slp1 (Cdc20/Fizzy ortholog) — review notes

Identity

UniProt P78972, SPAC821.08c, gene slp1. WD repeat-containing protein, 488 aa, 7 WD40 repeats (178–473). Belongs to the WD repeat CDC20/Fizzy family.
Fission-yeast ortholog of CDC20/p55CDC/Fizzy; the mitotic substrate-recognition co-activator of the anaphase-promoting complex/cyclosome (APC/C). PDB 4AEZ (88–488) is the WD40 beta-propeller; structure of the S. pombe MCC (Mad2-Mad3-Slp1/Cdc20) is PDB-related to PMID:22437499.

Slp1/Cdc20 is the transiently available APC/C activator early in mitosis; later in mitosis/G1 the activator switches to Ste9/Cdh1 PMID:18556659.
Cdc20Slp1-APC/C targets securin (Cut2) and cyclin B (Cdc13) for destruction by the 26S proteasome; once securin is destroyed, cohesin is cleaved and sister chromatids separate PMID:26882497.
Slp1 is essential for anaphase onset PMID:9461438.

Slp1/Cdc20 is the key target of the SAC; the mitotic checkpoint complex (MCC = Mad2-Mad3-Slp1/Cdc20 in S. pombe) binds and inhibits Cdc20-APC/C until kinetochores are correctly attached PMID:18556659.
Mad2 forms a complex with Slp1; disrupting the Slp1-Mad2 interaction abolishes the spindle checkpoint PMID:9461438. Mutation of Slp1 residues 131-132 (AF->PY) abrogates Mad2 binding and overrides checkpoint activation (UniProt MUTAGEN feature; PMID:9461438).
The S. pombe MCC crystal structure (Mad2, Mad3, Cdc20/Slp1) shows MCC inhibits the APC/C by obstructing degron recognition sites on Cdc20 and displacing Cdc20 from forming a bipartite D-box receptor with Apc10; C-Mad2 positions the Mad3 KEN-box to bind Cdc20 PMID:22437499.
Mad3 N-terminal KEN box (KEN1) is required for stable Mad3-Cdc20/Slp1 binding and MCC assembly PMID:18556659. Slp1-362 mutant cannot bind APC/C at restrictive temperature PMID:18556659.
Mps1Mph1 phosphorylation of Mad3 (near KEN2) directly potentiates inhibition of Cdc20Slp1-APC/C; in vitro Cdc20Slp1-APC/C ubiquitinates radiolabelled securin/Cut2 and Mad3 phosphomimics inhibit it PMID:26882497. This is the basis for the GO:1990757 (ubiquitin ligase activator activity) IDA annotation.

Fission-yeast MCC is exclusively Mad3-Mad2-Cdc20/Slp1 (no Bub3) PMID:18556659. Confirms part_of mitotic checkpoint complex (GO:0033597).

Slp1/Cdc20 is the only coactivator essential for meiosis I progression; slp1-NF410 ts cells delay anaphase I and stabilize Cut2/Cdc13 PMID:21389117. Supports GO:1990949 (metaphase/anaphase transition of meiosis I) and APC/C-dependent catabolic process IMP/EXP.
Mes1 is a meiosis-specific APC/C inhibitor and substrate that binds the WD40 domain of Fizzy-family activators including Slp1; ubiquitylation of Mes1 by APC/C-Slp1 relieves inhibition [PMID:18331722 "Mes1 directly binds the WD40 domain of the Fizzy family of APC/C activators"; PMID:21389117 "Slp1 alleviates Mes1 inhibition through its ubiquitylation"]. Mes1 (P41005, SPAC5D6.08c) interaction with Slp1 supports the protein-binding IPI annotations (PMID:15791259, PMID:21389117).
For meiosis II, Slp1 reactivation contributes; GO:1990950 (metaphase/anaphase transition of meiosis II) EXP from PMID:21389117 (both Slp1 and Fzr1/Mfr1 become active to complete anaphase II).

HDA proteome localization study (PMID:16823372) reports nucleus, cell division site, mitotic spindle pole body, mitotic spindle. Consistent with Cdc20 nuclear/mitotic-structure localization, but HDA from a global ORFeome-tagging study; nucleus is the core compartment (where APC/C substrates are degraded). Spindle/SPB/division-site are mitotic-apparatus locations consistent with function but are best kept as non-core given they come from a single high-throughput dataset.
Kinetochore (GO:0000776): NAS/TAS from PMID:15930132 (Mis6/Mad2 paper). This paper is about Mad2 kinetochore loading and does not directly assay Slp1 at kinetochore; the kinetochore localization annotation is curator-assigned (NAS/TAS). Cdc20/Slp1 transiently localizes to kinetochores in many systems. Keep as non-core (functions in checkpoint signalling that occurs at the kinetochore).

PMID:15791259 / PMID:21389117 with Mes1 (P41005, SPAC5D6.08c): bind to meiotic APC/C inhibitor Mes1 -> better captured as adaptor/APC/C activator function; replace bare "protein binding".
PMID:22437499 / PMID:9461438 with Mad2 (O14417): bind SAC protein Mad2 within MCC.
PMID:18331722 with SPAC5D6.08c (Mes1).
PMID:28178520 with SPAC18G6.15: Sgo2-dependent/Mad2-independent anaphase pathway paper. SPAC18G6.15 needs ID confirmation; treat the bare protein-binding term as over-annotation/uninformative -> replace with adaptor function or mark as over-annotated.

IBA (GO_REF:0000033) for anaphase-promoting complex (CC), positive regulation of APC/C catabolic process, APC/C binding, APC/C-dependent catabolic process, ubiquitin ligase activator activity: all consistent with strong experimental data; ACCEPT.
IEA InterPro2GO: GO:0010997 (APC/C binding) duplicate of experimental; GO:0097027 (ubiquitin-protein transferase activator activity) — APC/C is a RING E3 ligase, so "ubiquitin ligase activator activity" (GO:1990757) is the more accurate term; GO:0097027 is essentially equivalent/parent-ish and is supported. ARBA IEA GO:0031145 duplicate of experimental. Keep IEAs as ACCEPT (consistent) or note duplication.

MF: ubiquitin ligase activator activity (APC/C activator), APC/C binding (substrate-recognition WD40 co-activator/adaptor).
BP: APC/C-dependent catabolic process, positive regulation of APC/C-dependent catabolic process, metaphase/anaphase transition (mitosis and meiosis I/II), mitotic spindle assembly checkpoint signalling (as the SAC target).
CC: anaphase-promoting complex (slp1 variant), mitotic checkpoint complex, nucleus.