mph1 (Mps1 homolog, S. pombe) — Gene Review Notes

UniProt: O94235 (MPS1_SCHPO). PomBase: SPBC106.01. Gene name mph1, synonym mps1.
EC 2.7.12.1 (dual-specificity protein kinase). 678 aa, kinase domain 316–607, ATP binding
322–330 + 345, active-site (proton acceptor) D442 (PROSITE).

NOTE: S. pombe mph1 = the Mps1-family spindle assembly checkpoint (SAC) kinase. This is
unrelated to budding-yeast/human MPH1 (a DNA helicase). Do not confuse.

Identity and family

• Mph1 (Mps1p-like pombe homolog) is a member of the Mps1-like family of dual-specificity
  protein kinases, required for the spindle checkpoint in S. pombe; functions upstream of
  mad2 PMID:9601094.
• Functionally related to S. cerevisiae Mps1, but unlike Mps1, mph1 is NOT required for
  spindle pole body duplication and is not essential [PMID:9601094 "They differ in that Mps1p,
  but not mph1, has an additional essential role in spindle pole body duplication"; PMID:22281223
  "S. pombe Mph1 is the homolog of S. cerevisiae Mps1, but it is neither required for spindle
  pole duplication nor essential for cell viability"].

Catalytic activity (MF)

• Mph1 is an active kinase that phosphorylates itself and Mad2 in vitro; D459A "kinase-dead"
  allele abolishes activity PMID:22281223.
• Phosphorylates Spc7 in vitro on threonine residues of MELT motifs PMID:22521786.
• Dual specificity (Ser/Thr/Tyr) — UniProt assigns EC 2.7.12.1 with Ser, Thr, and Tyr Rhea
  reactions. The directly demonstrated targets in S. pombe are Ser/Thr (Spc7 MELT threonines,
  Mad2/Mad3 serine/threonine sites). Tyr autophosphorylation is a family property; no direct
  pombe Tyr substrate demonstrated in cached refs but family is dual-specificity PMID:9601094.

Core BP: spindle assembly checkpoint (mitotic)

• Mph1 phosphorylates kinetochore Spc7 (KNL1) MELT repeats; this promotes Bub1-Bub3 recruitment
  required for SAC PMID:22660415.
• Phosphodependent recruitment of Bub1 and Bub3 to Spc7 maintains the SAC PMID:22521786.
• mph1Δ and kinase-dead are checkpoint defective; required for checkpoint arrest PMID:22281223.
• Mph1 kinetochore localization is upstream/apical in SAC recruitment hierarchy [PMID:22825872
  "recruitment of the kinase Mph1 is of vital importance for a stable SAC arrest. An Mph1 mutant
  that eliminates kinetochore enrichment abolishes SAC signaling, whereas forced recruitment of
  this mutant to kinetochores restores SAC signaling"; "a three-layered hierarchy with Ark1 and
  Mph1 on top, Bub1 and Bub3 in the middle, and Mad3 as well as the Mad1-Mad2 complex at the
  lower end"].
• Centromere-tethered Mph1 is sufficient to recruit Bub1 (but not stable Mad1) and to arrest
  cells in a checkpoint-dependent manner [PMID:22184248 "localization of Mph1 at
  centromeres/kinetochores is sufficient to recruit Bub1"; "the fusion protein arrests cell
  cycle progression in a spindle-checkpoint dependent manner in fission yeast"].
• Mph1 kinase activity required for Mad2/Mad3 to stably bind APC/C (MCC-APC/C maintenance)
  [PMID:22281223 "Mad2p and Mad3p displayed a significantly reduced ability to bind stably to
  fission yeast APC/C in mph1-kd"; "Mph1 kinase activity is required to assemble and/or maintain
  MCC-APC/C complexes"].
• Mph1 phosphorylates Mad3 to inhibit Cdc20Slp1-APC/C and maintain checkpoint arrests
  PMID:26882497.
• Bub3-Bub1 binding to Spc7 MELT array toggles the checkpoint switch by licensing Bub1-Mad1-Mad2
  interaction, downstream of Mph1 phosphorylation [PMID:27618268 "Mph1 (Mps1) phosphorylates
  multiple conserved MELT motifs in the Spc7 (Spc105/KNL1) protein to recruit Bub1, Bub3, and
  Mad3 (BubR1) to kinetochores"; "multisite binding of Bub3 to the Spc7 MELT array toggles the
  spindle checkpoint switch by permitting Mph1 (Mps1)-dependent interaction of Bub1 with
  Mad1-Mad2"]. (This paper is a downstream-mechanism study; mph1 EXP MF annotation derived from it.)

Localization (CC)

• Localizes to kinetochore, especially unattached kinetochores [PMID:22660415 "MPS1/Mph1 kinase
  locating at the unattached kinetochores initially creates a mark"; this paper is the IDA
  source for kinetochore]. Also kinetochore IDA from PMID:22184248.
• Nucleus and cytosol from global YFP localization (HDA) [PMID:16823372 — genome-wide ORFeome
  localization study; mph1 scored nuclear/cytosolic].

Chromosome segregation / biorientation (BP)

• Mph1 kinase has chromosome segregation functions even without spindle perturbation; mph1Δ and
  kd elevate chromosome loss PMID:22281223. Phosphorylation of Spc7 MELT promotes chromosome alignment/
  biorientation PMID:22660415.

Meiosis (BP)

• mph1Δ shows meiosis I homolog non-disjunction; required for proper homolog disjunction at
  meiosis I [PMID:23370392 "Mph1, a member of the Mps1 family of spindle assembly checkpoint
  kinases, is required to prevent meiosis I homolog non-disjunction"; "We thus conclude that
  Mph1 is required for efficient homolog disjunction during meiosis I"]. Likely due to precocious
  anaphase I entry from a weakened SAC.
• Meiotic centromeric cohesion protection in anaphase I (GO:1990813), IMP from PMID:28497540
  (Miyazaki et al., meikin/Moa1-Plo1-Spc7-Bub1 pathway). Cached file is abstract-only; the SAC
  kinase Mph1 phosphorylating Spc7 in the Bub1-recruitment pathway is consistent with a meiotic
  cohesion-protection role via Bub1/Sgo1. Full text not in cache — defer to PomBase curator.

Summary of action decisions

• MF kinase activities (protein Ser/Thr kinase IDA/EXP; Ser/Thr/Tyr; protein kinase; ATP
  binding; protein Ser kinase RHEA; protein Tyr kinase RHEA) — ACCEPT core; consolidate.
  Generic "protein kinase activity" IEA kept as non-core (subsumed by specific term).
• BP mitotic SAC signaling (multiple IMP/EXP/IBA/IEA) — ACCEPT core.
• BP nuclear chromosome segregation / chromosome segregation — KEEP_AS_NON_CORE (general,
  downstream consequence; SAC + biorientation is the mechanistic core).
• BP protein localization to kinetochore (IBA) — ACCEPT (Mph1 recruits Bub1-Bub3 etc).
• BP meiosis I, meiotic SAC signaling, meiotic centromeric cohesion protection — KEEP_AS_NON_CORE
  (meiosis-specific, real but not the central mitotic function).
• CC kinetochore (IDA) — ACCEPT core. nucleus, cytosol (HDA) — KEEP_AS_NON_CORE.