UBAP2

UBAP2 is a ubiquitin-binding protein that couples recognition of polyubiquitinated proteins to substrate degradation and to higher-order assembly of metabolic condensates (PMID:37848033, PMID:35633597). As the human orthologue of yeast Def1 (together with its paralogue UBAP2L), it is required for UV-induced ubiquitylation and degradation of the largest RNA polymerase II subunit RPB1, acting through recruitment of the Elongin-Cul5 ubiquitin ligase (PMID:35633597). Beyond promoting degradation, its ubiquitin-binding domain reads K6-linked polyubiquitin on PAICS deposited by the Cul5/ASB11 ligase, and its intrinsically disordered regions then drive phase separation to nucleate purinosome assembly and increase de novo purine synthesis flux (PMID:37848033). UBAP2 also promotes ubiquitin-proteasome-dependent turnover of specific substrates—Annexin A2, where its degradation restrains hepatocellular carcinoma invasion and proliferation (PMID:27121050), and SLC27A5, whose loss lowers RAD51 to bias DNA repair toward homologous recombination and confer radioresistance (PMID:39186963). At the organismal level, UBAP2 supports NF-κB activation during pancreatitis (PMID:37865160) and influences bone remodeling by promoting osteoblastogenesis while restraining osteoclastogenesis (PMID:37339951).

1. 2016 Medium

   Established the first direct substrate relationship for UBAP2, showing it drives ubiquitination-dependent proteasomal degradation of a binding partner with a defined cellular consequence.

   Evidence Co-IP, ubiquitination assays, and knockdown/overexpression with invasion/proliferation readouts plus xenograft in hepatocellular carcinoma cells

   PMID:27121050

   Open questions at the time

   • Does not identify the cognate E3 ligase for Annexin A2
   • Single lab; ubiquitin-binding domain dependence not dissected
2. 2020 Low

   Addressed how UBAP2 protein levels are upregulated in cancer, identifying an upstream circRNA-miRNA regulatory axis controlling UBAP2 expression.

   Evidence Bioinformatics, miRNA binding assay, and UBAP2 protein measurement after circRNA knockdown with xenograft in hepatocellular carcinoma cells

   PMID:32464601

   Open questions at the time

   • Indirect; describes regulation of UBAP2 abundance, not UBAP2 molecular function
   • No enzymatic or structural characterization of UBAP2 itself
   • Single lab, single circRNA mechanism
3. 2022 High

   Defined UBAP2 as the human Def1 orthologue and placed it in a specific ubiquitin-ligase pathway, answering how transcription-blocking damage triggers RNA polymerase II degradation.

   Evidence UV irradiation, RPB1 ubiquitylation/degradation assays, UBAP2/UBAP2L knockdown, and Elongin-Cul5 recruitment assays

   PMID:35633597

   Open questions at the time

   • Functional redundancy versus distinct roles of UBAP2 and UBAP2L not resolved
   • Structural basis of Elongin-Cul5 recruitment not defined
4. 2023 High

   Revealed a non-degradative function: UBAP2 reads a specific ubiquitin signal and uses its disordered regions to nucleate a metabolic condensate, linking ubiquitin recognition to phase separation.

   Evidence Co-IP, ubiquitination assays, in vitro phase separation, purine flux measurements, and knockdown/overexpression with xenograft

   PMID:37848033

   Open questions at the time

   • Determinants of K6-linkage specificity by the ubiquitin-binding domain not mapped
   • Stoichiometry and regulation of purinosome dissolution unaddressed
5. 2023 Medium

   Extended UBAP2 function to inflammatory signaling in vivo, showing it is required for full NF-κB activation in the pancreas.

   Evidence Pancreas-specific Cre knockout mice in cerulein pancreatitis, NF-κB activation and promoter binding studies, cytokine/enzyme measurements

   PMID:37865160

   Open questions at the time

   • Molecular link between UBAP2 ubiquitin-binding activity and NF-κB not defined
   • Single lab
6. 2023 Medium

   Implicated UBAP2 in skeletal homeostasis by showing opposing effects on osteoblast and osteoclast differentiation.

   Evidence Ubap2 knockdown in mouse cells and zebrafish, osteoblastogenesis/osteoclastogenesis assays, expression correlation with Cdh1/Fosl1

   PMID:37339951

   Open questions at the time

   • Mechanistic connection to UBAP2's ubiquitin-binding/degradation activity unknown
   • Direct molecular targets in bone cells not identified
7. 2024 Medium

   Identified a second degradation substrate that links UBAP2 to DNA repair pathway choice and therapy resistance.

   Evidence UBAP2 knockdown with SLC27A5 rescue, proteasome inhibition, RAD51/CTIP analysis, and in vitro/in vivo radiation assays in hepatocellular carcinoma

   PMID:39186963

   Open questions at the time

   • E3 ligase mediating SLC27A5 ubiquitination not identified
   • Mechanism connecting SLC27A5 to RAD51 levels not resolved
   • Single lab

• How UBAP2's ubiquitin-binding activity is mechanistically partitioned between substrate degradation, condensate nucleation, and inflammatory/skeletal signaling, and how it differs functionally from UBAP2L, remains unresolved.
• No structure of the ubiquitin-binding domain or IDRs
• Linkage specificity rules across contexts unknown
• Division of labor between UBAP2 and UBAP2L undefined