PAL1 (Arabidopsis thaliana) — curation notes

UniProt: P35510 (PAL1_ARATH); locus At2g37040; 725 aa. EC 4.3.1.24.

Core enzymatic identity

• Catalyzes the first, rate-limiting step of the phenylpropanoid pathway: non-oxidative deamination of L-phenylalanine to (E)-cinnamate (trans-cinnamic acid) + NH4+. [UniProt P35510 CATALYTIC ACTIVITY: "Reaction=L-phenylalanine = (E)-cinnamate + NH4(+); ... EC=4.3.1.24; Evidence={ECO:0000269|PubMed:15276452}"]
• Enzyme is a homotetramer with an autocatalytically formed 4-methylidene-imidazol-5-one (MIO) cofactor from an Ala-Ser-Gly motif. [UniProt P35510 PTM/CROSSLNK 211..213 "5-imidazolinone (Ala-Gly)"; SUBUNIT "Homotetramer"]
• Belongs to the PAL/histidase family (Pfam Lyase_aromatic, TIGR01226 phe_am_lyase). [UniProt SIMILARITY]
• Recombinant AtPAL1 is catalytically competent for L-Phe deamination; L-Phe is the true physiological substrate (low/undetectable activity on L-Tyr). KM ~64-68 uM for L-Phe. [PMID:15276452 "recombinant native AtPAL1, 2, and 4 were demonstrated to be catalytically competent for l-phenylalanine deamination"; "establishing that l-Phe is the true physiological substrate"]

Biological role and redundancy

• Arabidopsis has four PAL genes (PAL1-PAL4). PAL1 and PAL2 act redundantly in flavonoid biosynthesis. PMID:20566705
• pal1 pal2 double mutants: yellow seeds (loss of condensed tannins), deficient anthocyanin, more UV-B sensitive, more drought tolerant. PMID:20566705
• pal1 pal2 pal3 pal4 quadruple mutants: stunted, sterile, ~10% residual PAL activity, reduced salicylic acid, increased susceptibility to Pseudomonas syringae. PMID:20566705
• Note: most BP phenotypes (pollen development, drought recovery, UV-B response, salicylic acid, lignin) in PMID:20566705 derive from double/triple/quadruple mutant combinations or are family-level, indicating redundancy; PAL1 contributes within a small family. The TAIR IMP annotations to "lignin catabolic process" (GO:0046274) and "salicylic acid catabolic process" (GO:0046244) are mislabeled — PAL produces precursors for lignin BIOsynthesis and feeds salicylate biosynthesis, it does not catabolize lignin or SA. These look like curation/automated-mapping errors.

Defense / immunity

• PAL1 is required for RPS2-mediated effector-triggered immunity (ETI); pal1 mutant shows reduced ETI-associated programmed cell death. [PMID:31568832 "we observed a significant reduction in ETI-associated PCD in pal1"; "phenylalanine ammonia lyase 1 (PAL1) for conversion of phenylalanine to phenylpropanoid compounds were among the RF up-regulated genes (EC 4.3.1.24)"]
• PAL1/PAL2 are oxidative-stress-responsive duplicated genes with divergent expression. PMID:15634198
• PAL gene family historically linked to defense response and wounding. PMID:7888622

Post-translational regulation (protein interactors)

• Kelch repeat F-box (KFB) proteins KFB01 (AT1G15670), KFB20 (AT1G80440), KFB50 (AT3G59940) physically interact with the four PAL isozymes and target them for ubiquitin-26S proteasome degradation. PMID:24363316
• SAGL1 (SMALL AND GLOSSY LEAVES1, AT1G55270), another Kelch repeat F-box protein, interacts with PAL1 for proteasome-mediated degradation. PMID:30542810
• These IPI "protein binding" annotations capture PAL1 as the SUBSTRATE of E3-ligase-mediated turnover. They are real interactions but uninformative as a molecular function ("protein binding"); the biologically meaningful description is that PAL1 is regulated by KFB/F-box-mediated ubiquitin-proteasome turnover. Marked over-annotated rather than accepted as core MF.

Localization

• Cytoplasm (no targeting signals; soluble cytosolic enzyme). [UniProt SUBCELLULAR LOCATION "Cytoplasm {ECO:0000305}"]

Curation decisions summary

• Core MF: GO:0045548 phenylalanine ammonia-lyase activity (ACCEPT EXP/IDA; verified by EC 4.3.1.24 + Rhea:21384).
• Generic MF terms GO:0003824 (catalytic activity) and GO:0016841 (ammonia-lyase activity): MODIFY -> GO:0045548 (too general; specific activity known).
• Core BP: GO:0006559 (L-phenylalanine catabolic process), GO:0009698 (phenylpropanoid metabolic process), GO:0009800 (cinnamic acid biosynthetic process) — ACCEPT.
• Defense/stress BP (defense response, response to wounding, oxidative stress, UV-B): downstream consequences of phenylpropanoid output; KEEP_AS_NON_CORE.
• GO:0046274 lignin catabolic process & GO:0046244 salicylic acid catabolic process: REMOVE/MODIFY — direction wrong (PAL feeds biosynthesis, not catabolism); these are TAIR IMP family-level over/mis-annotations.
• protein binding (GO:0005515) x several IPI: MARK_AS_OVER_ANNOTATED (PAL1 as proteasome-turnover substrate of KFB/SAGL1 F-box proteins; not an informative MF, no adapter function).

Deep research synthesis (Falcon / Edison Scientific)

Source: file:ARATH/PAL1/PAL1-deep-research-falcon.md (Edison Scientific Literature; 27 citations). The report corroborates the existing review and adds no contradictory evidence. Key corroborations folded into supported_by:

• Core reaction / MF: "PAL catalyzes the non-oxidative deamination of L-phenylalanine (Phe) to trans-cinnamic acid (t-cinnamate) and ammonia (NH3), representing the first committed step channelling carbon from primary metabolism into the phenylpropanoid pathway." Reinforces GO:0045548, GO:0006559, GO:0009800, GO:0009698 as core.
• Substrate specificity: "AtPAL1, AtPAL2, and AtPAL4 show robust phenylalanine deamination activity, whereas AtPAL3 has low activity" — independent support for PAL1 as a major catalytic isoform (consistent with PMID:15276452).
• Localization: corpus has no direct PAL1-protein localization experiment, but "PAL-dependent SA production is explicitly described as operating in a cytosolic PAL pathway" — pathway-level support for cytosolic action; consistent with UniProt cytoplasm. No basis to change the ACCEPT for GO:0005737.
• Catabolic mis-annotations confirmed wrong-direction: PAL1 "supplying precursors for lignin, flavonoids/anthocyanins, and contributing to salicylic acid (SA) biosynthesis" and participating in the "cytosolic 'PAL pathway' contributing to salicylic acid (SA) biosynthesis." This reinforces the REMOVE decisions for GO:0046274 (lignin catabolic) and GO:0046244 (SA catabolic) — PAL feeds biosynthesis, not catabolism.
• Post-translational regulation: "proteasome-dependent PAL degradation mediated by Kelch-domain F-box (KFB) proteins" — corroborates KFB/SAGL1 IPI annotations as PAL1-as-substrate (kept MARK_AS_OVER_ANNOTATED for bare protein binding).
• Transcriptional regulation (new context, not annotated): ERF114 directly binds the PAL1 promoter and activates expression during elicitor/pathogen defense ("ERF114 positively modulates elicitor-induced lignin and SA accumulation, likely via PAL1 activation"); WHY1 represses PAL1 indirectly via MYB15. These are regulators OF PAL1, not PAL1 functions, so they do not generate new PAL1 GO annotations.

No UNDECIDED annotations existed; none required resolution. No new GO term could be added with a GOA/UniProt-verifiable ID beyond those already present (regulatory/defense relationships are upstream-of-PAL1 and not appropriate as PAL1 MF/BP additions). proposed_new_terms therefore remains empty. status kept DRAFT.