mus81 (SPCC4G3.05c, P87231) — S. pombe — review notes

Summary

Mus81 is the catalytic subunit of the Mus81-Eme1 structure-specific DNA endonuclease (XPF/ERCC4 nuclease family). With its obligate partner Eme1 it cleaves branched DNA substrates — nicked Holliday junctions, 3'-flaps, D-loops, model replication forks — using an ERCC4 nuclease domain and a Mg2+ cofactor. Two catalytic aspartates (D395/D396) are essential. In fission yeast it is the principal/essential resolvase for meiotic crossover formation (no MSH4-MSH5 backup pathway) and processes stalled/collapsed replication forks in mitosis. Cds1-dependent phosphorylation of Mus81 promotes its dissociation from chromatin during replication stress, whereas Cdc2- and Rad3-dependent phosphorylation of Eme1 activates the complex after DNA damage.

Key evidence

Endonuclease activity / Holliday junction resolvase

• PMID:11719193 Mus81-Eme1 are subunits of a nuclear Holliday junction resolvase; required at a late step of meiotic recombination; mus81 meiotic defect rescued by a bacterial HJ resolvase. Catalytic mutant DD->AA (D395/D396) abrogates endonuclease activity (UniProt MUTAGEN).
• PMID:14527419 Nick-and-counternick mechanism; "HJs accumulate in a DNA polymerase alpha mutant that lacks Mus81, providing further evidence that the Mus81-Eme1 complex targets HJs in vivo."
• PMID:17363897 Robust intact-HJ cleavage; proposed as a failsafe backup to its main nicked-HJ cleavage.
• PMID:11741546 Human Mus81 (XPF homolog) cleaves HJs; supports cross-species MF.

Meiotic crossover / reciprocal recombination

• PMID:14704204 Mus81 essential for meiotic crossovers but not gene conversion.
• PMID:25414342 IGI with rad55 (SPAC3C7.03c) — paralogs/mediators promote Mus81-Eme1-dependent crossovers.
• PMID:15466419 Mus81-Eme1 resolves meiotic joint molecules (TAS for joint molecule formation pathway).

Replication fork processing / DSB repair / damage tolerance

• PMID:11073977 Mus81 interacts with Cds1 FHA1; "Inactivation of Mus81 triggers a checkpoint-dependent delay of mitosis." Damage tolerance; required in absence of Rqh1.
• PMID:19037101 Mus81 required for S-phase DNA damage checkpoint (replication slowing).
• PMID:17307401 mus81 required for repair of replication-associated DSBs (HR).
• [PMID:28586299 "loss of mus81 causes a > 2 fold reduction in SDDs ... indicating that Mus81-Eme1 specifically promotes SDDs"; "Mus81-Eme1 could indeed resolve an IFSA junction into two nicked/gapped linear duplex DNA products"] Mus81 processes replication-fork convergence junctions (replication fork processing IMP).
• PMID:14993467 DNA damage tolerance (TAS).

Localization / complex

• UniProt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:11719193}.
• PMID:16823372 PomBase derives both nucleus (HDA) and mitochondrion (HDA) from this genome-wide localization study. The cache is abstract-only and does not expose the Mus81 image or gene-specific assay details, so the experimental mitochondrial call must remain UNDECIDED rather than being rejected from incomplete evidence. The IC "mitochondrial DNA metabolic process" (GO:0032042) remains over-annotated because no direct Mus81 mtDNA phenotype or activity is known.
• PMID:10954073 The distinct resolvase SpCCE1/Ydc2 is directly localized to mitochondria; its loss causes aggregated mtDNA.
• PMID:12823554 Catalytically active, full-length Ydc2 rescues the mtDNA phenotype, establishing a dedicated mitochondrial resolvase. This is competing context but does not by itself exclude a minor mitochondrial Mus81 pool.
• ComplexPortal CPX-26589: MUS81-EME1 structure-specific endonuclease complex. PMID:17363897 supports complex + nuclear replication fork localization.

Checkpoint and cell-cycle regulation

• PMID:11073977 Mus81 physically interacts with the Cds1 checkpoint kinase.
• PMID:15805465 Cds1-dependent Mus81 phosphorylation restrains fork cleavage during acute HU arrest.
• PMID:23584455 DNA damage stimulates Mus81-Eme1 through phosphorylation of Eme1. The full text distinguishes this activating pathway from Cds1: Eme1 hyperphosphorylation still occurs without Cds1.

Curation considerations

• "protein binding" (GO:0005515, IPI with eme1 SPAPB1E7.06c, PMID:11719193) is uninformative; the informative call is the Mus81-Eme1 complex (GO:0048476) and endonuclease MF. Mark over-annotated; complex membership captures the meaningful interaction.
• GO:0033314 (mitotic DNA replication checkpoint signaling, IMP): keep as non-core. mus81 loss triggers a checkpoint-dependent delay, showing that the checkpoint responds to accumulated lesions; a direct signaling role rests on the authors' proposed recruitment of Cds1 to aberrant DNA structures PMID:11073977.
• GO:0006301 (DNA damage tolerance, TAS): keep as non-core because it is a broad PRR context rather than Mus81's direct catalytic role.
• GO:0006308 (DNA catabolic process, IEA/InterPro) is an over-general parent; the specific endonuclease/resolution terms are better.
• GO:0005739 (mitochondrion, HDA): UNDECIDED because the gene-specific image/full text is unavailable; focused nuclear evidence and Ydc2 biology cannot overrule an experimental curator from incomplete evidence.
• GO:0032042 (mitochondrial DNA metabolic process, IC): weak; rests on the HDA mitochondrial localization plus nuclease activity with no direct Mus81 mtDNA data. Mark as over-annotated / non-core.
• Core: structure-specific (crossover junction / HJ) endonuclease; Mus81-Eme1 complex; nucleus; meiotic crossover/resolution of recombination intermediates; replication fork processing & DSB repair.

2026-09-01 refresh provenance

• Refetched UniProt and GOA with just fetch-gene SCHPO mus81 --force; the new PAINT nucleus IBA is accepted because it agrees with direct S. pombe evidence PMID:11719193.
• Ran the OpenScientist hypothesis wrapper for mitochondrial localization. The report usefully surfaced Ydc2/SpCCE1 and the lack of focused Mus81 mtDNA evidence, but its "refuted" verdict exceeded the accessible evidence. The review therefore applies curator deference to the HDA call and records the functional IC inference separately.