rad21 (S. pombe) — curation notes

UniProt: P30776 (RAD21_SCHPO). PomBase: SPCC338.17c. 628 aa. Essential gene on chromosome III.
Rad21 = alpha-kleisin subunit of the mitotic cohesin complex (ortholog of S. cerevisiae Scc1/Mcd1, human RAD21). Bridges the Psm1(Smc1) and Psm3(Smc3) ATPase heads, and binds Psc3(Scc3), to close the tripartite cohesin ring that topologically entraps sister chromatids.

Identity / discovery

• First identified as a radiation-sensitivity / DNA double-strand-break (DSB) repair gene; rad21-45 mutant (I67T) is radiation-sensitive and DSB-repair-deficient but still arrests in G2. Plasmid rad21 fully restores DSB repair. Essential for mitotic growth. PMID:1480481 PMID:1480481
• Nuclear, cell-cycle-regulated phosphoprotein; NLS in C-terminal third; multiply phosphorylated on serines; mRNA/protein peak near G1/S. PMID:7706319 PMID:7706319

Cohesin complex / cohesion

• Rad21 forms a stable complex with Psm1 and Psm3 (IP); cohesin subunits essential for sister chromatid cohesion; mutants show precocious sister chromatid separation; loss of any subunit gives same phenotype. PMID:11069892 PMID:11069892
• Cohesin enriched at broad centromere regions; centromere-associated Rad21 unchanged metaphase->anaphase (unlike budding yeast). PMID:11069892
• Mis4 (Scc2) loads Rad21 onto chromatin. PMID:11069892
• Cohesin established during S phase by Eso1 (Eco1/Ctf7 homolog); Pds5 modulates. PMID:11598020 [PMID:10779336 mutant "arrested in M phase with a viability loss due to a failure to establish sister chromatid cohesion during S phase"]

Anaphase cleavage / separase

• Rad21 cleaved specifically in anaphase at sites similar to Scc1; cleavage essential though only <5% cleaved; protease likely Cut1 (separase); noncleavable R179E/R231E double mutant blocks sister separation. PMID:11069892 PMID:11069892
• Securin(Cut2)-separase(Cut1) cleaves cohesin at anaphase to allow segregation. PMID:15329725
• Interphase separase cleavage of cohesin required for DNA repair: rad21 mutant epistatic to cut2(EA2) in UV sensitivity; cohesin hyper-modified after UV in Rad3-dependent way. PMID:15329725 PMID:15329725

Segregation / kinetochore

• Cohesion mutants (rad21-K1, mis4-242) activate Mad2/Bub1 spindle checkpoint due to diminished microtubule-kinetochore interaction; needed for normal spindle-kinetochore interaction. PMID:11882285 PMID:11882285
• Mitotic Rad21 supports equational (bipolar) segregation; replaced by Rec8 in meiosis. Substitutive Rad21 relocates to centromeres in rec8∆ and gives equational meiosis I division; Rad21 supports bipolar attachment, Rec8 monopolar. PMID:12748297 PMID:12748297
• Meiosis: Rec8 (not Rad21) is the meiosis-specific kleisin. PMID:10440376
• Telomere/arm separation: Aurora B promotes dissociation of Swi6/HP1 and cohesin Rad21 from telomeres for telomere dispersion. PMID:25778919
• Anaphase merotely correction context (Ase1) - rad21 used as cohesion/separation marker. [PMID:19948483 abstract is about Ase1/Prc1 spindle elongation; full text assays rad21 cohesion]

Biochemistry / ring mechanism (in vitro fission-yeast cohesin)

• Reconstituted fission yeast cohesin (Psm1/Psm3/Rad21/Psc3) topologically loads onto DNA; loading stimulated by Mis4/Ssl3 loader; cohesin binds dsDNA (ATP-independent) and topological loading is ATP-hydrolysis dependent; TEV cleavage of modified Rad21 releases DNA. PMID:24291789 PMID:24291789 PMID:24291789
• DNA entry/exit by interlocking gate; Wapl disengages kleisin (Rad21). PMID:26687354 (IPI partner SPAC10F6.09c = wpl1/Wapl)
• DNA-DNA tethering by cohesin ring -> second DNA captured (basis of cohesion/DNA-DNA interactions). PMID:29358048
• Mis4 DNA binding promotes topological entrapment; ATP-dependent topological co-entrapment activity. PMID:33176147
• Suppressor/3D modeling: interfaces Psm1-Rad21, Psm3-Rad21; Cut1 dispensable when interfaces destabilized -> hold/release of DNA. PMID:29735656
• Cohesin ATPases interface with regulators (Eco1, Wpl1) to modulate DNA tethering (S. cerevisiae study, conserved). PMID:26583750

Chromatin organization / loop extrusion / localization

• Cohesin extrudes DNA loops (Brownian ratchet model; fission yeast cohesin) -> chromatin looping. PMID:34309513
• Cohesin Rad21 is enriched at centromeres, some RNA genes, and subtelomeric regions; the paper's text does not explicitly identify rDNA among the Rad21 sites, so the rDNA-heterochromatin annotation remains unresolved. PMID:25847133
• Heterochromatin island (facultative): Rad21 IDA localization at heterochromatin islands. [PMID:22144463 — facultative heterochromatin islands at meiotic genes]
• Global localization (ORFeome GFP) — nuclear. [PMID:16823372 proteome-scale localization study]

Curation summary

Core MF: structural constituent / protein-macromolecule adaptor bridging SMC heads (avoid bare protein binding); topological DNA entrapment activity; dsDNA binding (contributes_to, ring-level).
Core BP: mitotic sister chromatid cohesion (establishment + maintenance), mitotic sister chromatid segregation, DSB repair, chromatin looping.
Core CC: mitotic cohesin complex; nucleus; chromosome/chromatin including centromeric, pericentric, and subtelomeric regions. The precise rDNA-heterochromatin localization remains unresolved.

Notes on specific GOA entries:
- GO:0005515 protein binding (IPI PMID:26687354, with wpl1) — bare protein binding; MODIFY toward adaptor/structural MF; the actual finding is Wapl-kleisin interaction in ring gating.
- GO:0005730 nucleolus (IDA PMID:12748297) — that paper is meiosis kinetochore-focused and contains no nucleolar statement in searchable full text; UNDECIDED. The separate rDNA-heterochromatin annotation is also UNDECIDED because PMID:25847133 has no explicit Rad21-at-rDNA sentence.
- GO:0045143 homologous chromosome segregation (IMP PMID:12748297) — this is the meiosis I bipolar-attachment phenotype of substitutive Rad21; keep as non-core (meiotic/non-canonical role).
- GO:0044820 mitotic telomere tethering at nuclear periphery (IMP PMID:25778919) — supported; non-core peripheral role.

2026-09-01 re-review journal

• Refreshed UniProt and all 41 GOA rows through just fetch-gene SCHPO rad21 --force; annotation coverage was unchanged. UniProt advanced from entry version 161 to 162 and the four current IEA source dates advanced to 2026-07-27.
• Re-read the cached full texts underlying the ring mechanism and major localization claims. Replaced the truncated PMID:29358048 quotation with the complete sentence, and grounded PMID:25847133 localizations in the paper's Rad21 ChIP-seq results: centromere/RNA-gene enrichment and explicit subtelomeric enrichment. PMID:29358048 PMID:25847133
• GO:1990342 heterochromatin island: the local PMID:22144463 cache was initially abstract-only, so a focused OpenScientist hypothesis job was launched through the repository wrapper. It recovered PMC6338074 and the direct Fig. S2 result: "the cohesin sub unit Rad21 enriched at meiotic heterochromatin islands (fig. S2)." NCBI independently confirmed the PMCID-to-PMID mapping, and just fetch-pmid 22144463 --force refreshed the primary-paper cache with full text. The annotation is retained as KEEP_AS_NON_CORE. The report also flags the PomBase exists_during meiotic-cell-cycle extension because the ChIP used log-growing vegetative cells; this is recorded as a curator question rather than altering the machine-sourced GOA row.
• GO:0033553 rDNA heterochromatin was changed to UNDECIDED: PMID:25847133 reports Rad21 at centromeres, subtelomeres, and some RNA genes but contains no text explicitly placing Rad21 at rDNA. The precise experimental annotation is retained without claiming that the broader sentence verifies it.
• PMID:12748297 is full text in the cache and supports the meiotic segregation claim, but a text search found no nucleolar statement. The nucleolus IDA therefore remains UNDECIDED; it is not removed because the PomBase curator may have used figure-level/full-text evidence not recoverable by text search. PMID:12748297
• Added typed molecular-function/process/location/complex fields to all five core-function records so downstream tools can consume the already-supported biological synthesis rather than only prose; the fifth record makes the accepted chromatin-looping function explicit.
• Follow-up review removed residual claims that PMID:25847133 directly places Rad21 at rDNA, including in the standalone description and reference finding. Evidence quotations were also tightened to use the direct dsDNA-binding result from PMID:24291789 and the rad21-K1 result from PMID:19948483.