PBL1 (At3g55450; Q8H186) review notes

Identity

• PBS1-LIKE 1 / BIK1-like kinase (BLK) / CHANGED CALCIUM ELEVATION 5 (CCE5); RLCK subfamily VII, closest paralog of BIK1.
• N-terminal Gly2 myristoylation site; plasma-membrane targeting required for function
  PMID:25522736

Kinase activity (direct evidence)

• cce5 missense alleles abolish autophosphorylation
  PMID:25522736.
• Direct in vitro substrates: PHT1;4 / PHT1;1 cytosolic loops
  PMID:34919806;
  vacuolar CAX1/CAX3 S-cluster [PMID:38418878 "the associated cytoplasmic kinases BIK1 and PBL1, which phosphorylate the same S-cluster in CAXs to modulate Ca2+ signals in immunity" — abstract only].

Receptor coupling

• Associates with FLS2 and is phosphorylated after flg22 [PMID:20413097 "In unstimulated plants, BIK1 and PBL1 interact with FLS2 and are rapidly phosphorylated upon FLS2 activation by its ligand flg22." — abstract only].
• Interacts with PEPR1 PMID:23431184.
• MIK2/SCOOP signalling PMID:34535661.

Calcium signalling and ligand specificity (important for chitin module)

• cce5/pbl1 reduced Ca2+ responses to flg22, elf18, AtPep1, but NOT to chitin
  PMID:25522736
• PBL1 > BIK1 for flg22 root growth inhibition PMID:25522736.
• PBL1 is not required for flg22-induced resistance to Pseudomonas PMID:25522736.

RBOHD

• Deep research: PBL1 binds RBOHD N-terminus in vitro; RBOHD phosphosite reduction is only in the bik1 pbl1 double mutant; direct site-resolved RBOHD phosphorylation was shown for BIK1, not PBL1 (Kadota 2014, PMID:24630626, abstract-only cached; no PBL1 mention in abstract)
  [file:ARATH/PBL1/PBL1-deep-research-falcon.md "The evidence therefore supports PBL1 participation in an RBOHD-associated, reactive-oxygen signaling pathway, but is weaker for a claim that PBL1 itself phosphorylates RBOHD at S39 or S343."]
• Consequence for modules/chitin_perception.yaml: the "BIK1/PBL1 phosphorylation of RBOHD" annoton is not directly supported for PBL1, and the only chitin-specific PBL1 data (Ranf 2014) show PBL1 is dispensable for the chitin Ca2+ response. PBL1 is better grounded on LRR-RK (FLS2/EFR/PEPR/MIK2) signalling. Not edited (module out of scope).

Annotation decisions (summary)

• Kinase MF (IBA, IEA) accepted; generic GO:0004672 -> MODIFY to GO:0004674.
• PRR signalling IBA and response to molecule of bacterial origin IMP accepted (core).
• regulation of defense response to bacterium IMP: no bacterial infection assay for PBL1 in PMID:25522736; marked over-annotated.
• protein binding x2: over-annotated (FLS2, PEPR1), consistent with BIK1 review.
• chloroplast ISM: removed (myristoylated PM kinase, no transit peptide evidence).
• cytosol HDA: non-core (peripheral membrane protein; soluble pool plausible).