Keywords
CDPK, Ca²⁺ signaling, SnRK, CCaMK, CRK, CBL, CaM, EF-hand, Autophosphorylation, Calcium-Dependent Kinase, Calcium Regulation, Plant Protein Kinases
Reference
DOI: 10.1146/annurev.arplant.55.031903.141627
Abstract Summary
- Plant Ca²⁺-regulated kinases belong to the CDPK-SnRK superfamily.
- This family includes CDPK, CRK, CCaMK, SnRK, CBL, CBK, forming a diverse set of Ca²⁺-regulated and Ca²⁺-dependent kinases.
- Focus on structural features, activation models, functional roles, and evolutionary perspectives.
1. Members of the CDPK-SnRK Superfamily
| Family | Key Features |
|---|---|
| CDPK | Ca²⁺-dependent protein kinase; has EF hands and a kinase domain. |
| CRK | CDPK-related kinase; degenerated EF-hands (evolutionary note). |
| CCaMK | Ca²⁺ and Calmodulin-activated kinase; has 3 EF-hands; absent in Arabidopsis but present in rice, lily, moss. |
| SnRK | Snf1-related kinase (also called CIPK/PKS); interacts with CBL proteins. |
| CBL | Calcineurin B-like Ca²⁺-binding proteins; regulatory subunits for SnRK. |
| CBK | CaM-binding kinases; less defined. |
2. Speculation: Why CDPKs?
- Coevolution of kinase with divergent CaM-like domains (CaM-LD).
- Tight coupling to prevent nonresponsiveness caused by CaM dissociation.
- Co-targeting of kinase and its Ca²⁺ regulator to specific compartments (e.g., sieve tubes).
3. Ca²⁺ Regulation Models
- CDPK:
- Model A: Flexible tether, CaM-LD binds upon Ca²⁺, displacing autoinhibitor.
- Model B: CaM-LD pre-bound at basal Ca²⁺, Ca²⁺ binding to N-lobe activates.
- CRK:
- Activation possibly via CaM binding (e.g., OsCBK1, NtCaMK2), despite degenerated EF-hands.
- CCaMK:
- Activation via CaM binding to sequence between autoinhibitor and tether.
- Visinin-like domain as a second Ca²⁺ sensor.
- SnRK3:
- Activation via CBL interaction, overlaps with autoinhibitory domain.
- Phosphorylation cascades also activate SnRK3 (constitutive active mutants support this).
4. Phylogeny and Diversity
12 subfamilies of CDPKs.
Phylogenetic trees align kinase domains (KD) and CaM-like domains (CaM-LD) separately.
Example: DtCPK1 (green alga) contains an N-terminal C2-domain, absent in Arabidopsis CDPKs.
Figure: a pic from the paper (Click to enlarge)
5. Decoding Ca²⁺ Signals
| Type of Ca²⁺ Signal | Possible Kinase Response |
|---|---|
| Amplitude Modulation (AM) | Various CDPK isoforms activated by different Ca²⁺ levels. |
| Frequency Modulation (FM) | Likely decoded by CDPKs; analogous to CaMKII in animals for frequency sensing. |
CDPKs likely act as frequency decoders in plants, but detailed mechanisms remain unclear.
6. Autophosphorylation (AutoP) of CDPK and Related Kinases
| Kinase | Role of AutoP |
|---|---|
| CDPK (AtCPK1) | Enhances 14–3–3 binding and stimulation. |
| Groundnut CDPK | Essential for substrate phosphorylation. |
| Winged bean CDPK | Inhibitory effect. |
| CRK (NtCaMK1) | Activates kinase independently of Ca²⁺/CaM. |
| CCaMK | Increases CaM affinity, needed for maximal activation; T267 in lily CCaMK is key site. |
| Soybean, Groundnut CDPKs | AutoP does not confer Ca²⁺-independence, unlike CaMKII. |
7. Structural Insights
- Phospho-mimic residue (D-310 in AtCPK1) aligns with PKA’s T-197, possibly replacing the need for autoP.
- Exceptions:
- AtCPK22, 31, OsCPK-J2 use Arg, Lys, Gln at this site.
- CCaMKs use Ala or Gly, indicating distinct regulation.
8. Ca²⁺-Dependent vs. Ca²⁺-Regulated Activity
- CDPKs typically show 10–100x activation with Ca²⁺ increase from nM to low μM.
- Concern: Extreme Ca²⁺ levels (1 mM vs. 10 mM EGTA) in assays may misrepresent physiological conditions.
- Magnesium interference in these assays also a concern.
9. Other Key Insights
- 26S proteasome identified as a CDPK-interacting complex—first known example.
- Diverse N-terminal domains in CDPKs suggest multiple isoform-specific docking possibilities.
10. Discussion Points / Inspirations
- Functional diversity of autoP and regulatory models.
- CDPK as AM and FM signal decoders—a parallel to CaMKII in animal systems.
- Possible evolutionary divergence in EF-hand functionality and kinase-autoinhibitory regulation.
- Concept of “phospho-mimics” as pre-activated structural analogs—potential for synthetic modulation.
RD’s Takeaways
- Super rich functional landscape for CDPKs and relatives—fascinating cross-talk between structure and signaling!
- Interesting that CDPK’s “phospho-mimic” might functionally replace autoP—great for kinase engineering ideas.
- Important caution on interpreting in vitro Ca²⁺ assays—relevant to experimental designs.
