PYM1

PYM1 (PYM/WIBG) is a cytoplasmic factor that controls the lifecycle and positional specificity of the exon junction complex (EJC) on spliced mRNAs (PMID:14968132, PMID:19410547). Its N-terminal domain directly and simultaneously contacts the MAGOH–Y14 (RBM8A) heterodimer, capping their dimerization interface, while a separate domain engages the 40S ribosomal subunit and the 48S preinitiation complex, allowing PYM1 to bridge EJC-bearing mRNAs to the translation machinery and enhance translation of intron-derived transcripts (PMID:14968132, PMID:18026120). Acting on fully assembled EJCs (but not assembly intermediates), PYM1 dissociates the complex from mRNA to drive EJC recycling; overexpression strips EJCs and suppresses nonsense-mediated decay (NMD), whereas depletion causes EJCs to accumulate on spliced mRNAs (PMID:19410547). This recycling function enforces EJC deposition specificity: loss of the PYM1–EJC interaction permits non-canonical EJCs to accumulate away from canonical exon-exon junctions, especially on intron-poor transcripts, where they trigger aberrant NMD, and PYM1 depletion stabilizes mRNAs with few, long exons that localize to ER-associated TIS-granules (PMID:40885765). PYM1 activity is autoregulated: its first 160 residues are intrinsically disordered and bind RNA in a sequence-independent, fuzzy manner that is sterically incompatible with EJC engagement, providing a negative regulatory input on recycling (PMID:37065448). PYM1 is a host target hijacked by viruses—KSHV ORF57 recruits PYM1 to intronless viral mRNAs to promote their translation (PMID:20436455), and the flavivirus capsid protein binds PYM1 to mimic its depletion and reshape host EJC occupancy and gene expression (PMID:40885765, PMID:40161626).

1. 2004 High

   Established the structural basis of how PYM recognizes the EJC core: a single N-terminal domain that simultaneously contacts Mago and Y14 by capping their heterodimer interface.

   Evidence 1.9 Å crystal structure of the Drosophila PYM–Mago–Y14 ternary complex with in vitro pulldowns and nuclear export assays

   PMID:14968132

   Open questions at the time

   • Did not establish the cellular consequence of EJC binding (recycling vs. disassembly)
   • Functional role of regions outside the N-terminal Mago-Y14-binding domain unresolved
2. 2007 High

   Showed PYM is a bifunctional bridge, using one domain for the EJC and a separate domain for the 40S/48S ribosomal complex to couple EJC-marked spliced mRNAs to enhanced translation.

   Evidence Reciprocal Co-IP, domain mapping, ribosome sedimentation, and siRNA knockdown with intron-containing vs. intronless translation reporters

   PMID:18026120

   Open questions at the time

   • Did not determine whether ribosome engagement is required for EJC removal
   • Mechanism linking translation initiation to EJC fate not defined
3. 2009 High

   Defined PYM as an EJC disassembly and recycling factor, linking its activity directly to suppression of NMD.

   Evidence In vitro EJC disassembly reconstitution plus gain- and loss-of-function in cells with NMD reporter assays

   PMID:19410547

   Open questions at the time

   • Did not separate translation-dependent from translation-independent disassembly
   • Genome-wide consequences for EJC positioning not addressed
4. 2014 High

   Demonstrated in vivo that PYM regulates EJC homeostasis and is autoregulated by its own C-terminus, with disruption causing mRNA mislocalization and developmental defects.

   Evidence Drosophila transgenic overexpression/loss-of-function, ternary complex biochemistry, RIP, and genetic epistasis with y14/mago

   PMID:24967911

   Open questions at the time

   • DmPYM lacks ribosome binding, so translation-coupling conclusions do not transfer directly to humans
   • C-terminal autoregulatory mechanism not structurally resolved
5. 2023 Medium

   Revealed an intrinsic autoinhibitory mechanism: the disordered N-terminal region binds RNA sequence-independently in a manner mutually exclusive with EJC binding.

   Evidence NMR spectroscopy and RNA-binding assays on the PYM1-160 disordered region

   PMID:37065448

   Open questions at the time

   • Functional downregulation of EJC recycling by RNA binding is inferential, not directly demonstrated in cells
   • Physiological RNA targets and stoichiometry unknown
6. 2025 High

   Separated PYM1's two activities and established its role as an EJC positional-specificity factor that prevents non-canonical EJC deposition and aberrant NMD, a function viruses exploit.

   Evidence PYM1 interaction-deficient EJC mutants in HEK293 cells, eCLIP/iCLIP occupancy mapping, RNA-seq, NMD reporters, and flavivirus infection

   PMID:40885765

   Open questions at the time

   • Mechanism by which PYM1 selectively targets non-canonical EJCs not defined
   • Link between stabilized long-exon mRNAs and TIS-granule localization mechanistically unresolved
7. 2025 Medium

   Identified the flavivirus capsid protein as a direct PYM1 interactor that phenocopies PYM1 depletion to reshape host mRNA regulation.

   Evidence Co-IP of PYM1–capsid interaction with viral infection and RNA-seq comparison (preprint)

   PMID:40161626

   Open questions at the time

   • Preprint version; binding interface on PYM1 not mapped
   • Functional consequence for viral replication not established

• How PYM1 distinguishes canonical from non-canonical EJC positions and how its RNA-binding autoinhibition is regulated in vivo remain unresolved.
• No structural model of PYM1 acting on a non-canonically positioned EJC
• Trigger that switches PYM1 between RNA-bound (autoinhibited) and EJC-bound states unknown
• Physiological regulators of PYM1 recycling activity uncharacterized