iwr1 (SPAC23H4.08 / O13951) — S. pombe — curation notes

Identity (verified from UniProt record iwr1-uniprot.txt)

• UniProt: O13951, IWR1_SCHPO, 277 aa, reviewed. PE level 3 (Inferred from homology).
• Gene: iwr1; systematic/ORF name SPAC23H4.08; Chromosome I.
• Taxon: Schizosaccharomyces pombe 972 / ATCC 24843, NCBITaxon:284812.
• RecName: "RNA polymerase II nuclear localization protein iwr1".
• Family: IWR1/SLC7A6OS family (UniProt SIMILARITY, ECO:0000305).
• Domains/family signatures (DR lines):
• Pfam PF08574 (Iwr1)
• InterPro IPR040150 (Iwr1), IPR013883 (TF_Iwr1_dom / "Interactor of Rpb1")
• PANTHER PTHR28063 = "RNA POLYMERASE II NUCLEAR LOCALIZATION PROTEIN IWR1" (SF1 same name)
• Feature table: C-terminal disordered region 222–277; MobiDB-lite compositional bias — basic+acidic (225–234), strongly acidic (251–277). No annotated structured/globular domain outside the family fold.
• UniProt curated statements (all by similarity, ECO:0000250 — i.e. NOT direct pombe experiment):
• FUNCTION: "Directs RNA polymerase II nuclear import."
• SUBUNIT: "Associates with RNA polymerase II."
• SUBCELLULAR LOCATION: Cytoplasm + Nucleus; "shuttles between the nucleus and cytoplasm."

GOA annotation landscape (iwr1-goa.tsv) — 9 annotations, ALL inferred

No experimental (IDA/IMP/IGI/IPI/IEP) annotation exists for pombe iwr1. Evidence is entirely:
- IBA (GO_REF:0000033, PANTHER PTN002821512 | SGD:S000002273): nucleus (is_active_in), cytoplasm (is_active_in), protein import into nucleus (involved_in).
- ISO (GO_REF:0000024, from SGD:S000002273 = S. cerevisiae IWR1/YDL115C): nucleus (is_active_in), protein import into nucleus (involved_in).
- IEA (GO_REF:0000044, UniProt-SubCell): nucleus, cytoplasm (located_in).
- IEA (GO_REF:0000002, InterPro IPR040150): protein import into nucleus (involved_in).
- ND (GO_REF:0000015, PomBase): molecular_function root GO:0003674 — i.e. PomBase records no experimentally-supported MF.

=> This is a genuinely MF-dark / mechanistically understudied gene in pombe: its curated
function is inferred by orthology and family signatures, although high-throughput deletion
phenotypes exist.

Ortholog literature (S. cerevisiae IWR1 = SGD:S000002273; the source of the ISO transfers)

The pombe annotations transfer from experimentally characterized S. cerevisiae IWR1.

• Czeko et al. 2011, Mol Cell PMID:21504834 — S. cerevisiae (abstract-only in cache; full_text_available: false):
• "Pol II nuclear import requires the protein Iwr1" PMID:21504834
• Mechanism: "Iwr1 binds Pol II in the active center cleft between the two largest subunits, maybe facilitating or sensing complete Pol II assembly in the cytoplasm." PMID:21504834
• "Iwr1 then uses an N-terminal bipartite nuclear localization signal that is recognized by karyopherin α to direct Pol II nuclear import." PMID:21504834
• Recycling: "In the nucleus, Iwr1 is displaced from Pol II by transcription initiation factors and nucleic acids, enabling its export and recycling." PMID:21504834

• Specificity/conservation: "Iwr1 function is Pol II specific, transcription independent, and apparently conserved from yeast to human." PMID:21504834

• Esberg et al. 2011, PLoS ONE PMID:21695216 — S. cerevisiae (full text in cache):

• Originally identified via physical interaction with Pol II: "Iwr1, a protein conserved throughout eukaryotes, was originally identified by its physical interaction with RNA polymerase (Pol) II." PMID:21695216
• Broader PIC role: "Iwr1 plays an important role in preinitiation complex formation by all three nuclear RNA polymerases." PMID:21695216
• iwr1 mutant defects: "an iwr1 mutant strain shows reduced association of TBP and Pol III at Pol III promoters, a decreased rate of Pol III transcription, and lower steady-state levels of Pol III transcripts." PMID:21695216

• The paper explicitly argues that the Pol III defect is not merely an indirect consequence
  of reduced Pol II transcription and reports a weak or indirect Pol III association. These
  are additional budding-yeast roles; conservation in pombe is untested.

• Gómez-Navarro & Estruch 2015 PMID:26455955 — S. cerevisiae:

• IWR1 is non-essential and Iwr1-independent import routes exist; Pol II subunits can enter as
  partial assemblies or individually. Iwr1 is therefore a facilitating, not obligate, importer.

• Gómez-Navarro et al. 2017 PMID:28258010 — S. cerevisiae:

• Iwr1 interacts with elongating Pol II and contributes to polymerase disassembly from
  chromatin, especially after DNA damage. Import is not its only demonstrated direct role.

KNOWN (by orthology + family signature, not pombe experiment)

• Molecular role: facilitating RNA polymerase II assembly/import chaperone — binds the assembled 12-subunit Pol II in its active-center cleft; not a general karyopherin but an adaptor that presents its own bipartite NLS to karyopherin-α. Alternative import routes exist in budding yeast.
• Biological role: protein import into nucleus (of Pol II specifically), enabling nuclear transcription; cyclic loading/unloading and recycling.
• Localization: nucleocytoplasmic shuttling (cytoplasm + nucleus).

NOT known for pombe iwr1 (knowledge gaps — primary deliverable)

• No direct S. pombe experimental evidence for ANY molecular function (PomBase MF = ND).
• Whether the Pol II import mechanism is conserved in pombe. PomBase records a viable vegetative deletion population, but viability does not establish the molecular route.
• Whether pombe iwr1 physically binds pombe Pol II (Rpb1/Rpb2 cleft) — no pombe IPI.
• Targeted mechanistic deletion analysis in pombe. PomBase does record a viable deletion plus
  high-throughput decreased mating efficiency and multiseptate cells [PMID:28410370,
  "a systematic screen for morphological abnormalities of the mating process in fission yeast"],
  loss of stationary-phase viability [PMID:34250083, "to screen for mutants showing altered
  lifespan during stationary phase"], and hydroxyurea/MMS resistance [PMID:37787768,
  "We assayed colony-growth phenotypes to measure the fitness of deletion mutants for 3509
  non-essential genes in 131 conditions with different nutrients, drugs, and stresses."].
  PomBase's current SPAC23H4.08 API record links these exact FYPO calls to the
  cited high-throughput studies; none establishes the import mechanism.
• Whether it has any Pol I/Pol III-related or transcription-elongation roles in pombe.
• Precise molecular-function curation remains absent (MF ND). This review proposes GO:0000993
  RNA polymerase II complex binding by ISS. GO:0030674 protein-macromolecule adaptor activity
  was considered but not proposed because cargo-to-karyopherin bridging has not been directly
  shown in S. pombe and GO:0000993 more directly captures the ortholog-supported interaction.
• PMID:16823372 was checked as a possible direct S. pombe localization source. Its cached
  abstract does not identify iwr1, and the current iwr1 GOA has no HDA annotation citing it;
  therefore it is not treated as gene-specific localization evidence.

Curation reasoning for actions

• BP GO:0006606 protein import into nucleus: well-supported by orthology + family; the IBA (best phylogenetic evidence) is the core representative; ISO and InterPro-IEA are corroborating duplicates — KEEP core one, mark the duplicate lower-evidence ones as non-core/over-annotated as appropriate (do not REMOVE, all consistent).
• CC GO:0005634 nucleus + GO:0005737 cytoplasm: consistent with shuttling; IBA is_active_in reflects site of action. Keep; SubCell IEA duplicates are redundant but not wrong.
• MF GO:0003674 root with ND: keep the imported GOA row as reviewed, but recognize ND as a
  no-data placeholder rather than a negative assertion. Propose GO:0000993 as NEW with ISS,
  consistent with the orthology evidence and without implying direct S. pombe experimentation.
• Never REMOVE the ISO/IBA import annotation: it is a defensible orthology transfer from an experimentally solid budding-yeast function; the family signature (Pfam Iwr1) directly supports it.