PBS2 (YJL128C, UniProt P08018) curation notes

Identity

• MAP kinase kinase of the HOG (high-osmolarity glycerol) pathway; aliases HOG4, SFS4, SSK4.
  UniProt: "Belongs to the protein kinase superfamily. STE Ser/Thr" (MAP kinase kinase subfamily);
  668 aa, C-terminal kinase domain (360-623), long disordered N-terminal region (PBS2_N, PF31257).
• PANTHER PTHR48013:SF25 "MAP KINASE KINASE PBS2".

Core catalytic function: MAPKK for Hog1

• PMID:7681220
• PMID:7681220
• UniProt FUNCTION: activates HOG1 by concomitant phosphorylation of Thr-174 and Tyr-176
  (PubMed:38270553, 7681220); Rhea Thr and Tyr reactions carry experimental evidence
  (PubMed:38270553); the Ser reaction (RHEA:17989) carries no experimental evidence tag.
• PMID:38270553
  -> Pbs2 used as in vitro activator of Hog1 (basis for EXP tyrosine kinase activity row).

Upstream activation - convergence of two branches

• PMID:7624781
• SLN1 branch: PMID:7624781
• SHO1 branch: PMID:7624781
• PMID:9180081
• Deep research (falcon; Tatebayashi 2020/2023, not cached): Ste11 phosphorylates Pbs2 mainly at
  Thr518, Ssk2/Ssk22 at Ser514+Thr518; Hog1 feeds back on Pbs2 Ser248 (Mosbacher 2023).
  PP2Cs Ptc1-4 dephosphorylate Pbs2; [file deep-research "Nbp2 recruits Ptc1 to Pbs2, providing
  interaction specificity for dephosphorylation."]

Scaffold function

• PMID:9180081
• PMID:15200959 and Sho1 and Pbs2 are co-scaffolds:
  PMID:15200959
• Pbs2 Pro-rich motif (PLPPLP, res 94-99) is the high-specificity ligand of the Sho1 SH3 domain:
  PMID:14668868
• Affinity of Sho1 SH3 for Pbs2 tunes HOG output PMID:15200958
• Pbs2 also binds the SH3 of the negative regulator Nbp2 via a distinct Pro-rich motif:
  PMID:14685261
• Conclusion: GO:0005078 MAP kinase scaffold activity is supported by cached evidence (SGD IPI
  annotations with Sho1, Ste11, Hog1). Partner-specific MF for the IntAct/SGD protein-binding rows:
  GO:0017124 SH3 domain binding (Sho1 SH3, Nbp2 SH3).

Localization

• PMID:9755161
• PMID:10980703
• Transient polarized (bud tip/bud neck) pool only resolved with a kinase-dead trap:
  PMID:10980703

Peripheral / downstream phenotypes

• Actin repolarization after osmotic shock needs the HOG pathway PMID:7941729 -> downstream, non-core.
• Hog1 nuclear import: Hog1 phosphorylation is sufficient PMID:9755161; Pbs2 C-terminal chimera study (PMID:15707964)
  therefore reflects Hog1 activation, Pbs2 is not part of the import machinery -> over-annotation.
• Macroautophagy (2025): PMID:40524543 Late-stage defect; mechanism
  unknown -> non-core, acts upstream.
• Stress granule core proteomics (PMID:26777405, HDA): single HT dataset, no follow-up.

Problematic annotations: NatB (PMID:12783868)

• UniProt-assigned (2006/2013) rows on P08018: GO:0004596 N-terminal acetyltransferase (IMP),
  GO:0031416 NatB complex (IDA), protein binding with MDM20 (IPI).
• The abstract explicitly defines the complex: PMID:12783868
• Pbs2 has no GNAT/acetyltransferase domain (InterPro: kinase domain only). UniProt's own P08018
  SUBUNIT line ("Interacts with NBP2, PTC1, SHO1 and STE11.") does not mention NatB, and SGD's live
  curated PBS2 GO set (checked via SGD API 2026-09-24) does not contain any of these rows, whereas SGD
  annotates NAT3 (Q06504) to NatB from this same paper. Strongly suggests the rows were entered on
  the wrong accession (NAT3 intended). Removed; flagged as a question for UniProt.

GO-CAM / module context

• modules/scer_hog1_cascade.yaml uses GO:0004708 MAP kinase kinase activity for Pbs2.