ppk34 / ckk2 (SPCC1919.01) - Research Notes

Gene Identity

• UniProt: Q9UU87
• PomBase: SPCC1919.01
• Current PomBase/GOA symbol: ppk34
• Literature alias: ckk2 (calcium/calmodulin-dependent protein kinase kinase Ckk2)
• ORF names: SPCC1919.01, SPCC830.12
• 354 amino acids, serine/threonine kinase domain (residues 40-331)

Nomenclature History

ppk34 was the systematic name assigned by Bimbo et al. 2005 PMID:15821139 in their "Systematic deletion analysis of fission yeast protein kinases." Cisneros-Barroso et al. 2014 PMID:25081204 renamed the gene ckk2 after identifying it as a CaMKK2 homolog: "In addition to Ssp1, we found a second sequence homologous to mammalian CaMKK: SPC1919.01, named Ppk34 for the uncharacterised putative protein kinase 34 in a systematic deletion analysis of fission yeast kinases. Due to its homology to the CaMKK proteins, we have renamed it Ckk2." The current PomBase GOA export fetched on 2026-09-01 nevertheless uses ppk34 as the gene-product symbol, so this review retains ppk34 and records ckk2 as an alias.

2026-09-01 — current GOA and evidence refresh

• Refreshed UniProt and all 14 GOA rows with just fetch-gene SCHPO ppk34 --force;
  qualifiers were backfilled for every annotation and no GOA rows were added or removed.
• Re-read the full text of PMID:25639242 and corrected the nitrogen-stress synthesis:
  Ppk34 is required for the stress-induced increase in Ssp2 T189 phosphorylation, but
  direct Ppk34 phosphorylation of Ssp2 is not demonstrated and indirect regulation
  through an AMPK phosphatase remains possible PMID:25639242.

Domain Architecture

• CDD: cd14008 (STKc_LKB1_CaMKK) - classifies it in the LKB1/CaMKK subfamily
• PANTHER: PTHR43895 (Calcium/calmodulin-dependent protein kinase kinase-related)
• PANTHER subfamily: PTHR43895:SF150 (Serine/threonine-protein kinase STK11). This
  automated subfamily placement may be imprecise at the ortholog level, but is consistent
  with a broader evolutionary AMPK-kinase relationship; it does not establish that
  Ppk34 directly phosphorylates Ssp2.
• The target-specific full-text evidence remains decisive: Ppk34 is required for
  stress-induced Ssp2 activation, while PMID:25639242.
• InterPro: IPR000719 (Protein kinase domain), IPR008271 (Ser/Thr kinase active site)
• Pfam: PF00069 (Pkinase)

Homology to Mammalian CaMKKs

From PMID:25081204: "Ckk2 is most homologous to mammalian CaMKK2 (34%), followed by Ssp1 (32%) and CaMKK1 (28%)."

S. pombe has two CaMKK-like kinases:
1. Ssp1 - the primary CaMKK ortholog, constitutively required for AMPK (Ssp2) T-loop phosphorylation
2. Ckk2/ppk34 - a second CaMKK, with specific roles in calcium and nitrogen stress signaling

Core Functions (Experimentally Demonstrated)

Function 1: Activation of Cmk1 in calcium signaling (PMID:25081204)

• Ckk2 phosphorylates and activates the CaM-dependent kinase Cmk1 in response to Ca2+ stress
• Active Cmk1 then phosphorylates and inactivates the Prz1 transcription factor (calcineurin/NFAT homolog)
• This constitutes a negative feedback loop: Ca2+ activates both calcineurin (which activates Prz1) and the Ckk2/Cmk1 cascade (which inactivates Prz1)
• "Ckk2 counteracts calcineurin function by negatively regulating Prz1 activity" PMID:25081204
• Delta-ckk2 cells show phenotypes similar to delta-cmk1: Cdc25 fails to accumulate during Ca2+ response

Supporting text from PMID:25081204: "we have identified a second CaMKK in fission yeast, the Ckk2 kinase, which is involved in the activation of Cmk1 in response to Ca2+"

Function 2: Activation of AMPK (Ssp2) in nitrogen stress (PMID:25639242)

• Ppk34/Ckk2 is specifically required for nitrogen-stress-induced activation of AMPKα (Ssp2)
• This is distinct from Ssp1, which is constitutively required for Ssp2 T-loop phosphorylation
• Active AMPK then inhibits TORC1 signaling via the Tsc1/2-Rhb1 axis
• "CaMKKPpk34 is required to induce AMPKαSsp2 activation following nitrogen stress" PMID:25639242
• Overexpression of Ppk34 induces growth arrest dependent on nutrient environment

Supporting text from PMID:25639242: "a second homolog CaMKK(Ppk34) is specifically required to stimulate AMPKα(Ssp2) activation in response to nitrogen stress"

The full text does not establish Ssp2 as a direct Ppk34 substrate. The authors instead
state that "CaMKKPpk34 may regulate AMPKαSsp2 activity via inhibition of an AMPK
phosphatase" PMID:25639242. Review wording should therefore distinguish the directly
supported requirement for stress-induced Ssp2 T189 phosphorylation from an unproven
direct Ppk34→Ssp2 kinase reaction.

Signaling pathway summary:

Ca2+ stress: Ca2+/CaM -> Ckk2 -> Cmk1 -> phospho-Prz1 (inactive) = negative regulation of calcineurin signaling
N stress:    N stress -> Ckk2 --(direct reaction unresolved)--> Ssp2/AMPK -> Tsc1/2 -> Rhb1 -> TORC1 inhibition

Subcellular Localization (PMID:16823372)

• Cytosol and nucleus (HDA evidence from large-scale GFP localization study by Matsuyama et al. 2006)

Viability (PMID:15821139)

• ppk34 deletion is viable under standard growth conditions (Bimbo et al. 2005)

Withdrawn Reactome projections

• Earlier UniProt versions carried R-SPO-111932, R-SPO-442729, and R-SPO-9619229,
  orthology projections from human CREB/CaMKIV/NMDAR pathways that did not reflect
  demonstrated S. pombe biology. The refreshed UniProt entry (version 162, fetched
  2026-09-01) no longer contains these Reactome cross-references.

Key Interaction Partners

• Cmk1 (SPACUNK12.02c) - direct substrate, phosphorylated by Ckk2 in Ca2+ signaling
• Ssp2 (SPCC74.03c) - AMPKalpha whose stress-induced T189 phosphorylation requires
  Ppk34; Ppk34 is not established as its direct kinase PMID:25639242
• Ssp1 (SPCC297.03) - the other CaMKK, constitutively required for basal Ssp2
  T-loop phosphorylation
• Prz1 - indirect target (phosphorylated by Cmk1, not directly by Ckk2)
• Ppb1 (calcineurin) - the phosphatase whose signaling is counteracted by the Ckk2/Cmk1 axis

Verification of BioReason Claims

What BioReason got right:

1. Ser/Thr kinase activity - correct
2. Negative regulation of calcineurin-mediated signaling - correct
3. Negative regulation of TORC1 signaling - correct, but mechanism is indirect (via AMPK/Ssp2)
4. Cellular response to calcium ion - correct
5. Cytosol and nucleus localization - correct

What BioReason got wrong or exaggerated:

1. "Chronological aging" - BioReason mentions "chronological aging" and "lifespan traits" but there is NO published evidence for ppk34/ckk2 directly regulating aging. This appears to be an extrapolation from TORC1 inhibition (which CAN affect aging in other contexts) but no such data exists for this gene.
2. "Phosphorylates calcineurin's regulatory subunit" - Incorrect. Ckk2 does NOT phosphorylate calcineurin directly. It phosphorylates Cmk1, which then phosphorylates Prz1 (the calcineurin target). The BioReason model confuses the indirect pathway.
3. "14-3-3 proteins that read phospho-motifs" as likely interaction partners - While Rad24/Rad25 (14-3-3 proteins) are involved in Prz1 regulation, there is no evidence they directly interact with Ckk2/ppk34.
4. "PP2A-family phosphatases" as interaction partners - Speculative; while PP2C phosphatases regulate Ssp2 phosphorylation state, they are not demonstrated Ckk2 interactors.
5. The Reactome pathway annotations are projected from human and do not reflect actual S. pombe biology.
6. The UniProt Summary ("Involved in chronological aging and growth control") is fabricated by BioReason and is NOT from UniProt -- UniProt does not contain this text.

Empty GO predictions:

The BioReason GO Term Predictions sections (MF, BP, CC) are all empty, which is notable.