SYP121 (PEN1/SYR1; At3g11820; Q9ZSD4) curation notes

2026-10-02 session

Sources: UniProt Q9ZSD4, GOA (58 rows), SYP121-deep-research-falcon.md, cached publications (many abstract-only; checked full_text_available). Added PMIDs 18678865, 20884800, 19794113, 17241452 (verified via PubMed MCP).

Identity / core activity

• Plasma-membrane Qa-SNARE. Forms ternary SNARE complexes with SNAP33 and VAMP721/722 PMID:18273019.
• SNARE-domain variants that fail ternary assembly lose defense activity PMID:18678865.
• Constitutive secretion: SYP121ΔC blocks secretion at PM PMID:32699172; secGFP traffic suppressed by SYP121 Sp2 PMID:17662029.
• Cargo: PIP2;7 with SYP61 PMID:25082856.

Immunity (project curation question 3: necessity vs participation)

• PEN1 recruited to papillae; 2-h delay in papilla formation in pen1-1 PMID:15342780.
• Judgement: PEN1 participates (executes the fusion step of focal secretion), so defense response to fungus (IGI) is ACCEPTED alongside exocytosis as the mechanistic core. response to fungus (IMP) and defense response (TAS) kept as non-core.
• Negative regulation of defense: SYP121+SYP122 suppress SA/JA/ET/PCD PMID:17241452 -> kept non-core (likely indirect).

K+ channels

• SYP121 binds KC1, promotes AKT1 gating PMID:19794113; FxRF motif required PMID:20884800. Added NEW potassium channel regulator activity (GO:0015459).
• protein-membrane adaptor activity (IDA, PMID:16531497, full text available) REMOVED: authors state "These observations do not necessarily confirm an immediate role for the SNAREs in K + channel anchoring, nor do they imply direct protein–protein interaction".
• Stomatal phenotypes (delayed reopening, KAT1 recycling) PMID:21914010 kept non-core.

Other

• Protein binding IPIs -> SNARE binding (SNAP33, SYP61, VAMP721) or transmembrane transporter binding (KAT1, PIP2;7); SEC11 and EXO70A1 rows removed (no informative MF term).
• Cytosol HDA marked over-annotated (tail-anchored membrane protein).
• Validation ontology still labels GO:0005484 "SNAP receptor activity" (QuickGO: "helical-bundle zippering fusogenic activity"); core_functions use the validator label.