GAI (GIBBERELLIC ACID INSENSITIVE) — research notes
UniProt: Q9LQT8 (GAI_ARATH), gene AT1G14920, 533 aa. GRAS family, DELLA subfamily.
Synonyms: GRAS family protein 3 / AtGRAS-3; RGA2; "Restoration of growth on ammonia protein 2".
Core biology
GAI is one of five Arabidopsis DELLA proteins (GAI, RGA, RGL1, RGL2, RGL3) that act as
nuclear negative regulators (repressors) of the gibberellin (GA) signaling pathway.
- DELLA proteins are nuclear and lack a DNA-binding domain; they regulate transcription
via protein–protein interactions rather than by binding DNA directly.
PMID:24989044
PMID:25035403
- DELLA structural similarity is to mammalian STAT proteins, not classic TFs.
PMID:25035403
Domain architecture (UniProt)
- N-terminal DELLA domain (DELLA motif 28-32, LEXLE 50-54, VHYNP 73-77): GA/GID1 receiver domain.
PMID:17416730
- C-terminal GRAS domain (160-529; LRI, VHIID, LRII, PFYRE, SAW subdomains): protein interaction / repressor activity.
Function / mechanism
- GAI represses GA responses; gai loss-of-function alone is near-WT but synergizes with rga.
PMID:11606552
PMID:11606552
- Notably, in the rga gai double null, "seed germination and flower development defects were not restored" — GAI is not the principal DELLA for germination/floral development (RGL2/RGL1 dominate).
PMID:11606552
- Green revolution orthologues: GAI orthologues (wheat Rht, maize d8) are GA response modulators resembling nuclear TFs.
PMID:10421366
Note: the "nuclear transcription factor" description here is a resemblance inference (1999), predating the finding that DELLAs lack a DNA-binding domain. The TAS/ISS GO:0003700 DNA-binding TF activity annotations derive from this older inference and are not supported by later mechanistic work.
Dual transcriptional mechanism (key paper PMID:25035403)
- GAF1/IDD2 (and ENY/IDD1) are DNA-binding IDD-family TFs; GAI binds the GAF1 PAM/SAW region and acts as a coactivator of GAF1 at GA feedback genes (e.g. GA20OX2).
PMID:25035403
PMID:25035403
- GA degrades DELLAs → converts GAF1-DELLA activator to GAF1-TPR repressor; this is the mechanism for GA homeostasis / feedback.
PMID:25035403
- DELLAs thus have dual roles: titration of activators (PIFs/BZR1) AND coactivation of GAF1.
PMID:25035403
- Activity regulation: transcription activation repressed by GA(3) in presence of TPR4 (UniProt ACTIVITY REGULATION).
Epidermal germination module (PMID:24989044)
- GAI (DELLA) interacts directly with HD-ZIP TFs ATML1 and PDF2; GA-driven DELLA loss releases ATML1/PDF2 to activate L1-box genes (LIP1) in epidermis, enhancing germination.
PMID:24989044
PMID:24989044
GID1 receptor interaction
Other interactions / processes
- Prefoldin (PFD3/PFD5): GAI/DELLA sequesters prefoldin in nucleus in absence of GA, affecting tubulin folding / cortical microtubule organization.
PMID:23583555
- CONSTANS (CO): DELLA-CO interaction links GA and photoperiod flowering.
PMID:26801684
- ANAC106: DELLA-ANAC106 reciprocal negative feedback for GA homeostasis / male fertility.
PMID:40755058
- Meiotic cytokinesis / 2n pollen: rga gai double mutant shows defective male meiotic cytokinesis (RMA organization); GA/DELLA acts largely in somatic anther tissue.
PMID:27621423
- Far-red / red light & germination: PIL5 (PIF1) directly binds GAI and RGA promoters and upregulates them in dark; phytochrome light signaling represses GAI/RGA transcription.
PMID:17449805
Note: GO:0010218 (response to far red light) IEP for GAI reflects GAI being a transcriptional target of light/PIL5, i.e. its expression responds; this is a "response to" annotation supported by the data.
- Phloem RNA trafficking: GAI mRNA moves long-distance via phloem and affects leaf development (engineered dominant ΔDELLA-gai construct).
PMID:15773853
- Nitrogen: GAI (RGA2) was originally cloned by complementation of a yeast gln3 gdh1 nitrogen-regulation mutant.
PMID:9237632
Note: this is a heterologous yeast complementation phenotype; GO:0006808 regulation of nitrogen utilization is weakly supported as a native Arabidopsis function — treat as non-core / over-annotation candidate.
- Secondary cell wall network (PMID:25533953): large Y1H gene regulatory network; GRAS family over-represented among TFs binding cell wall promoters. The GO:0000976 "transcription cis-regulatory region binding" IPI to GAI derives from a Y1H protein-DNA screen. DELLAs lack a DNA-binding domain, so a direct, sequence-specific cis-region binding by GAI is mechanistically questionable; Y1H positives for non-DNA-binding cofactors can occur via bridging. Treat as over-annotation / MODIFY away from direct DNA binding.
GO annotation assessment summary
- Core MF: GAI has no catalytic/DNA-binding activity of its own; its molecular function is transcription coactivator activity (GO:0003713, IDA, PMID:25035403) exerted via protein-protein interactions, and adapter/co-regulator binding (GID1, IDD TFs, PIFs, prefoldin).
- Core BP: negative regulation of GA signaling (GO:0009938), GA homeostasis (GO:0010336), regulation of transcription/gene expression.
- Core CC: nucleus (GO:0005634), strongly supported (IDA multiple).
- GO:0003700 DNA-binding transcription factor activity (TAS/ISS x4): over-annotation; DELLAs lack a DNA-binding domain (see above). Recommend MARK_AS_OVER_ANNOTATED / MODIFY.
- Many GO:0005515 "protein binding" IPI: real interactions but uninformative as a core MF; KEEP_AS_NON_CORE.