iwr1 is the fission-yeast member of the conserved eukaryotic Iwr1/SLC7A6OS family (Pfam PF08574; InterPro IPR040150/IPR013883, "Interactor of Rpb1"), represented by a single copy per genome. In the well-characterized budding-yeast ortholog the protein is an RNA polymerase II biogenesis factor: it binds the fully assembled 12-subunit Pol II in the active-center cleft between the two largest subunits in the cytoplasm, and presents its own N-terminal bipartite nuclear localization signal to karyopherin-alpha to drive nuclear import of the polymerase. Inside the nucleus it is displaced from Pol II by transcription initiation factors and nucleic acids, allowing its re-export and recycling, so it acts as a cyclic, Pol II-specific, transcription-independent import adaptor rather than a stable subunit. It facilitates this route but is not obligate, because assembled subunits can also enter by Iwr1-independent pathways; budding-yeast Iwr1 additionally acts during Pol II elongation and disassembly from chromatin. Consistent with this shuttling behaviour the protein distributes between cytoplasm and nucleus. The S. pombe protein (277 aa) carries the family fold plus a long, acidic, intrinsically disordered C-terminus. Its function in S. pombe has not been assayed directly and is inferred from orthology and family conservation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Nuclear localization is consistent with the conserved function of the Iwr1 family: the import adaptor accompanies RNA polymerase II into the nucleus and is displaced there before recycling. IBA (phylogenetic) is the strongest of the several redundant evidence lines supporting nuclear localization. Reason: Phylogenetic inference over the Iwr1 family (PTHR28063) supports nuclear localization; consistent with UniProt SUBCELLULAR LOCATION (nucleus) and the nucleocytoplasmic shuttling described for the S. cerevisiae ortholog (PMID:21504834). Accepted as a site intrinsic to the import cycle. Supporting Evidence: PMID:21504834 Iwr1 then uses an N-terminal bipartite nuclear localization signal that is recognized by karyopherin alpha to direct Pol II nuclear import. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasmic localization is consistent with the family function: Iwr1 binds the newly assembled Pol II in the cytoplasm before escorting it into the nucleus, and the protein shuttles between the two compartments. Reason: Phylogenetic inference plus UniProt SUBCELLULAR LOCATION supports the cytoplasm as the site where the ortholog loads assembled Pol II before nuclear import. Supporting Evidence: PMID:21504834 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. |
| GO:0006606 protein import into nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: This is the core biological role assigned by orthology: the Iwr1 family directs nuclear import of RNA polymerase II. The IBA annotation is the best-evidence (phylogenetically inferred) representative of this function for S. pombe iwr1, whose own role has not been assayed directly. Reason: Well supported by the defining S. cerevisiae study (PMID:21504834) and by the Iwr1 Pfam/InterPro family signature (PF08574 / IPR040150), which InterPro maps to this same term. Represents the core function; note the substrate is specifically Pol II, which GO:0006606 does not itself express (see knowledge gaps). Supporting Evidence: PMID:21504834 Here we show that Pol II nuclear import requires the protein Iwr1 and provide evidence for cyclic Iwr1 function. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation from the UniProt Subcellular Location vocabulary mapping, mirroring the nucleus localization already captured by the stronger IBA evidence above. Correct but redundant with the phylogenetic annotation. Reason: Consistent with the IBA nuclear annotation and UniProt SUBCELLULAR LOCATION. The electronic line is redundant, but the underlying location claim is accepted. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation from the UniProt Subcellular Location vocabulary mapping, mirroring the cytoplasm localization already captured by the stronger IBA evidence above. Correct but redundant with the phylogenetic annotation. Reason: Consistent with the IBA cytoplasm annotation and UniProt SUBCELLULAR LOCATION. The electronic line is redundant, but the underlying location claim is accepted. |
| GO:0006606 protein import into nucleus | IEA GO_REF:0000002 | ACCEPT | Summary: Electronic InterPro2GO annotation mapping the Iwr1 family signature (IPR040150) to protein import into nucleus. Correct in essence and consistent with the orthology-based function; redundant with the IBA/ISO annotations to the same core term. Reason: InterPro2GO (GO_REF:0000002) transfer of the Iwr1-family-to-GO:0006606 mapping; biologically defensible and consistent with the stronger IBA and curator ISO annotations for the inferred core process. |
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: PomBase records the molecular_function root with No Data, correctly reflecting that no molecular activity has been experimentally determined for S. pombe iwr1 and that no specific MF term has been curated. This gene is MF-dark at the level of direct evidence. Reason: Standard ND placeholder indicating that no molecular-function data were available to the curator. It is not a negative assertion and should be retired if the proposed orthology-supported RNA polymerase II complex binding annotation is adopted. |
| GO:0006606 protein import into nucleus | ISO GO_REF:0000024 | ACCEPT | Summary: Manual orthology transfer (ISO) from the experimentally characterized S. cerevisiae ortholog IWR1 (SGD:S000002273), which was directly shown to be required for RNA polymerase II nuclear import. This is the annotation that most directly reflects the curatorial basis for iwr1's assigned function. Reason: Curator-judged sequence-similarity transfer from S. cerevisiae IWR1, whose Pol II nuclear-import role is experimentally established (PMID:21504834). A defensible, high-confidence orthology assignment for a single-copy conserved family; do not remove. Supporting Evidence: PMID:21504834 Iwr1 function is Pol II specific, transcription independent, and apparently conserved from yeast to human. |
| GO:0005634 nucleus | ISO GO_REF:0000024 | ACCEPT | Summary: Manual orthology transfer (ISO) of nuclear localization from S. cerevisiae IWR1. Consistent with the IBA nucleus annotation and with the shuttling behaviour of the ortholog; corroborating the core nuclear-localization call. Reason: Orthology transfer duplicating the nucleus localization already accepted from IBA; redundant as an evidence line, but the underlying site-of-action claim is accepted consistently. |
| GO:0000993 RNA polymerase II complex binding | ISS PMID:21504834 Iwr1 directs RNA polymerase II nuclear import. | NEW | Summary: Proposed informative molecular-function annotation: the conserved Iwr1 activity is direct binding of assembled RNA polymerase II in its active-center cleft, the physical basis of its import-adaptor role. This is inferred for S. pombe by orthology and family signature, not demonstrated experimentally in this species. Reason: GO:0000993 specifically captures the experimentally established Pol II interaction of the budding-yeast ortholog. An ISS proposal is coherent with the existing ISO biological-process transfer; the molecular_function-root ND is a no-data placeholder, not evidence against adding an informative inferred term. Supporting Evidence: PMID:21504834 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. |
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Download this section (compressed HTML)Q: Does S. pombe iwr1 physically associate with fission-yeast RNA polymerase II, and is it required for the polymerase's nuclear localization as in S. cerevisiae?
Q: Which of the existing high-throughput deletion phenotypes reproduce in targeted assays, and do any track with defective Pol II nuclear import?
Q: Does S. pombe Iwr1 also act during Pol II elongation and disassembly from damaged chromatin, as demonstrated for the budding-yeast ortholog?
Experiment: Affinity-purify tagged iwr1 and test for co-purification of Rpb1/Rpb2; assess Rpb1 nuclear/cytoplasmic distribution by microscopy in wild-type vs iwr1-deletion cells; test genetic interaction with karyopherin-alpha (imp1/cut15).
Hypothesis: iwr1 binds assembled S. pombe Pol II and mediates its karyopherin-alpha-dependent nuclear import.
Experiment: Reproduce S. pombe iwr1-deletion resistance to hydroxyurea and MMS alongside matched S. cerevisiae sensitivity assays; quantify Pol II nuclear localization, chromatin occupancy, damage-arrest release, and nascent transcription; and test whether an Rpb3-NLS fusion separates import defects from nuclear elongation or chromatin-release functions. Treat failure to reproduce the high-throughput resistance calls as evidence for a screen-specific effect.
Hypothesis: The opposite genotoxin phenotypes of fission- and budding-yeast iwr1 mutants reflect a species-specific difference in Pol II import, elongation, or damage-arrest release, rather than a conserved directional response.
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