutation of Glu304 significantly reduces USP21 activity (ye2011polyubiquitinbindingand pages 6-7).

Validated Protein Substrates

USP21 deubiquitinates a diverse array of protein substrates across multiple cellular compartments and pathways:

Substrate Name	Ubiquitin Chain Type Removed (K48, K63, etc.)	Functional Consequence of Deubiquitination	Cellular Process/Pathway Affected	Key References
Nanog	Primarily K48-linked polyubiquitin; little effect on K63-linked or monoubiquitination	Stabilizes Nanog protein by preventing proteasomal degradation	Embryonic stem cell pluripotency and self-renewal	(liu2016usp21deubiquitylatesnanog pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6)
MEK2	K48-linked polyubiquitin, not K63-linked	Stabilizes MEK2 and increases ERK pathway signaling	ERK/MAPK signaling, hepatocellular carcinoma proliferation and tumor growth	(li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5)
RIG-I	K63-linked polyubiquitin on activated RIG-I/RIG-I-CARD	Dampens RIG-I activation, lowers IRF3 phosphorylation and type I IFN production	Antiviral innate immunity / RIG-I–MAVS–IRF3 signaling	(fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 4-5, fan2014usp21negativelyregulates pages 6-7)
GATA3	Reported substrate; chain type not specified in retrieved context	Deubiquitinates and stabilizes GATA3 in vitro/cell-based studies	Transcriptional regulation, T-cell biology, hematopoietic regulation	(pannu2015ubiquitinspecificprotease pages 1-2)
RIPK1	Reported substrate; linked to removal of activating ubiquitin chains, exact linkage not specified in retrieved context	Negative regulation of RIPK1 activity and suppression of downstream signaling	TNFR1/NF-κB inflammatory signaling	(pannu2015ubiquitinspecificprotease pages 1-2)
Histone H2A (H2AK119ub)	Monoubiquitin on H2A K119 in chromatin/nucleosomes	Removes repressive histone ubiquitination mark, promoting transcriptional activation	Chromatin regulation / epigenetic control of transcription	(ye2011polyubiquitinbindingand pages 1-2, pannu2015ubiquitinspecificprotease pages 1-2)
MARK3	Ubiquitin linkage not specified in retrieved context	Stabilizes/maintains MARK3 function, promoting macropinocytosis and restricting YAP/TAZ activity depending on context	Macropinocytosis, cytoskeleton regulation, Hippo pathway	(hou2021usp21deubiquitinaseelevates pages 1-2, park2023synergisticeffectof pages 1-2)
YOD1	Mutual deubiquitination reported; linkage not specified in retrieved context	USP21 deubiquitinates YOD1, but does not regulate YOD1 stability; reciprocal regulation mainly affects USP21 stability	Hippo pathway-associated DUB network	(park2023synergisticeffectof pages 1-2)
IL-33	Ubiquitin linkage not specified in retrieved context	Reported USP21 substrate/interactor; deubiquitination implicated in nuclear IL-33 regulation	Nuclear cytokine/transcriptional regulation, inflammation	(pannu2015ubiquitinspecificprotease pages 1-2)
TCF7	Ubiquitin linkage not specified in retrieved context	Stabilizes TCF7 in the nucleus	Wnt/TCF transcriptional program, pancreatic cancer stemness	(hou2021usp21deubiquitinaseelevates pages 1-2)
ISG15-conjugated substrates (cross-reactivity, not a single canonical substrate protein)	ISG15 conjugates	USP21 can cleave ISG15 with lower activity than ubiquitin; demonstrates modifier cross-reactivity	Ubiquitin-like modifier processing / enzyme specificity	(ye2011polyubiquitinbindingand pages 1-2)

Table: This table compiles the major reported USP21 substrates and substrate classes, the ubiquitin linkages removed where known, and the functional/pathway consequences of deubiquitination. It is useful for quickly distinguishing well-supported direct activities from cases where the substrate is reported but linkage specificity remains incompletely defined in the retrieved evidence.

Key substrates include:

Nanog: USP21 specifically removes K48-linked polyubiquitin from the pluripotency transcription factor Nanog, stabilizing the protein and maintaining embryonic stem cell self-renewal (liu2016usp21deubiquitylatesnanog pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6). USP21 interacts with Nanog through its C-terminal USP domain binding to Nanog's C-domain, and only wild-type USP21 (not the catalytically inactive C221A mutant) effectively deubiquitinates Nanog (liu2016usp21deubiquitylatesnanog pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6).

MEK2: USP21 directly binds and deubiquitinates MEK2 (mitogen-activated protein kinase kinase 2), removing K48-linked (but not K63-linked) polyubiquitin chains (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5). This stabilization of MEK2 activates downstream ERK signaling and promotes cell proliferation and tumor growth, particularly in hepatocellular carcinoma (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5).

RIG-I: USP21 binds to and deubiquitinates the pattern recognition receptor RIG-I (retinoic acid-inducible gene I), removing K63-linked polyubiquitin from the activated RIG-I CARD domains (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 4-5, fan2014usp21negativelyregulates pages 6-7). This deubiquitination dampens RIG-I activation, reduces IRF3 phosphorylation, and suppresses type I interferon production in response to RNA virus infection (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 4-5, fan2014usp21negativelyregulates pages 6-7).

Histone H2A: USP21 removes monoubiquitin from histone H2A at lysine 119 (H2AK119ub) in nucleosomal contexts, with strong specificity for nucleosomal rather than free histone substrates (pannu2015ubiquitinspecificprotease pages 1-2). This activity promotes transcriptional activation and has been implicated in liver regeneration (pannu2015ubiquitinspecificprotease pages 1-2).

MARK3: In pancreatic cancer, cytoplasmic USP21 regulates microtubule affinity-regulating kinase 3 (MARK3), promoting MARK3-dependent macropinocytosis and nutrient scavenging that supports KRAS-independent tumor growth (hou2021usp21deubiquitinaseelevates pages 1-2).

Other substrates: Additional validated substrates include GATA3 (transcription factor), RIPK1 (death-domain kinase), IL-33 (nuclear cytokine), TCF7 (Wnt pathway transcription factor), and YOD1 (another DUB with which USP21 exhibits mutual deubiquitination) (hou2021usp21deubiquitinaseelevates pages 1-2, park2023synergisticeffectof pages 1-2, pannu2015ubiquitinspecificprotease pages 1-2).

Subcellular Localization

USP21 exhibits a unique dual localization pattern that reflects functional compartmentalization:

Cytoplasmic Localization

USP21 is the only deubiquitinase in a systematic survey of 66 mammalian DUBs to display clear association with both microtubules and centrosomes (urbe2012systematicsurveyof pages 1-2, urbe2012systematicsurveyof pages 2-3). In vitro binding assays demonstrate that USP21 directly binds microtubules through a novel microtubule-binding motif encompassed within amino acids 59-75 of its N-terminus (urbe2012systematicsurveyof pages 1-2, urbe2012systematicsurveyof pages 2-3). At low expression levels, USP21 predominantly accumulates at the centrosome, while at higher levels it extends along the microtubular network (urbe2012systematicsurveyof pages 1-2, urbe2012systematicsurveyof pages 2-3).

Functional studies indicate USP21 plays key roles in microtubule-associated processes: USP21 depletion compromises the reestablishment of a radial microtubule array during recovery from cold-induced depolymerization, reduces primary cilium formation, and inhibits nerve growth factor-induced neurite outgrowth in PC12 cells (urbe2012systematicsurveyof pages 1-2, urbe2012systematicsurveyof pages 2-3).

Nuclear Localization

USP21 exists in at least two splice isoforms with distinct localizations (pannu2015ubiquitinspecificprotease pages 1-2). The short isoform localizes primarily to the nucleus and nuclear membrane, where it functions as a deubiquitinase for histone H2A (pannu2015ubiquitinspecificprotease pages 1-2). In contrast, the long isoform localizes to the cytosol and associates with microtubules (pannu2015ubiquitinspecificprotease pages 1-2). USP21 contains a CRM1-dependent nuclear export sequence (NES) at amino acids 134-147; deletion of the N-terminal 184 amino acids (including the NES) causes nuclear accumulation with enrichment in nucleoli (urbe2012systematicsurveyof pages 1-2, urbe2012systematicsurveyof pages 2-3).

Signaling and Biochemical Pathways

USP21 participates in multiple major signaling pathways as both a positive and negative regulator:

Pathway Name	USP21's Role in Pathway (activator/inhibitor)	Key Substrates in Pathway	Molecular Mechanism	Biological Outcome
NF-κB / TNFR1 inflammatory signaling	Inhibitor	RIPK1	USP21 deubiquitinates RIPK1, suppressing its signaling activity downstream of TNFR1 and thereby reducing NF-κB activation and inflammatory cytokine production (pannu2015ubiquitinspecificprotease pages 1-2)	Negative regulation of inflammation and TNFα-driven signaling; likely context-dependent restraint of innate immune activation (pannu2015ubiquitinspecificprotease pages 1-2)
RIG-I / type I interferon antiviral pathway	Inhibitor	RIG-I	USP21 binds RIG-I and removes activating K63-linked polyubiquitin from RIG-I/RIG-I-CARD, reducing IRF3 phosphorylation and IFN signaling (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 4-5, fan2014usp21negativelyregulates pages 6-7)	Decreased IFN-α/β production and dampened antiviral innate immune responses to RNA viruses (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 6-7)
ERK / MAPK signaling	Activator	MEK2	USP21 directly deubiquitinates MEK2 by removing K48-linked polyubiquitin, stabilizing MEK2 and enhancing ERK pathway output (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5)	Increased cell proliferation, cell-cycle progression, and tumor growth, especially in hepatocellular carcinoma models (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5)
Hippo signaling	Generally inhibitor of Hippo growth-suppressive output / activator of proliferative signaling	MARK; USP21 also functionally cooperates with YOD1	Recent work places USP21 in Hippo regulation through deubiquitination of MARK and interaction with YOD1; this modulates YAP/p-YAP signaling and cell proliferation (park2023synergisticeffectof pages 1-2)	Enhanced proliferation and migration in cultured cancer cells through altered Hippo pathway signaling (park2023synergisticeffectof pages 1-2)
mTORC1 signaling	Activator	MARK3-associated nutrient-scavenging axis; downstream mTORC1 effectors	In KRAS-independent pancreatic cancer growth, cytoplasmic USP21 promotes MARK3-dependent macropinocytosis, sustaining intracellular amino acid supply and enabling mTORC1 activation (hou2021usp21deubiquitinaseelevates pages 1-2)	Supports nutrient scavenging, anabolic growth, and bypass of KRAS dependency in pancreatic ductal adenocarcinoma (hou2021usp21deubiquitinaseelevates pages 1-2)
Wnt / TCF transcriptional pathway	Activator	TCF7	USP21 stabilizes TCF7 in the nucleus by deubiquitination, supporting a transcriptional program linked to cancer stemness (hou2021usp21deubiquitinaseelevates pages 1-2)	Promotes pancreatic cancer stemness and contributes to tumor recurrence/resistance programs (hou2021usp21deubiquitinaseelevates pages 1-2)
Stem cell pluripotency pathway	Activator	Nanog	USP21 interacts with Nanog and specifically removes K48-linked polyubiquitin, stabilizing Nanog; USP21 depletion reduces Nanog and drives differentiation (liu2016usp21deubiquitylatesnanog pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6)	Maintenance of embryonic stem cell self-renewal and pluripotency; loss of USP21 promotes ESC differentiation (liu2016usp21deubiquitylatesnanog pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6)

Table: This table summarizes the main signaling pathways currently linked to USP21, indicating whether USP21 acts as a positive or negative regulator, the relevant substrates, and the biological consequences. It is useful for quickly connecting USP21's deubiquitinase activity to specific cellular pathways and phenotypes.

Antiviral Immunity and Interferon Signaling

USP21 functions as a negative regulator of RIG-I-mediated antiviral responses (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 4-5, fan2014usp21negativelyregulates pages 6-7). By deubiquitinating RIG-I, USP21 inhibits downstream MAVS (mitochondrial antiviral-signaling protein) activation, reduces TBK1 and IRF3 phosphorylation, and suppresses type I interferon (IFN-α/β) production (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 6-7). USP21-deficient mice exhibit elevated RIG-I polyubiquitination, enhanced IRF3 phosphorylation, increased IFN production, and greater resistance to vesicular stomatitis virus (VSV) infection compared to wild-type littermates (fan2014usp21negativelyregulates pages 6-7). Importantly, USP21's deubiquitinase activity is required for this inhibitory effect on antiviral signaling (fan2014usp21negativelyregulates pages 1-2, fan2014usp21negativelyregulates pages 6-7).

NF-κB and Inflammatory Signaling

USP21 negatively regulates NF-κB pathway activation downstream of TNF receptor 1 (TNFR1) by deubiquitinating RIPK1 (receptor-interacting protein kinase 1) (pannu2015ubiquitinspecificprotease pages 1-2). This regulation suppresses inflammatory cytokine production, including IL-6 and TNF-α (pannu2015ubiquitinspecificprotease pages 1-2). However, notably, USP21-knockout mice are viable and fertile with no significant defects in responses of antigen-presenting cells to TLR and TNFR stimulation under standard conditions, suggesting functional redundancy or context-dependent regulation (pannu2015ubiquitinspecificprotease pages 1-2).

ERK/MAPK Signaling

In hepatocellular carcinoma and other cancer types, USP21 acts as an activator of ERK/MAPK signaling by stabilizing MEK2 (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5). USP21 overexpression increases MEK2 protein levels, enhances phosphorylation of downstream targets (phospho-S6 kinase, phospho-S6, phospho-ERK1/2), and promotes cell proliferation, anchorage-independent growth, and tumor formation (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5). Conversely, USP21 depletion decreases cell proliferation and in vivo tumor growth (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5). Gene set enrichment analysis reveals that high USP21 expression correlates with activation of cell cycle progression pathways (li2018thedeubiquitinaseusp21 pages 1-2).

Hippo Signaling Pathway

USP21 regulates the Hippo pathway through multiple mechanisms (hou2021usp21deubiquitinaseelevates pages 1-2, park2023synergisticeffectof pages 1-2). USP21 interacts with and deubiquitinates YOD1 (another DUB), and both enzymes synergistically influence cell proliferation by modulating Hippo signaling and YAP/p-YAP levels (park2023synergisticeffectof pages 1-2). Additionally, USP21 regulates MARK (microtubule affinity-regulating kinase) proteins, which in turn restrict YAP/TAZ activity (hou2021usp21deubiquitinaseelevates pages 1-2, park2023synergisticeffectof pages 1-2).

mTOR Signaling and Macropinocytosis

Recent work demonstrates that USP21 enables KRAS-independent pancreatic cancer growth by promoting mTORC1 pathway activation (hou2021usp21deubiquitinaseelevates pages 1-2). Cytoplasmic USP21 regulates MARK3-dependent macropinocytosis, elevating nutrient scavenging (particularly extracellular protein uptake) to maintain intracellular amino acid levels and support mTORC1-driven anabolic growth and protein synthesis (hou2021usp21deubiquitinaseelevates pages 1-2). This function is independent of USP21's nuclear role in stabilizing TCF7 for cancer stemness (hou2021usp21deubiquitinaseelevates pages 1-2).

Stem Cell Pluripotency

USP21 is a critical regulator of embryonic stem cell (ESC) pluripotency through its stabilization of the master transcription factor Nanog (liu2016usp21deubiquitylatesnanog pages 1-2, liu2016usp21deubiquitylatesnanog pages 5-6). USP21 is highly expressed in undifferentiated mouse ESCs and its expression decreases upon differentiation induced by LIF withdrawal or retinoic acid treatment (liu2016usp21deubiquitylatesnanog pages 5-6). Knockdown of USP21 in ESCs leads to Nanog degradation, loss of alkaline phosphatase activity, morphological changes consistent with differentiation, decreased expression of pluripotency markers (Oct4, Sox2, Nanog), and increased expression of differentiation markers (liu2016usp21deubiquitylatesnanog pages 5-6).

Biological Processes Regulated by USP21

Based on experimental evidence, USP21 regulates diverse biological processes:

Cell Proliferation and Cell Cycle: USP21 promotes cell proliferation across multiple cancer types by stabilizing oncogenic signaling molecules like MEK2 and activating growth-promoting pathways (li2018thedeubiquitinaseusp21 pages 1-2, li2018thedeubiquitinaseusp21 pages 4-5). USP21 overexpression significantly increases G0/G1 to S phase progression and enhances colony formation in cancer cell lines (li2018thedeubiquitinaseusp21 pages 1-2).

Tumor Growth and Metastasis: USP21 is frequently amplified and overexpressed in hepatocellular carcinoma, pancreatic ductal adenocarcinoma, cholangiocarcinoma, cervical cancer, and other malignancies, where it promotes tumorigenesis, invasion, and therapeutic resistance (li2018thedeubiquitinaseusp21 pages 1-2, hou2021usp21deubiquitinaseelevates pages 1-2). High USP21 expression correlates with poor clinical outcomes in multiple cancer datasets (li2018thedeubiquitinaseusp21 pages 1-2).

Microtubule and Centrosome Function: USP21 regulates microtubule network regeneration, centrosome function, primary cilia formation, and neurite outgrowth, positioning it as a key regulator of cytoskeletal dynamics (urbe2012systematicsurveyof pages 1-2, urbe2012systematicsurveyof pages 2-3).

Epigenetic Regulation: Through deubiquitination of histone H2A at K119, USP21 modulates chromatin structure and transcriptional activation, particularly during liver regeneration (pannu2015ubiquitinspecificprotease pages 1-2).

Clinical Significance: Recent studies from 2023-2026 highlight USP21's emerging importance in cancer biology, with roles identified in biliary tract cancer (cholangiocarcinoma and gallbladder cancer), where USP21 regulates aerobic glycolysis and proliferation (li2018thedeubiquitinaseusp21 pages 1-2), and in cervical cancer, where USP21 expression is positively associated with radioresistance (liu2016usp21deubiquitylatesnanog pages 1-2).

Structural and Mechanistic Insights

The crystal structure of USP21 in complex with linear diubiquitin aldehyde reveals the molecular basis of polyubiquitin recognition (ye2011polyubiquitinbindingand pages 1-2, ye2011polyubiquitinbindingand pages 6-7). USP21 binds ubiquitin at two distinct sites: the canonical active site (S1) and a secondary binding surface (S2) on the USP domain core (ye2011polyubiquitinbindingand pages 1-2). The ubiquitin C-terminal LRLRGG sequence interacts extensively with the enzyme, with ubiquitin Arg72 forming critical contacts with the conserved USP21 Glu304 residue (ye2011polyubiquitinbindingand pages 6-7). This interaction is essential for processing ubiquitin (and ISG15) modifications and distinguishes USP21's ability to cleave ubiquitin/ISG15 from its inability to process NEDD8, which contains Ala72 instead of Arg72 (ye2011polyubiquitinbindingand pages 6-7).

Knockout Mouse Phenotype

USP21-knockout mice are viable and fertile with no significant developmental abnormalities, in contrast to knockout models of some USP21 substrates like GATA3 and RIPK1, which exhibit embryonic or perinatal lethality (pannu2015ubiquitinspecificprotease pages 1-2, fan2014usp21negativelyregulates pages 6-7). However, USP21-deficient mice display spontaneous splenomegaly with increased macrophages and neutrophils, and show enhanced resistance to VSV infection with elevated interferon production (fan2014usp21negativelyregulates pages 6-7). Aged USP21-knockout mice exhibit spontaneous T cell activation, though this is not linked to altered GATA3 levels (pannu2015ubiquitinspecificprotease pages 1-2). These findings suggest that while USP21 is dispensable for normal development, it plays important regulatory roles in immune homeostasis and antiviral responses.

Summary

USP21 is a multifunctional deubiquitinating enzyme with broad substrate specificity that operates across nuclear and cytoplasmic compartments to regulate diverse cellular processes. Its primary catalytic function is to remove K48-linked and K63-linked ubiquitin chains from specific protein substrates, thereby modulating protein stability, activity, and localization. Through deubiquitination of key substrates including Nanog, MEK2, RIG-I, histone H2A, and MARK3, USP21 impacts stem cell pluripotency, cell proliferation, innate immunity, epigenetic regulation, and metabolic adaptation. The enzyme's dual localization to microtubules/centrosomes and the nucleus reflects its functional diversity. Recent studies (2023-2026) increasingly implicate USP21 in cancer biology, highlighting its potential as a therapeutic target in hepatocellular carcinoma, pancreatic cancer, cholangiocarcinoma, and other malignancies where it promotes tumor growth and therapeutic resistance.

References

(liu2016usp21deubiquitylatesnanog pages 1-2): Xingyu Liu, Yuying Yao, Huiguo Ding, Chuanchun Han, Yuhan Chen, Yuan Zhang, Chanjuan Wang, Xin Zhang, Yiling Zhang, Yun Zhai, Ping Wang, Wenyi Wei, Jing Zhang, and Lingqiang Zhang. Usp21 deubiquitylates nanog to regulate protein stability and stem cell pluripotency. Signal Transduction and Targeted Therapy, Nov 2016. URL: https://doi.org/10.1038/sigtrans.2016.24, doi:10.1038/sigtrans.2016.24. This article has 58 citations and is from a peer-reviewed journal.
(fan2014usp21negativelyregulates pages 1-2): Yihui Fan, Renfang Mao, Yang Yu, Shangfeng Liu, Zhongcheng Shi, Jin Cheng, Huiyuan Zhang, Lei An, Yanling Zhao, Xin Xu, Zhenghu Chen, Mari Kogiso, Dekai Zhang, Hong Zhang, Pumin Zhang, Jae U. Jung, Xiaonan Li, Guotong Xu, and Jianhua Yang. Usp21 negatively regulates antiviral response by acting as a rig-i deubiquitinase. The Journal of Experimental Medicine, 211:313-328, Feb 2014. URL: https://doi.org/10.1084/jem.20122844, doi:10.1084/jem.20122844. This article has 193 citations.
(li2018thedeubiquitinaseusp21 pages 1-2): Wenjuan Li, Kaisa Cui, Edward V Prochownik, and Youjun Li. The deubiquitinase usp21 stabilizes mek2 to promote tumor growth. Cell Death & Disease, Apr 2018. URL: https://doi.org/10.1038/s41419-018-0523-z, doi:10.1038/s41419-018-0523-z. This article has 81 citations and is from a peer-reviewed journal.
(ye2011polyubiquitinbindingand pages 1-2): Yu Ye, Masato Akutsu, Francisca Reyes‐Turcu, Radoslav I Enchev, Keith D Wilkinson, and David Komander. Polyubiquitin binding and cross‐reactivity in the usp domain deubiquitinase usp21. EMBO reports, Apr 2011. URL: https://doi.org/10.1038/embor.2011.17, doi:10.1038/embor.2011.17. This article has 233 citations and is from a highest quality peer-reviewed journal.
(pannu2015ubiquitinspecificprotease pages 1-2): Jaspreet Pannu, Jad I. Belle, Michael Förster, Claudia U. Duerr, Shiyang Shen, Leanne Kane, Katherine Harcourt, Jörg H. Fritz, Simon Clare, and Anastasia Nijnik. Ubiquitin specific protease 21 is dispensable for normal development, hematopoiesis and lymphocyte differentiation. PLoS ONE, 10:e0117304, Feb 2015. URL: https://doi.org/10.1371/journal.pone.0117304, doi:10.1371/journal.pone.0117304. This article has 41 citations and is from a peer-reviewed journal.
(liu2016usp21deubiquitylatesnanog pages 5-6): Xingyu Liu, Yuying Yao, Huiguo Ding, Chuanchun Han, Yuhan Chen, Yuan Zhang, Chanjuan Wang, Xin Zhang, Yiling Zhang, Yun Zhai, Ping Wang, Wenyi Wei, Jing Zhang, and Lingqiang Zhang. Usp21 deubiquitylates nanog to regulate protein stability and stem cell pluripotency. Signal Transduction and Targeted Therapy, Nov 2016. URL: https://doi.org/10.1038/sigtrans.2016.24, doi:10.1038/sigtrans.2016.24. This article has 58 citations and is from a peer-reviewed journal.
(li2018thedeubiquitinaseusp21 pages 4-5): Wenjuan Li, Kaisa Cui, Edward V Prochownik, and Youjun Li. The deubiquitinase usp21 stabilizes mek2 to promote tumor growth. Cell Death & Disease, Apr 2018. URL: https://doi.org/10.1038/s41419-018-0523-z, doi:10.1038/s41419-018-0523-z. This article has 81 citations and is from a peer-reviewed journal.
(ye2011polyubiquitinbindingand pages 6-7): Yu Ye, Masato Akutsu, Francisca Reyes‐Turcu, Radoslav I Enchev, Keith D Wilkinson, and David Komander. Polyubiquitin binding and cross‐reactivity in the usp domain deubiquitinase usp21. EMBO reports, Apr 2011. URL: https://doi.org/10.1038/embor.2011.17, doi:10.1038/embor.2011.17. This article has 233 citations and is from a highest quality peer-reviewed journal.
(fan2014usp21negativelyregulates pages 4-5): Yihui Fan, Renfang Mao, Yang Yu, Shangfeng Liu, Zhongcheng Shi, Jin Cheng, Huiyuan Zhang, Lei An, Yanling Zhao, Xin Xu, Zhenghu Chen, Mari Kogiso, Dekai Zhang, Hong Zhang, Pumin Zhang, Jae U. Jung, Xiaonan Li, Guotong Xu, and Jianhua Yang. Usp21 negatively regulates antiviral response by acting as a rig-i deubiquitinase. The Journal of Experimental Medicine, 211:313-328, Feb 2014. URL: https://doi.org/10.1084/jem.20122844, doi:10.1084/jem.20122844. This article has 193 citations.
(fan2014usp21negativelyregulates pages 6-7): Yihui Fan, Renfang Mao, Yang Yu, Shangfeng Liu, Zhongcheng Shi, Jin Cheng, Huiyuan Zhang, Lei An, Yanling Zhao, Xin Xu, Zhenghu Chen, Mari Kogiso, Dekai Zhang, Hong Zhang, Pumin Zhang, Jae U. Jung, Xiaonan Li, Guotong Xu, and Jianhua Yang. Usp21 negatively regulates antiviral response by acting as a rig-i deubiquitinase. The Journal of Experimental Medicine, 211:313-328, Feb 2014. URL: https://doi.org/10.1084/jem.20122844, doi:10.1084/jem.20122844. This article has 193 citations.
(hou2021usp21deubiquitinaseelevates pages 1-2): Pingping Hou, Xingdi Ma, Zecheng Yang, Qiang Zhang, Chang-Jiun Wu, Jun Li, Lin Tan, Wantong Yao, Liang Yan, Xin Zhou, Alec C. Kimmelman, Philip L. Lorenzi, Jianhua Zhang, Shan Jiang, Denise Spring, Y. Alan Wang, and Ronald A. DePinho. Usp21 deubiquitinase elevates macropinocytosis to enable oncogenic kras bypass in pancreatic cancer. Genes & Development, 35:1327-1332, Sep 2021. URL: https://doi.org/10.1101/gad.348787.121, doi:10.1101/gad.348787.121. This article has 37 citations and is from a highest quality peer-reviewed journal.
(park2023synergisticeffectof pages 1-2): Sang-Soo Park and Kwang-Hyun Baek. Synergistic effect of yod1 and usp21 on the hippo signaling pathway. Cancer Cell International, Sep 2023. URL: https://doi.org/10.1186/s12935-023-03078-3, doi:10.1186/s12935-023-03078-3. This article has 10 citations and is from a peer-reviewed journal.
(urbe2012systematicsurveyof pages 1-2): Sylvie Urbé, Han Liu, Sebastian D. Hayes, Claire Heride, Daniel J. Rigden, and Michael J. Clague. Systematic survey of deubiquitinase localization identifies usp21 as a regulator of centrosome- and microtubule-associated functions. Molecular Biology of the Cell, 23:1095-1103, Mar 2012. URL: https://doi.org/10.1091/mbc.e11-08-0668, doi:10.1091/mbc.e11-08-0668. This article has 156 citations and is from a domain leading peer-reviewed journal.
(urbe2012systematicsurveyof pages 2-3): Sylvie Urbé, Han Liu, Sebastian D. Hayes, Claire Heride, Daniel J. Rigden, and Michael J. Clague. Systematic survey of deubiquitinase localization identifies usp21 as a regulator of centrosome- and microtubule-associated functions. Molecular Biology of the Cell, 23:1095-1103, Mar 2012. URL: https://doi.org/10.1091/mbc.e11-08-0668, doi:10.1091/mbc.e11-08-0668. This article has 156 citations and is from a domain leading peer-reviewed journal.

Artifacts

Citations

ye2011polyubiquitinbindingand pages 6-7
pannu2015ubiquitinspecificprotease pages 1-2
park2023synergisticeffectof pages 1-2
ye2011polyubiquitinbindingand pages 1-2
urbe2012systematicsurveyof pages 1-2
urbe2012systematicsurveyof pages 2-3
https://doi.org/10.1038/sigtrans.2016.24,
https://doi.org/10.1084/jem.20122844,
https://doi.org/10.1038/s41419-018-0523-z,
https://doi.org/10.1038/embor.2011.17,
https://doi.org/10.1371/journal.pone.0117304,
https://doi.org/10.1101/gad.348787.121,
https://doi.org/10.1186/s12935-023-03078-3,
https://doi.org/10.1091/mbc.e11-08-0668,

📚 Additional Documentation

Notes

(USP21-notes.md)

USP21 (Q9UK80, UBP21_HUMAN) review notes

NOTE: This is the CORRECT USP21 (UniProt Q9UK80). An earlier batch-6 fetch under the symbol
"USP21" mis-resolved (UniProt synonym collision) to USP25 and was relabeled; this review is
freshly fetched for Q9UK80.

Identity / overview

Ubiquitin carboxyl-terminal hydrolase 21 (USP21), a deubiquitinating enzyme (DUB) of the
peptidase C19 / USP family. EC 3.4.19.12. 565 aa. Gene on chr 1q21. HGNC:12620.
[file:human/USP21/USP21-uniprot.txt "Belongs to the peptidase C19 family. USP21 subfamily."]
Catalytic USP domain spans residues 212-558; catalytic nucleophile Cys-221 (mutating to A/S
abolishes activity); proton-acceptor His-518; a structural Zn(2+) site (C384/C387/C437/C440).
[file:human/USP21/USP21-uniprot.txt "MUTAGEN 221 ... C->A,S: Abolishes ubiquitin thioesterase activity."]
Dual subcellular localization: cytoplasm and nucleus; has a CRM1-dependent nuclear export
signal (motif 134-152). [PMID:21888622 — "A global survey of CRM1-dependent nuclear export
sequences in the human deubiquitinase family"]

Core molecular function: cysteine-type deubiquitinase (GO:0004843)

Originally identified as an isopeptidase with DUAL specificity, removing ubiquitin from
ubiquitinated proteins AND NEDD8 from NEDD8 conjugates (but NOT SUMO/Sentrin-1). Cys-221
required. PMID:10799498 -> supports both GO:0004843 (deubiquitinase) and GO:0019784
(deNEDDylase activity), both IDA.
Catalytic activity re-confirmed with active-site mapping and Cys-221 mutagenesis.
[PMID:32011234 — active site, mutagenesis of Cys-221]
USP21 USP domain crystallized in complex with ubiquitin (PDB 3I3T/2Y5B/3MTN); structural Zn
site. PMID:21399617

Substrate / process contexts (BP)

Ribosome-associated quality control (RQC), negative regulator. USP21 (with OTUD3)
deubiquitinates 40S ribosomal proteins eS10/RPS10 and uS10/RPS20, antagonizing
ZNF598-mediated 40S ubiquitylation and limiting RQC activation.
PMID:32011234
PMID:32011234
This is the IDA-supported BP behind GO:0016579 protein deubiquitination (PMID:32011234).
rDNA silencing / NoRC stabilization. USP21 deubiquitinates and stabilizes BAZ2A/TIP5
(a NoRC component), acting with BEND3 to repress rRNA gene transcription.
PMID:26100909 Supports GO:0016579 (IMP/IPI; substrate BAZ2A/TIP5).
Histone H2A deubiquitination / transcription coactivation. UniProt (By similarity, from
mouse Q9QZL6) describes deubiquitination of histone H2A relieving repression, acting as a
transcriptional coactivator and regulating transcription initiation. This underlies the
ISS terms GO:0003713 (transcription coactivator activity), GO:0045815 (transcription
initiation-coupled chromatin remodeling) and the GOC IEA GO:0045893 (positive regulation of
DNA-templated transcription). These are ortholog-transferred (mouse) / inferred, not direct
human experimental evidence.
[file:human/USP21/USP21-uniprot.txt "Deubiquitinates histone H2A ... thereby acting as a
coactivator (By similarity)."]
Reported but not in this GOA set: deubiquitination of RIPK1/RIP1 (TNF/NF-kB signaling),
RIG-I, GATA3, MARK3, ACLY, microtubule/centrosome regulation. Mentioned as prior work in
PMID:26100909 ("regulate the deubiquitination of several other substrates including RIG-1,
RIP1, and GATA-3"). These broaden the substrate range but the specific GO BP terms for them
are not in the seeded GOA.

Interactions (IPI protein binding annotations)

All bare GO:0005515 protein binding annotations from high-throughput interactome screens:
PMID:19615732 (Sowa et al. DUB interaction landscape) WITH UCHL1 (P09936)
PMID:25416956 (HuRI/CCSB Y2H) WITH KRT40 (Q6A162)
PMID:32296183 (binary interactome HuRI) WITH EFEMP2/O95967, CPSF6/Q16630-2,
ADAMTSL4/Q6UY14-3, KCTD9/Q7L273, HOXC10/Q9NYD6
PMID:32814053 (neurodegeneration interactome) WITH UCHL1 (P09936)
PMID:26100909 IPI WITH BEND3 (Q5T5X7) and BAZ2A (Q9UIF9) — these two are functionally
meaningful (BEND3 = interactor; BAZ2A/TIP5 = substrate), unlike the generic screen hits.
Per curation guidelines, bare "protein binding" is uninformative and should not be elevated to
core. The BEND3/BAZ2A interactions (PMID:26100909) are biologically meaningful but are still
recorded as bare GO:0005515; they are best captured via the deubiquitination BP context.

Localization annotations

Cytoplasm + nucleus, both experimentally supported (PMID:21888622, EXP). HPA IDA: cytosol
(GO:0005829) and nucleoplasm (GO:0005654). IBA cytoplasm. These are consistent.
Centrosome (CD-CODE Centrosome cross-ref in UniProt) consistent with reported
microtubule/centrosome role, but no centrosome GO term is in the seeded GOA.

Reactome TAS

Reactome annotations (R-HSA-5688426, -5690157, -5690159, -6783177) place USP21 DUB activity in
Ub-specific processing protease and TNFR1 signaling pathways (consistent with RIPK1
deubiquitination role). These are TAS general DUB-activity / localization terms — acceptable as
curated but non-specific.

Curation decisions summary

Core MF: cysteine-type deubiquitinase activity (GO:0004843) — strong IDA/IMP human evidence.
Secondary MF: deNEDDylase activity (GO:0019784) — IDA, accept (genuine dual specificity).
BP: protein deubiquitination (GO:0016579) — accept as core process (multiple direct
substrates: 40S RPs, BAZ2A).
GO:0008234 cysteine-type peptidase activity — parent of GO:0004843; KEEP_AS_NON_CORE (less
precise).
Transcription-related terms (GO:0003713, GO:0045815, GO:0045893) — mouse ISS / inferred;
KEEP_AS_NON_CORE (real but ortholog-transferred / indirect, not the primary human-demonstrated
function).
protein binding (GO:0005515) IPI — KEEP_AS_NON_CORE (records real interactions) or
MARK_AS_OVER_ANNOTATED for generic screen hits unrelated to function; never core.
Localizations — ACCEPT (cytosol, nucleoplasm, nucleus, cytoplasm all supported).

Pn Notes

(USP21-pn-notes.md)

USP21 PN Consistency Notes

Generated: 2026-06-18
Project: PROTEOSTASIS
Scope: PN consistency rereview against local AIGR review and available deep-research artifacts
UniProt: Q9UK80
AIGR review status: COMPLETE
Review batch: proteostasis-batch-2026-06-07c
Batch change status: added

Source Files Checked

AIGR review YAML: present - genes/human/USP21/USP21-ai-review.yaml
AIGR review HTML: present - genes/human/USP21/USP21-ai-review.html
Gene notes: present - genes/human/USP21/USP21-notes.md
GOA TSV: present - genes/human/USP21/USP21-goa.tsv
UniProt record: present - genes/human/USP21/USP21-uniprot.txt
PN dossier: projects/PROTEOSTASIS/reports/phase1_dossiers/USP21.md
PN consistency section: projects/PROTEOSTASIS/reports/phase1_dossiers/_sections/USP21.md
PN projection report: projects/PROTEOSTASIS/reports/pn_projection/pn_projected_gene_go_summary.tsv
PN mapping audit: projects/PROTEOSTASIS/reports/pn_mapping_audit/current_mapping_scrutiny.tsv

Deep Research Files

No *-deep-research*.md file found in this gene directory.

AIGR Review Snapshot

Description: USP21 (ubiquitin carboxyl-terminal hydrolase 21) is a cysteine-protease deubiquitinating enzyme (DUB) of the peptidase C19 / ubiquitin-specific protease (USP) family. Its catalytic USP domain (residues ~212-558) uses a Cys-221 nucleophile and a His-518 proton acceptor and contains a structural zinc site; mutation of Cys-221 abolishes activity. USP21 hydrolyzes isopeptide and peptide bonds at the C-terminal Gly of ubiquitin to remove ubiquitin from conjugated substrates, and it has dual specificity, also removing the ubiquitin-like modifier NEDD8 (but not SUMO/Sentrin-1). It localizes to both the cytoplasm and the nucleus and carries a CRM1-dependent nuclear export signal. Through its DUB activity USP21 antagonizes ubiquitin-dependent signaling and degradation of diverse substrates. It deubiquitinates 40S ribosomal proteins RPS10/eS10 and RPS20/uS10 to counteract ZNF598-mediated ribosomal ubiquitylation and limit ribosome-associated quality control (RQC); it deubiquitinates and stabilizes the NoRC component BAZ2A/TIP5 to promote rDNA silencing together with BEND3; and (by similarity to the mouse ortholog) it deubiquitinates histone H2A to relieve transcriptional repression and act as a transcriptional coactivator. Additional reported substrates include RIPK1, RIG-I, GATA3, MARK3 and ACLY, consistent with broad roles in innate immune signaling, transcription and metabolism.
Existing/core annotation action counts: ACCEPT: 22; KEEP_AS_NON_CORE: 11

PN Consistency Summary

Consistency: Consistent. Notes, review, and PN agree USP21 is a dual ubiquitin/NEDD8 cysteine-type DUB (GO:0004843, GO:0019784) that, per PMID:32011234, deubiquitinates 40S eS10/RPS10 and uS10/RPS20 to antagonize ZNF598 and limit RQC. The RQC role is captured in the review as GO:0016579 protein deubiquitination (IDA). MF-granularity note: PN projects GO:0101005 (parent) while GOA/review use child GO:0004843; "entailed_by_goa_closure" is the correct status, no conflict.
PN story / NEW pressure: PN asserts an RQC process role. GOA/review for USP21 contain GO:0016579 but NOT the projected GO:0006515 (verified real, non-obsolete) nor any rescue/RQC-specific process term. Unlike ZNF598/USP10, USP21 has no GO:0072344 or GO:1990116. GO:0006515 (or the more specific GO:1990116 ribosome-associated ubiquitin-dependent protein catabolic process, verified real) is a defensible ADD: USP21's RQC role is a negative regulator (it reverses the mark), so a generic "involved_in protein QC" term is biologically defensible. Conclude: defensible ADD (broad), but the negative-regulator nuance means the bare process term slightly over-states; GO:0016579 already partly captures it.
Evidence alignment: PN UPS row cites "19734957/rev" (Komander/Clague DUB review) — not in review references; the RQC TR row has no titled reference. Review's RQC evidence (PMID:32011234) is more specific and absent from the PN rows. Strong divergence: the key RQC paper is in the review but not cited by PN.
Verdict: Consistent; RQC role captured via GO:0016579 (IDA). PN's GO:0006515 is a defensible-but-broader ADD; no NEW term required.

Full Consistency Review

UniProt: Q9UK80 · batch: proteostasis-batch-2026-06-07c · review status: COMPLETE
PN placement: TR Cytosolic translation|Ribosome-associated QC|Deubiquitination; UPS DUBs and UBL demodifiers|USP|other|other ; PN-node mapping: RQC type→GO:0101005 deubiquitinase activity (mapped, ok_for_propagation); RQC group→GO:0006515 protein QC for misfolded/incompletely synthesized proteins (new_to_goa); UPS group→GO:0101005 (entailed_by_goa_closure); type/subtype/class/branch = no_mapping.
Consistency: Consistent. Notes, review, and PN agree USP21 is a dual ubiquitin/NEDD8 cysteine-type DUB (GO:0004843, GO:0019784) that, per PMID:32011234, deubiquitinates 40S eS10/RPS10 and uS10/RPS20 to antagonize ZNF598 and limit RQC. The RQC role is captured in the review as GO:0016579 protein deubiquitination (IDA). MF-granularity note: PN projects GO:0101005 (parent) while GOA/review use child GO:0004843; "entailed_by_goa_closure" is the correct status, no conflict.
PN story / NEW pressure: PN asserts an RQC process role. GOA/review for USP21 contain GO:0016579 but NOT the projected GO:0006515 (verified real, non-obsolete) nor any rescue/RQC-specific process term. Unlike ZNF598/USP10, USP21 has no GO:0072344 or GO:1990116. GO:0006515 (or the more specific GO:1990116 ribosome-associated ubiquitin-dependent protein catabolic process, verified real) is a defensible ADD: USP21's RQC role is a negative regulator (it reverses the mark), so a generic "involved_in protein QC" term is biologically defensible. Conclude: defensible ADD (broad), but the negative-regulator nuance means the bare process term slightly over-states; GO:0016579 already partly captures it.
Mapping strategy: USP21 does not change the node. PN-projected GO:0006515 is broader than the review's GO:0016579/GO:0004843 — a "broader than review" case (cf. TOMM20/HSPA8 precedent). UPS group→GO:0101005 entailed by GOA closure (GO:0004843 child present). Status/scope correct.
Evidence alignment: PN UPS row cites "19734957/rev" (Komander/Clague DUB review) — not in review references; the RQC TR row has no titled reference. Review's RQC evidence (PMID:32011234) is more specific and absent from the PN rows. Strong divergence: the key RQC paper is in the review but not cited by PN.
Verdict: Consistent; RQC role captured via GO:0016579 (IDA). PN's GO:0006515 is a defensible-but-broader ADD; no NEW term required.

PN Dossier Context

review_batch: proteostasis-batch-2026-06-07c
review_yaml: genes/human/USP21/USP21-ai-review.yaml
PN workbook rows: 2

PN row 1: Translation | Cytosolic translation | Ribosome-associated QC | Deubiquitination

UniProt: Q9UK80
In branches: TR, UPS
PN-node mapping records (path + ancestors):
- [type] Translation|Cytosolic translation|Ribosome-associated QC|Deubiquitination
  status=mapped scope=ok_for_propagation_to_go GO=[GO:0101005 deubiquitinase activity]
  rationale: This PN RQC type denotes deubiquitinases acting in ribosome-associated quality control. Deubiquitinase activity is the shared molecular-function target.
- [group] Translation|Cytosolic translation|Ribosome-associated QC
  status=mapped scope=ok_for_propagation_to_go GO=[GO:0006515 protein quality control for misfolded or incompletely synthesized proteins]
  rationale: The PN ribosome-associated quality-control group covers surveillance and disposal of stalled or defective nascent-chain translation products. GO lacks a dedicated ribosome-associated QC term in the local cache, so the broader protein-quality-control process is the best supported target.
- [class] Translation|Cytosolic translation
  status=context_only scope=too_broad_to_propagate GO=[GO:0002181 cytoplasmic translation]
  rationale: The PN class Cytosolic translation is centered on the cytoplasmic translation apparatus and process, but it also houses supporting machinery such as ribosome biogenesis factors. The GO process term is a useful high-level label for the class, but propagating it to all members would over-annotate genes whose PN placement is through assembly or maturation context rather than core cytoplasmic translation.
- [branch] Translation
  status=context_only scope=too_broad_to_propagate GO=[GO:0006412 translation]
  rationale: The PN Translation branch is organized around the translation apparatus and immediately associated cotranslational quality-control systems. GO translation is the closest high-level process label, but the PN branch also contains adjacent machinery such as ribosome biogenesis and nascent-chain handling. Keeping this relationship is useful for interpretation, but it is too broad to project safely onto every member.

PN row 2: Ubiquitin Proteasome System | DUBs and UBL demodifiers | USP | other | other

UniProt: Q9UK80
In branches: TR, UPS
Signature domains: IPR028889
Auxiliary domains: (none)
PN references (titles):
- 19734957 / rev
PN-node mapping records (path + ancestors):
- [subtype] Ubiquitin Proteasome System|DUBs and UBL demodifiers|USP|other|other
  status=no_mapping scope= GO=[]
  rationale: Reviewed as a narrower active DUB family/domain subdivision already covered by the curated parent DUB-family mapping. No additional direct GO mapping is needed at this node.
- [type] Ubiquitin Proteasome System|DUBs and UBL demodifiers|USP|other
  status=no_mapping scope= GO=[]
  rationale: Reviewed as a narrower active DUB family/domain subdivision already covered by the curated parent DUB-family mapping. No additional direct GO mapping is needed at this node.
- [group] Ubiquitin Proteasome System|DUBs and UBL demodifiers|USP
  status=mapped scope=ok_for_propagation_to_go GO=[GO:0101005 deubiquitinase activity]
  rationale: This PN group is an active deubiquitinase family bucket. The shared molecular-function assertion is deubiquitinase activity.
- [class] Ubiquitin Proteasome System|DUBs and UBL demodifiers
  status=no_mapping scope= GO=[]
  rationale: Reviewed as a UPS taxonomy container. Its descendants mix catalytic roles, complex membership, binding domains, regulators, adaptors, and substrate-context labels, so a single propagating GO assertion would overstate the shared biology.
- [branch] Ubiquitin Proteasome System
  status=no_mapping scope= GO=[]
  rationale: Reviewed as the top-level UPS branch. It is a project taxonomy umbrella rather than a direct GO assertion; UPS propagation must come from manually curated child nodes.

Projected GO annotations (3)

GO:0006515 protein quality control for misfolded or incompletely synthesized proteins | scope=ok_for_propagation_to_go | goa_status=new_to_goa | from=Translation|Cytosolic translation|Ribosome-associated QC
GO:0101005 deubiquitinase activity | scope=ok_for_propagation_to_go | goa_status=entailed_by_goa_closure | from=Translation|Cytosolic translation|Ribosome-associated QC|Deubiquitination
GO:0101005 deubiquitinase activity | scope=ok_for_propagation_to_go | goa_status=entailed_by_goa_closure | from=Ubiquitin Proteasome System|DUBs and UBL demodifiers|USP

Note

This file is generated from the current PROTEOSTASIS phase-1 dossier and local gene-review artifacts. Edit the source review, PN mapping, or dossier rather than this generated note when correcting the underlying curation.

📄 View Raw YAML

id: Q9UK80
gene_symbol: USP21
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  USP21 (ubiquitin carboxyl-terminal hydrolase 21) is a cysteine-protease
  deubiquitinating enzyme (DUB) of the peptidase C19 / ubiquitin-specific
  protease (USP) family. Its catalytic USP domain (residues ~212-558) uses a
  Cys-221 nucleophile and a His-518 proton acceptor and contains a structural
  zinc site; mutation of Cys-221 abolishes activity. USP21 hydrolyzes isopeptide
  and peptide bonds at the C-terminal Gly of ubiquitin to remove ubiquitin from
  conjugated substrates, and it has dual specificity, also removing the
  ubiquitin-like modifier NEDD8 (but not SUMO/Sentrin-1). It localizes to both
  the cytoplasm and the nucleus and carries a CRM1-dependent nuclear export
  signal. Through its DUB activity USP21 antagonizes ubiquitin-dependent
  signaling and degradation of diverse substrates. It deubiquitinates 40S
  ribosomal proteins RPS10/eS10 and RPS20/uS10 to counteract ZNF598-mediated
  ribosomal ubiquitylation and limit ribosome-associated quality control (RQC);
  it deubiquitinates and stabilizes the NoRC component BAZ2A/TIP5 to promote
  rDNA silencing together with BEND3; and (by similarity to the mouse ortholog)
  it deubiquitinates histone H2A to relieve transcriptional repression and act
  as a transcriptional coactivator. Additional reported substrates include
  RIPK1, RIG-I, GATA3, MARK3 and ACLY, consistent with broad roles in innate
  immune signaling, transcription and metabolism.
alternative_products:
- name: '1'
  id: Q9UK80-1
- name: '2'
  id: Q9UK80-2
  sequence_note: VSP_036717, VSP_036718
- name: '3'
  id: Q9UK80-3
  sequence_note: VSP_036719
existing_annotations:
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Phylogenetically inferred cytoplasmic localization. USP21 is experimentally documented in the cytoplasm and acts on cytoplasmic substrates (e.g. 40S ribosomal proteins), so this is well supported.
    action: ACCEPT
    reason: Cytoplasmic localization is corroborated by direct experimental evidence (EXP, PMID:21888622) and HPA cytosol IDA, and is where USP21 deubiquitinates ribosomal substrates.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm'
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Electronic (InterPro/EC-based) assignment of the core DUB activity. This is the central, experimentally proven molecular function of USP21.
    action: ACCEPT
    reason: The cysteine-type deubiquitinase activity is directly demonstrated in human cells (IDA, PMID:10799498; PMID:32011234) with Cys-221 as the catalytic nucleophile; the IEA assignment is correct and corroborated by experiment.
    supported_by:
    - reference_id: PMID:10799498
      supporting_text: USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Automated subcellular-location assignment of nuclear localization, consistent with USP21's documented nuclear pool and nuclear substrates (BAZ2A/TIP5, histone H2A).
    action: ACCEPT
    reason: Nuclear localization is experimentally supported (EXP, PMID:21888622) and consistent with nucleoplasm IDA; USP21 shuttles via a CRM1-dependent NES.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: Nucleus
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Automated subcellular-location assignment of cytoplasmic localization, redundant with but consistent with the IBA and experimental cytoplasm annotations.
    action: ACCEPT
    reason: Correct compartment; agrees with stronger experimental (EXP) and IBA evidence for the same location.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm'
- term:
    id: GO:0008234
    label: cysteine-type peptidase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: enables
  review:
    summary: Family-level (ARBA) assignment of cysteine-type peptidase activity, a parent of the more specific deubiquitinase activity that USP21 actually performs.
    action: KEEP_AS_NON_CORE
    reason: Correct but less precise than GO:0004843 cysteine-type deubiquitinase activity, which is the experimentally established function. Retained as a true but non-core (general) molecular-function annotation.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: Belongs to the peptidase C19 family. USP21 subfamily.
- term:
    id: GO:0016579
    label: protein deubiquitination
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: InterPro-based assignment of the protein deubiquitination process, which is the direct biological outcome of USP21's DUB activity. Beyond the cache-verified substrates (40S ribosomal proteins, BAZ2A/TIP5), the Falcon deep-research report compiles a broad reported substrate repertoire (e.g. RIG-I, MEK2, Nanog, histone H2A, MARK3, TCF7) processed via multiple ubiquitin chain types, all of which fall under this general process term.
    action: ACCEPT
    reason: Protein deubiquitination is directly demonstrated for multiple human substrates (40S ribosomal proteins, BAZ2A/TIP5); this process annotation is well supported. The Falcon report (UNVERIFIED LLM synthesis; primary substrate papers not in cache) further supports breadth of substrate processing and chain-type promiscuity but does not alter this annotation.
    supported_by:
    - reference_id: PMID:32011234
      supporting_text: cells lacking USP21 or OTUD3 have altered RQC activity and delayed eS10 deubiquitylation
    - reference_id: file:human/USP21/USP21-deep-research-falcon.md
      supporting_text: The enzyme exhibits promiscuous activity toward multiple ubiquitin chain types, including K6-, K11-, K29-, K48-, K63-linked, and linear ubiquitin conjugates
- term:
    id: GO:0045893
    label: positive regulation of DNA-templated transcription
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  qualifier: involved_in
  review:
    summary: Automated inference (from the transcription coactivator activity term) that USP21 positively regulates transcription. This derives ultimately from the mouse "by similarity" histone H2A deubiquitination / coactivator role rather than direct human experimental evidence.
    action: KEEP_AS_NON_CORE
    reason: A plausible downstream process inherited from the ortholog-transferred coactivator role (histone H2A deubiquitination relieving repression), but not directly demonstrated for human USP21 and peripheral to its core DUB function. USP21 acts as both a coactivator (H2A) and a repressor (rDNA via BAZ2A), so a single positive-regulation term is an oversimplification. The Falcon report adds (UNVERIFIED) that USP21 removes the repressive H2AK119ub mark with specificity for nucleosomal substrate, consistent with a transcriptional-activation role, though the primary paper is not cached.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: resulting in regulation of transcriptional initiation
    - reference_id: file:human/USP21/USP21-deep-research-falcon.md
      supporting_text: USP21 removes monoubiquitin from histone H2A at lysine 119 (H2AK119ub) in nucleosomal contexts, with strong specificity for nucleosomal rather than free histone substrates
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19615732
  qualifier: enables
  review:
    summary: High-throughput DUB interaction-landscape screen (Sowa et al.) capturing a USP21-UCHL1 (P09936) interaction. The bare protein binding term is uninformative and the partner does not define a specific USP21 function.
    action: KEEP_AS_NON_CORE
    reason: Records a real physical interaction but bare protein binding (GO:0005515) is uninformative per curation guidelines; not a core function. The UCHL1 partner is an unrelated DUB from a global screen.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: PMID:19615732 UniProtKB:P09936
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  qualifier: enables
  review:
    summary: Proteome-scale yeast two-hybrid interactome (HuRI) capturing a USP21-KRT40 (Q6A162) interaction. Bare protein binding from a global screen; uninformative.
    action: KEEP_AS_NON_CORE
    reason: Single high-throughput Y2H interaction with a keratin (KRT40) that has no established functional relationship to USP21; bare protein binding is uninformative and not part of the core function.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: PMID:25416956 UniProtKB:Q6A162
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: Binary reference interactome (HuRI) capturing several USP21 interactions (EFEMP2, CPSF6, ADAMTSL4, KCTD9, HOXC10). These are bare protein binding hits from a systematic screen.
    action: KEEP_AS_NON_CORE
    reason: Records real binary interactions but bare protein binding (GO:0005515) is uninformative; the partners are from a global screen and do not individually define a specific USP21 molecular function. Not core.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: PMID:32296183 UniProtKB:O95967
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32814053
  qualifier: enables
  review:
    summary: Neurodegeneration interactome screen capturing a USP21-UCHL1 (P09936) interaction. Bare protein binding from a high-throughput map.
    action: KEEP_AS_NON_CORE
    reason: Records a real interaction but bare protein binding (GO:0005515) is uninformative and the partner does not define a specific USP21 function; not core.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: PMID:32814053 UniProtKB:P09936
- term:
    id: GO:0016579
    label: protein deubiquitination
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5688426
  qualifier: involved_in
  review:
    summary: Reactome (TAS) curation of USP21 protein deubiquitination, here in the context of ubiquitin-specific processing protease pathways. Consistent with its core process.
    action: ACCEPT
    reason: Protein deubiquitination is the genuine process carried out by USP21 and is directly supported by experimental evidence elsewhere; the curated TAS term is appropriate.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0016579 protein deubiquitination
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5690157
  qualifier: enables
  review:
    summary: Reactome (TAS) annotation of the core DUB activity. Correct and experimentally corroborated.
    action: ACCEPT
    reason: Reflects USP21's experimentally proven cysteine-type deubiquitinase activity; curated TAS evidence is consistent with direct IDA evidence.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0004843 cysteine-type deubiquitinase activity
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5690159
  qualifier: enables
  review:
    summary: Reactome (TAS) annotation of the core cysteine-type deubiquitinase activity.
    action: ACCEPT
    reason: Correct core molecular function, corroborated by direct experimental evidence.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0004843 cysteine-type deubiquitinase activity
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-6783177
  qualifier: enables
  review:
    summary: Reactome (TAS) annotation of the core cysteine-type deubiquitinase activity.
    action: ACCEPT
    reason: Correct core molecular function, corroborated by direct experimental evidence.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0004843 cysteine-type deubiquitinase activity
- term:
    id: GO:0005654
    label: nucleoplasm
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) evidence for nucleoplasmic localization, consistent with USP21's nuclear substrates (BAZ2A/TIP5, histone H2A).
    action: ACCEPT
    reason: IDA-supported nuclear localization agrees with the documented nuclear pool of USP21 and its nuclear functions.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0005654 nucleoplasm cellular_component
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) evidence for cytosolic localization, consistent with USP21's cytoplasmic role in ribosome quality control.
    action: ACCEPT
    reason: IDA-supported cytosolic localization agrees with the documented cytoplasmic site of action.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0005829 cytosol cellular_component
- term:
    id: GO:0005654
    label: nucleoplasm
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5690157
  qualifier: located_in
  review:
    summary: Reactome (TAS) nucleoplasm localization, consistent with the experimentally supported nuclear pool.
    action: ACCEPT
    reason: Agrees with HPA IDA nucleoplasm and EXP nucleus evidence.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0005654 nucleoplasm cellular_component
- term:
    id: GO:0005654
    label: nucleoplasm
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-6783177
  qualifier: located_in
  review:
    summary: Reactome (TAS) nucleoplasm localization, redundant with the other nucleoplasm annotations and consistent with experimental evidence.
    action: ACCEPT
    reason: Agrees with HPA IDA nucleoplasm and EXP nucleus evidence.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0005654 nucleoplasm cellular_component
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5690159
  qualifier: located_in
  review:
    summary: Reactome (TAS) cytosol localization, consistent with the experimentally supported cytoplasmic pool.
    action: ACCEPT
    reason: Agrees with HPA IDA cytosol and EXP cytoplasm evidence.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: GO:0005829 cytosol cellular_component
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: EXP
  original_reference_id: PMID:21888622
  qualifier: located_in
  review:
    summary: Experimental (EXP) evidence for nuclear localization from a study of CRM1-dependent nuclear export signals across the human DUB family; USP21 has a functional NES.
    action: ACCEPT
    reason: Direct experimental evidence places USP21 in the nucleus with a CRM1-dependent export signal, consistent with its nuclear substrates.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: Nucleus {ECO:0000269|PubMed:21888622}
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: EXP
  original_reference_id: PMID:21888622
  qualifier: located_in
  review:
    summary: Experimental (EXP) evidence for cytoplasmic localization, consistent with USP21's CRM1-dependent nuclear export and cytoplasmic substrates. The Falcon deep-research report notes a more specific cytoplasmic distribution from a systematic DUB-localization survey (Urbe et al. 2012), in which USP21 uniquely associates with microtubules and centrosomes.
    action: ACCEPT
    reason: Direct experimental evidence supports cytoplasmic localization; consistent with IBA, IEA and HPA IDA cytosol. A more granular cytoplasmic localization (microtubule/centrosome) is reported in the Falcon synthesis but the primary paper is not in the cache, so no more-specific CC term is asserted here.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: Cytoplasm {ECO:0000269|PubMed:21888622}
    - reference_id: file:human/USP21/USP21-deep-research-falcon.md
      supporting_text: USP21 is the only deubiquitinase in a systematic survey of 66 mammalian DUBs to display clear association with both microtubules and centrosomes
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: IDA
  original_reference_id: PMID:32011234
  qualifier: enables
  review:
    summary: Direct (IDA) demonstration that USP21 is a cysteine-type deubiquitinase acting on ubiquitylated 40S ribosomal proteins, with catalytic Cys-221 required.
    action: ACCEPT
    reason: Strong direct human experimental evidence for the core DUB activity; USP21 antagonizes ZNF598-mediated 40S ubiquitylation. This is a defining piece of evidence for the core molecular function.
    supported_by:
    - reference_id: PMID:32011234
      supporting_text: we identify OTUD3 and USP21 as deubiquitylating enzymes that antagonize ZNF598-mediated 40S ubiquitylation and can limit RQC activation
- term:
    id: GO:0016579
    label: protein deubiquitination
  evidence_type: IDA
  original_reference_id: PMID:32011234
  qualifier: involved_in
  review:
    summary: Direct (IDA) evidence that USP21 carries out protein deubiquitination of 40S ribosomal proteins (eS10/RPS10, uS10/RPS20) within the RQC pathway, acting as a negative regulator of RQC.
    action: ACCEPT
    reason: Directly demonstrated process; cells lacking USP21 show delayed eS10 deubiquitylation and altered RQC activity. This is a core, well-supported biological process for USP21.
    supported_by:
    - reference_id: PMID:32011234
      supporting_text: cells lacking USP21 or OTUD3 have altered RQC activity and delayed eS10 deubiquitylation
- term:
    id: GO:0045815
    label: transcription initiation-coupled chromatin remodeling
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: Sequence-similarity (ISS, from mouse Q9QZL6) transfer of a chromatin-remodeling / transcription-initiation role based on histone H2A deubiquitination. Not directly demonstrated for human USP21.
    action: KEEP_AS_NON_CORE
    reason: A plausible role inherited by ortholog transfer from mouse (histone H2A deubiquitination relieving H3K4 methylation repression), but not directly shown in human and secondary to USP21's core DUB function across multiple substrates.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: Deubiquitination of histone H2A releaves the repression of di- and trimethylation of histone H3 at 'Lys-4'
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: Sequence-similarity (ISS, from mouse Q9QZL6) assignment of the core DUB activity, which is independently proven by direct human experimental evidence.
    action: ACCEPT
    reason: The cysteine-type deubiquitinase activity is the core molecular function and is directly demonstrated in human (IDA); the ISS transfer is concordant.
    supported_by:
    - reference_id: PMID:10799498
      supporting_text: USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: IMP
  original_reference_id: PMID:26100909
  qualifier: enables
  review:
    summary: Mutational/functional (IMP) evidence supporting USP21 deubiquitinase activity in the context of deubiquitinating and stabilizing the NoRC component BAZ2A/TIP5.
    action: ACCEPT
    reason: USP21 interacts with and deubiquitinates Tip5/BAZ2A, stabilizing it; this supports the core DUB activity acting on a defined nuclear substrate.
    supported_by:
    - reference_id: PMID:26100909
      supporting_text: USP21 can interact with and deubiquitinate Tip5, thereby stabilizing the total levels of Tip5.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:26100909
  qualifier: enables
  review:
    summary: IPI interactions (WITH BEND3/Q5T5X7 and BAZ2A/Q9UIF9) underlying the rDNA-silencing study. Unlike generic screen hits, these partners are functionally meaningful (BEND3 regulates USP21 stability; BAZ2A/TIP5 is a USP21 substrate), but the term itself is uninformative bare protein binding.
    action: KEEP_AS_NON_CORE
    reason: Records biologically meaningful interactions (BEND3 interactor and BAZ2A substrate), but bare protein binding (GO:0005515) is uninformative and the relevant biology is better captured by the deubiquitination process annotation; not core.
    supported_by:
    - reference_id: file:human/USP21/USP21-goa.tsv
      supporting_text: PMID:26100909 UniProtKB:Q9UIF9
- term:
    id: GO:0008234
    label: cysteine-type peptidase activity
  evidence_type: IMP
  original_reference_id: PMID:10799498
  qualifier: enables
  review:
    summary: Mutational (IMP) evidence (Cys-221 required) for cysteine-type peptidase activity, a parent of the specific deubiquitinase activity USP21 performs.
    action: KEEP_AS_NON_CORE
    reason: Correct but less precise than GO:0004843; the same Cys-221 mutagenesis supports the more specific deubiquitinase activity term, which should be the core MF.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: 'C->A,S: Abolishes ubiquitin thioesterase activity.'
- term:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  evidence_type: IDA
  original_reference_id: PMID:10799498
  qualifier: enables
  review:
    summary: Original direct (IDA) demonstration that USP21 removes ubiquitin from ubiquitinated proteins in cells. Defines the core molecular function.
    action: ACCEPT
    reason: First direct evidence of USP21's cysteine-type deubiquitinase activity; this is the gene's core molecular function.
    supported_by:
    - reference_id: PMID:10799498
      supporting_text: USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
- term:
    id: GO:0019784
    label: deNEDDylase activity
  evidence_type: IDA
  original_reference_id: PMID:10799498
  qualifier: enables
  review:
    summary: Direct (IDA) evidence that USP21 also removes NEDD8 from NEDD8 conjugates (but not SUMO/Sentrin-1), giving it dual ubiquitin/NEDD8 isopeptidase specificity.
    action: ACCEPT
    reason: A genuine, experimentally demonstrated secondary catalytic activity; USP21 was the first USP shown to have dual ubiquitin/NEDD8 specificity.
    supported_by:
    - reference_id: PMID:10799498
      supporting_text: USP21 is capable of removing NEDD8 from NEDD8 conjugates but has no effect on Sentrin-1 conjugates.
- term:
    id: GO:0003713
    label: transcription coactivator activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: Sequence-similarity (ISS, from mouse Q9QZL6) assignment of transcription coactivator activity, based on histone H2A deubiquitination relieving repression. Not a direct catalytic/binding activity and not demonstrated in human.
    action: KEEP_AS_NON_CORE
    reason: Inherited by ortholog transfer; reflects an indirect (via H2A deubiquitination) coactivator role rather than USP21's core enzymatic function. USP21 can also repress transcription (rDNA via BAZ2A), so coactivator is context-specific.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: thereby acting as a coactivator
- term:
    id: GO:0008234
    label: cysteine-type peptidase activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: Sequence-similarity (ISS, from mouse Q9QZL6) assignment of the general cysteine-type peptidase activity, a parent of the specific deubiquitinase activity.
    action: KEEP_AS_NON_CORE
    reason: Correct but less precise than GO:0004843 cysteine-type deubiquitinase activity; retained as a general non-core MF annotation.
    supported_by:
    - reference_id: file:human/USP21/USP21-uniprot.txt
      supporting_text: Belongs to the peptidase C19 family. USP21 subfamily.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on inter-ontology links
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:10799498
  title: Identification of a novel isopeptidase with dual specificity for ubiquitin- and NEDD8-conjugated proteins.
  findings:
  - statement: USP21 is a cysteine-protease isopeptidase that removes ubiquitin from ubiquitinated proteins and removes NEDD8 from NEDD8 conjugates, but has no effect on Sentrin-1/SUMO conjugates (dual ubiquitin/NEDD8 specificity).
    reference_section_type: ABSTRACT
  - statement: Catalytic Cys-221 is required; its mutation abolishes ubiquitin thioesterase activity.
    reference_section_type: RESULTS
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: "Cached PMID_10799498 title/body match; establishes USP21 as the founding dual ubiquitin/NEDD8 isopeptidase (supports GO:0004843 and GO:0019784, consistent with the GOA evidence rows for this PMID)."
- id: PMID:19615732
  title: Defining the human deubiquitinating enzyme interaction landscape.
  findings: []
- id: PMID:21888622
  title: A global survey of CRM1-dependent nuclear export sequences in the human deubiquitinase family.
  findings:
  - statement: USP21 contains a functional CRM1-dependent nuclear export signal and localizes to both nucleus and cytoplasm.
    reference_section_type: RESULTS
- id: PMID:25416956
  title: A proteome-scale map of the human interactome network.
  findings: []
- id: PMID:26100909
  title: BEND3 represses rDNA transcription by stabilizing a NoRC component via USP21 deubiquitinase.
  findings:
  - statement: USP21 interacts with and deubiquitinates BAZ2A/TIP5 (a NoRC component), stabilizing it; SUMOylated BEND3 stabilizes USP21, and together they promote rDNA silencing.
    reference_section_type: RESULTS
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: "Cached PMID_26100909 title/body match; supports USP21's nuclear deubiquitination of BAZ2A/TIP5 substrate (a basis for the protein-deubiquitination core function)."
- id: PMID:32011234
  title: Distinct regulatory ribosomal ubiquitylation events are reversible and hierarchically organized.
  findings:
  - statement: USP21 (with OTUD3) is a deubiquitylating enzyme that antagonizes ZNF598-mediated 40S ribosomal ubiquitylation (on eS10/RPS10 and uS10/RPS20) and limits ribosome-associated quality control (RQC) activation.
    reference_section_type: ABSTRACT
  - statement: Cells lacking USP21 have altered RQC activity and delayed eS10 deubiquitylation, establishing a functional role for USP21 within the RQC pathway.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: "Cached PMID_32011234 title/body match; body explicitly names USP21 (with OTUD3) antagonizing ZNF598-mediated 40S ubiquitylation, supporting the RQC deubiquitination function."
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
- id: PMID:32814053
  title: Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.
  findings: []
- id: Reactome:R-HSA-5688426
  title: Ub-specific processing proteases (protein deubiquitination)
  findings: []
- id: Reactome:R-HSA-5690157
  title: Ub-specific processing proteases (USP21 deubiquitinase activity)
  findings: []
- id: Reactome:R-HSA-5690159
  title: Ub-specific processing proteases (USP21 deubiquitinase activity)
  findings: []
- id: Reactome:R-HSA-6783177
  title: Ub-specific processing proteases (USP21 deubiquitinase activity)
  findings: []
- id: file:human/USP21/USP21-deep-research-falcon.md
  title: Falcon deep research report for USP21
  findings:
  - statement: USP21 is a broad-specificity DUB that processes multiple polyubiquitin chain types (K6, K11, K29, K48, K63, and linear) and cross-reacts with ISG15, with substrate-stabilizing deubiquitination reported for RIG-I (K63), MEK2 (K48), Nanog (K48), histone H2A (H2AK119ub), MARK3, TCF7, GATA3 and RIPK1.
    reference_section_type: OTHER
  reference_review:
    relevance: HIGH
    correctness: UNVERIFIED
    review_notes: >-
      LLM-synthesized (Edison/Falcon) literature report; useful for substrate
      breadth and chain-type/pathway context, but treated as UNVERIFIED. USP21-SPECIFIC
      substrate evidence that the report attributes to dedicated primary papers includes:
      RIG-I deubiquitination / negative regulation of antiviral RIG-I-IFN signaling
      (Fan et al. 2014, J Exp Med, doi:10.1084/jem.20122844), MEK2 stabilization driving
      ERK/MAPK signaling (Li et al. 2018, doi:10.1038/s41419-018-0523-z), Nanog stabilization
      in ESC pluripotency (Liu et al. 2016), histone H2A K119ub deubiquitination, MARK3/macropinocytosis in
      pancreatic cancer (Hou et al. 2021), TCF7, and microtubule/centrosome association
      (Urbe et al. 2012). These primary papers are NOT in the local publications cache
      and could not be verified here, so substrate identities and chain-type assignments
      are recorded but not used to change existing experimental annotations. DUB-FAMILY-LEVEL
      inference (broad chain-type promiscuity, ISG15 cross-reactivity, general USP catalytic
      mechanism) should not be over-read as USP21-specific physiological function. NOTE a
      conflict to flag for experts: this report (citing Ye et al. 2011 structural work) states
      USP21 is "inactive against NEDD8 conjugates", whereas the curated IDA annotation
      GO:0019784 (PMID:10799498) records deNEDDylase activity; the existing experimental
      deNEDDylase annotation is retained and NOT weakened on the basis of this report.
core_functions:
- description: Cysteine-type deubiquitinating enzyme that hydrolyzes isopeptide/peptide bonds at the C-terminal Gly of ubiquitin to remove ubiquitin from conjugated substrates, using a Cys-221 nucleophile within its USP domain.
  molecular_function:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  locations:
  - id: GO:0005829
    label: cytosol
  - id: GO:0005654
    label: nucleoplasm
  supported_by:
  - reference_id: PMID:10799498
    supporting_text: USP21 is capable of removing ubiquitin from ubiquitinated proteins as expected.
  - reference_id: PMID:32011234
    supporting_text: we identify OTUD3 and USP21 as deubiquitylating enzymes that antagonize ZNF598-mediated 40S ubiquitylation and can limit RQC activation
- description: Removes the ubiquitin-like modifier NEDD8 from NEDD8-conjugated proteins (deNEDDylase activity), a genuine secondary isopeptidase specificity distinct from SUMO; USP21 was the first USP shown to act on both ubiquitin and NEDD8.
  molecular_function:
    id: GO:0019784
    label: deNEDDylase activity
  supported_by:
  - reference_id: PMID:10799498
    supporting_text: USP21 is capable of removing NEDD8 from NEDD8 conjugates but has no effect on Sentrin-1 conjugates.
- description: Through its deubiquitinase activity USP21 carries out protein deubiquitination of specific substrates, including 40S ribosomal proteins (eS10/RPS10, uS10/RPS20) to negatively regulate ribosome-associated quality control, and BAZ2A/TIP5 to stabilize the NoRC complex and promote rDNA silencing.
  molecular_function:
    id: GO:0004843
    label: cysteine-type deubiquitinase activity
  locations:
  - id: GO:0005829
    label: cytosol
  - id: GO:0005654
    label: nucleoplasm
  supported_by:
  - reference_id: PMID:32011234
    supporting_text: cells lacking USP21 or OTUD3 have altered RQC activity and delayed eS10 deubiquitylation
  - reference_id: PMID:26100909
    supporting_text: USP21 can interact with and deubiquitinate Tip5, thereby stabilizing the total levels of Tip5.
  directly_involved_in:
  - id: GO:0016579
    label: protein deubiquitination
proposed_new_terms: []
suggested_questions:
- question: What is the in vivo substrate hierarchy and selectivity of USP21 across its reported targets (40S ribosomal proteins, BAZ2A/TIP5, histone H2A, RIPK1, RIG-I, GATA3, MARK3, ACLY), and which interactions reflect direct catalysis versus indirect stabilization?
- question: How is USP21's dual ubiquitin/NEDD8 specificity regulated in cells, and does its deNEDDylase activity have a distinct physiological substrate set?
- question: Does USP21 nucleocytoplasmic shuttling (via its CRM1-dependent NES) partition its ribosome-quality-control (cytoplasmic) versus chromatin/transcription (nuclear) functions?
suggested_experiments:
- description: Catalytically-dead (C221A) versus wild-type USP21 rescue in USP21-knockout cells combined with diGly-ubiquitin proteomics to define the direct, catalysis-dependent deubiquitinated substrate repertoire genome-wide.
- description: Quantitative kinetic assays comparing USP21 activity on ubiquitin- versus NEDD8-isopeptide substrates (and on different polyubiquitin linkage types) to characterize its dual specificity and linkage preference.
- description: Time-resolved measurement of eS10/uS10 (RPS10/RPS20) ubiquitylation and ribosome stalling / RQC reporter readouts upon USP21 depletion or overexpression to quantify its role as a negative regulator of RQC.

USP21

Gene Description

Existing Annotations Review

Core Functions

Cysteine-type deubiquitinating enzyme that hydrolyzes isopeptide/peptide bonds at the C-terminal Gly of ubiquitin to remove ubiquitin from conjugated substrates, using a Cys-221 nucleophile within its USP domain.

Removes the ubiquitin-like modifier NEDD8 from NEDD8-conjugated proteins (deNEDDylase activity), a genuine secondary isopeptidase specificity distinct from SUMO; USP21 was the first USP shown to act on both ubiquitin and NEDD8.

Through its deubiquitinase activity USP21 carries out protein deubiquitination of specific substrates, including 40S ribosomal proteins (eS10/RPS10, uS10/RPS20) to negatively regulate ribosome-associated quality control, and BAZ2A/TIP5 to stabilize the NoRC complex and promote rDNA silencing.

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Comprehensive Research Report: USP21 (Ubiquitin Carboxyl-Terminal Hydrolase 21)

Gene and Protein Identity

Primary Catalytic Function and Substrate Specificity

Enzymatic Activity

Validated Protein Substrates

Subcellular Localization

Cytoplasmic Localization

Nuclear Localization

Signaling and Biochemical Pathways

Antiviral Immunity and Interferon Signaling

NF-κB and Inflammatory Signaling

ERK/MAPK Signaling

Hippo Signaling Pathway

mTOR Signaling and Macropinocytosis

Stem Cell Pluripotency

Biological Processes Regulated by USP21

Structural and Mechanistic Insights

Knockout Mouse Phenotype

Summary

Artifacts

Citations

📚 Additional Documentation

Notes

USP21 (Q9UK80, UBP21_HUMAN) review notes

Identity / overview

Core molecular function: cysteine-type deubiquitinase (GO:0004843)

Substrate / process contexts (BP)

Interactions (IPI protein binding annotations)

Localization annotations

Reactome TAS

Curation decisions summary

Pn Notes

USP21 PN Consistency Notes

Source Files Checked

Deep Research Files

AIGR Review Snapshot

PN Consistency Summary

Full Consistency Review

PN Dossier Context

PN row 1: Translation | Cytosolic translation | Ribosome-associated QC | Deubiquitination

PN row 2: Ubiquitin Proteasome System | DUBs and UBL demodifiers | USP | other | other

Projected GO annotations (3)

Note

📄 View Raw YAML