CIPK24 / SOS2 (Q9LDI3, At5g35410) — research notes

Identity

Core biology (synthesized)

CIPK24/SOS2 is a genuine, catalytically active Ser/Thr protein kinase that is the central kinase of the SOS (Salt Overly Sensitive) pathway. It is held inactive by an autoinhibitory C-terminal domain (the NAF/FISL motif). Binding of a Ca2+-loaded calcineurin-B-like (CBL) calcium sensor to the NAF motif relieves autoinhibition and activates the kinase in a Ca2+-dependent manner. The activated CBL–CIPK24 complex phosphorylates and activates downstream membrane transporters to restore Na+/K+ homeostasis under salt stress.

Evidence by reference

PMID:10725382 (Liu et al. 2000, PNAS) — full text available

Positional cloning of SOS2; encodes a Ser/Thr kinase with N-terminal catalytic domain similar to yeast SNF1. "Autophosphorylation assays show that SOS2 is an active protein kinase." Kinase activity required for salt tolerance; sos2-5 G197E and K40N abolish autophosphorylation. PMID:10725382 Supports protein kinase activity (IDA) and ATP binding (catalytic Lys-40). Transcript "up-regulated by salt stress in the root."

PMID:10725350 (Halfter et al. 2000, PNAS) — full text available

SOS3 physically interacts with and activates SOS2 in a Ca2+-dependent manner; interaction via the C-terminal regulatory domain of SOS2; sos2 sos3 double mutant epistasis shows same pathway. PMID:10725350 Supports functional CBL4/SOS3 (O81223) binding and Ca2+-dependent kinase activation.

PMID:11402167 (Guo et al. 2001, Plant Cell) — full text available

Identified the 21-aa SOS3-binding motif (NAF) in SOS2 that is also the autoinhibitory domain; removal of the regulatory domain gives constitutive (SOS3-independent) kinase activity; T168D in activation loop also constitutively activates. PMID:11402167 Supports CBL4/SOS3 (O81223) and CBL1 (O81445) binding via NAF motif and the autoinhibition/activation mechanism.

PMID:14504388 (Ohta et al. 2003, PNAS) — full text available

Identified the 37-aa PPI motif of SOS2 necessary and sufficient for interaction with PP2C phosphatase ABI2 (O04719). PMID:14504388 Supports specific ABI2 interaction (negative regulation by PP2C).

PMID:14730064 (Kolukisaoglu et al. 2004, Plant Physiol) — full text available

Genomics of the CBL-CIPK network; 10 CBLs and 25 CIPKs in Arabidopsis. IntAct entries map CIPK24 interactions with CBL1 (O81445) and CBL9 (Q9LTB8). Establishes CIPK24 as a member of CBL-interacting kinase family. NOTE: this paper now carries an Expression of Concern and an Erratum per the cached record. PMID:14730064

PMID:17449811 (Quan et al. 2007, Plant Cell) — abstract only (full_text_available: false)

SCABP8/CBL10 binds Ca2+, interacts with SOS2 in vitro and in vivo, recruits SOS2 to the plasma membrane, enhances SOS2 activity in a Ca2+-dependent manner, activates SOS1 in yeast; CBL10 acts mainly in shoots, SOS3 mainly in roots. IntAct maps CBL4/SOS3 (O81223) and CBL10 (Q7FRS8). PMID:17449811

PMID:17825054 (Kim et al. 2007, Plant J) — abstract only (full_text_available: false)

CBL10 interacts with CIPK24/SOS2; CBL10-CIPK24 complex associated with vacuolar compartments responsible for salt storage/detoxification. Basis for the IDA "plant-type vacuole membrane" localization annotation. PMID:17825054

PMID:19448033 (Lin et al. 2009, Plant Cell) — full text available

SOS2 phosphorylates SCaBP8/CBL10 at C-terminal Ser-237 (does NOT phosphorylate SOS3); phosphorylation is salt-induced, occurs at the membrane, stabilizes the SCaBP8-SOS2 complex, and enhances plasma membrane Na+/H+ exchange. PMID:19448033 Supports kinase activity acting on a CBL substrate (CBL10 isoform Q7FRS8-2).

PMID:21262798 (Quintero et al. 2011, PNAS) — full text available

Mechanism of SOS1 activation: SOS1 held in resting state by C-terminal autoinhibitory domain; SOS2-SOS3 phosphorylates it to relieve autoinhibition; mutation of the SOS2 phosphorylation/recognition site impairs SOS1 activation in vivo and in vitro. IntAct maps SOS3/CBL4 (O81223) and SOS1/NHX7 (Q9LKW9). PMID:21262798 Strongest direct support for SOS2 substrate = SOS1 and for regulation of sodium ion transport.

PMID:17785451 (Verslues et al. 2007, MCB) — full text available

SOS2 interacts with NDPK2 (O64903) and catalases, linking salt stress and H2O2 signaling; sos2 ndpk2 double mutant more salt sensitive. SOS2 inhibits NDPK2 autophosphorylation. PMID:17785451 Supports a specific protein interaction beyond CBL/transporters; a secondary (non-core) branch.

PMID:9668136 (Zhu et al. 1998, Plant Cell) — abstract only (full_text_available: false)

Genetic screen defining the SOS2 locus; sos2 mutants specifically hypersensitive to Na+/Li+ (not general osmotic stress), and impaired in K+ nutrition; SOS1/SOS2/SOS3 act in the same pathway. Basis for IMP "response to salt stress." PMID:9668136

PMID:40726285 (Wang et al. 2025, Plant J) — abstract only (full_text_available: false)

Nuclear branch: salt-induced SSN1 (AT2G36080) condensation; SSN1 co-condenses with PIF4 via assembling a SOS2-PIF4 complex in nuclear salt bodies to facilitate PIF4 degradation. Supports a nuclear role for SOS2 and the nucleus localization annotation. PMID:40726285

PMID:17360592 (high-density protein microarray) — full text available; PMID:32612234 (interactome/signal integration) — abstract only; PMID:21798944 (Arabidopsis interactome map) — abstract only; PMID:11006339 (SOS3 myristoylation) — full text available

These are high-throughput / interactome papers contributing the IntAct GO:0005515 "protein binding" IPI entries (CAM4/CAM7/CML9, CBL1, CBL9). 11006339 is primarily about SOS3 N-myristoylation but its IntAct entry maps the SOS2–SOS3 (O81223) binding pair.

GO term definitions verified (QuickGO API, June 2026)

Localization synthesis

Curation decisions summary