PSME3IP1

PSME3IP1 (NIP30/PIP30/FAM192A) is a bifunctional nuclear protein operating in two distinct machineries: the nuclear PA28γ (REGγ)–20S proteasome and the spliceosome (PMID:29934401, PMID:35705093). As a proteasome regulator, it binds PA28γ directly through its C-terminal serine-rich domain in a manner stabilized by CK2 phosphorylation of four conserved serines, thereby increasing PA28γ association with the 20S proteasome, altering PA28γ-dependent peptide degradation, and inhibiting PA28γ recruitment to Cajal bodies by blocking its association with coilin (PMID:29934401, PMID:32764536). This phosphorylation switch is controlled by the phosphatase CDC25A, coupling PSME3IP1 activity to the cell cycle and DNA damage response and tuning REGγ-mediated, ubiquitin-independent degradation of substrates such as p21 (PMID:32764536). The same CK2-dependent phosphoswitch operates in osteoclasts, where dephosphorylated NIP30 promotes REGγ-20S degradation of TRAF6 to suppress osteoclast activity (PMID:39536082). In its second role, PSME3IP1 acts as a second-step (exon-ligation) spliceosomal factor: it contacts the U2 snRNA–branch site duplex in C* intermediate complexes (PMID:35705093) and, as established for its yeast ortholog Fyv6, uniquely contacts the Prp22 ATPase in a manner mutually exclusive with the first-step factor Yju2, promoting exon ligation and favoring consensus, branch point-distal 3' splice site usage (PMID:37625852, PMID:39688371).

1. 2018 High

   Established PSME3IP1 as a direct, phosphorylation-dependent binding partner and modulator of the PA28γ-20S proteasome, defining its first molecular function.

   Evidence Direct binding, co-IP, in vitro peptide degradation, CK2 phosphorylation assays, and Cajal body localization imaging

   PMID:29934401

   Open questions at the time

   • Did not define the upstream regulator of the phosphorylation state
   • Physiological substrate consequences of altered PA28γ activity not addressed
2. 2020 High

   Identified the conserved serine-rich domain and its 4-serine phosphorylation as the functional switch, placing PSME3IP1 under CDC25A control during cell cycle and DNA damage and linking it to REGγ-mediated p21 degradation in vivo.

   Evidence Reciprocal co-IP, serine mutagenesis, phosphomimetic/phosphodead mutants, and p53−/− and p53/REGγ double-KO mice

   PMID:32764536

   Open questions at the time

   • Structural basis of the NIP30-REGγ interface not resolved
   • Full substrate repertoire beyond p21 not mapped
3. 2022 High

   Revealed an unanticipated second function by showing FAM192A is a structural component of exon-ligation-stage spliceosomes, contacting the U2 snRNA-branch site duplex.

   Evidence Cryo-EM of human pre-C*-I and pre-C*-II complexes at 3.6 Å

   PMID:35705093

   Open questions at the time

   • Did not establish functional consequence of the contact for splicing outcome
   • Connection between proteasome and spliceosome roles unaddressed
4. 2023 High

   Provided functional and genetic proof via the yeast ortholog Fyv6 that the protein is required for exon ligation and proper 3' splice site selection.

   Evidence FYV6 deletion, genetic interactions with Prp8/Prp16/Prp22, in vitro splicing and in vivo reporter assays

   PMID:37625852

   Open questions at the time

   • Direct human FAM192A functional splicing assays not performed
   • Mechanism of 3' splice site discrimination not yet structurally defined
5. 2024 High

   Defined the structural mechanism: Fyv6 uniquely contacts the Prp22 ATPase mutually exclusively with the first-step factor Yju2, enforcing consensus, branch point-distal 3' splice site usage transcriptome-wide.

   Evidence 2.3 Å cryo-EM of yeast product complex, RNA-seq, suppressor screen, structure-guided mutagenesis

   PMID:39688371

   Open questions at the time

   • Human FAM192A-Prp22 contact not directly demonstrated
   • How the same protein partitions between spliceosomal and proteasomal pools is unknown
6. 2024 Medium

   Extended the proteasome-regulatory axis to a physiological context, showing the CK2-dependent NIP30 phosphoswitch governs REGγ-20S degradation of TRAF6 to control osteoclast activity.

   Evidence BMM-specific REGγ and Traf6 conditional KO mice, CKII inhibitor (TTP22), bone loss assays, proteomics

   PMID:39536082

   Open questions at the time

   • Biochemical dissection of NIP30 phosphorylation in this context is limited to one study
   • Direct demonstration that NIP30 bridges REGγ to TRAF6 is incomplete

• How a single protein is partitioned and regulated between its nuclear proteasome-regulatory and spliceosomal functions, and whether these activities are mechanistically coupled, remains unresolved.
• No study integrates the proteasome and spliceosome roles
• Determinants of pool allocation unknown
• Human splicing function shown only structurally, inferred functionally from yeast