Keywords

CaMKII, Structure, Autoinhibition, Autophosphorylation, Isoforms, Calcium Signaling, Ca2+/CaM, Holoenzyme Assembly, Frequency Decoding, Switch-like Behavior

Reference

DOI: 10.1042/BJ20020228

Abstract

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous, multifunctional calcium-activated kinase that regulates diverse cellular processes through substrate phosphorylation. Notably, its multimeric structure and complex autoregulatory mechanisms render it a key decoder of Ca2+ oscillations and mediator of molecular memory. CaMKII:

  • Exhibits Ca2+-spike frequency-dependent activation.
  • Generates autonomous activity independent of Ca2+/CaM following autophosphorylation.
  • Behaves as a molecular switch, critical for processes such as synaptic plasticity and learning.
    Isoforms (α, β, γ, δ) arise from four genes and over 30 spliced variants, showing cell type- and compartment-specific expression. This review elegantly dissects the structure-function relationships that underlie CaMKII’s diverse roles, highlighting autoinhibition, autophosphorylation, isoform targeting, and self-association as key regulatory features.

Notes

1. CaMKII Isoform Diversity and Structure

  • Four genes (α, β, γ, δ) with ~30 spliced isoforms, varying by alternative splicing in variable linker and targeting domains.
  • Brain-specific α and β isoforms are dominant (up to 2% of total protein); α isoform concentration in forebrain: 19–37 μM.
  • High sequence similarity (~89–93%) in catalytic/autoregulatory domains across isoforms.
  • Some isoforms (e.g., αB) contain NLS, enabling nuclear targeting regulated by phosphorylation.
  • βM isoform contains multiple targeting domains, supporting subcellular specificity.

Key idea: Isoform diversity enables cellular compartmentalization, fine-tuning CaMKII function via specific localization and interaction partners.

2. Autoinhibition and Activation by Ca2+/CaM

  • Autoinhibitory domain (residues 281–317) contains pseudosubstrate elements and CaM-binding sequence.
  • Inhibitory mechanism: autoinhibitory domain blocks catalytic site via pseudosubstrate mimicry and interference with ATP binding (e.g., His282 may mimic ATP).
  • Ca2+/CaM binding displaces autoinhibitory segment, releasing active site and enabling substrate phosphorylation.
  • CaMKII activation is fully CaM-dependent, unlike CaMKI/IV, which require phosphorylation by upstream CaMKK for full activation.

Important: CaMKII does NOT require phosphorylation for activation, but autophosphorylation (e.g., Thr286) provides autonomous activity and CaM trapping.

3. Autophosphorylation and Autonomous Activity

  • Thr286 phosphorylation creates Ca2+/CaM-independent activity (molecular memory).
  • AutoP of Thr286 enhances CaM binding by exposing residues 293–295.
  • Thr305/306 phosphorylation prevents CaM binding — key for resetting kinase.
  • Autophosphorylation on Thr306 occurs in basal/inactive state and may block subsequent Ca2+/CaM binding.

Switch-like behavior: Once phosphorylated at Thr286, CaMKII remains active, integrating past Ca2+ events, thus serving as a cellular register.

4. Holoenzyme Structure and Self-association

  • Holoenzyme = dodecamer (12-mer) of subunits arranged around a hub domain; requires residues 344–478 for assembly.
  • Self-association into larger aggregates occurs in vitro and in neurons, regulated by isoform composition and biochemical conditions (pH, ATP).
  • α or α/β holoenzymes self-associate robustly; β-only holoenzymes do not.
  • Self-association may limit activation, contribute to targeting, or regulate subunit exchange.

Note: “Pseudoanchor” sequences may serve as latent targeting motifs, exposed only upon activation.

5. Frequency Detection and Molecular Mechanisms

  • Autonomous activity generation and response to Ca2+ spike frequency depend on inter- and intra-holoenzyme autophosphorylation.
  • Frequency detection models:
    1. Interval detection: rate of dephosphorylation vs. stimulation interval.
    2. Sequential autophosphorylation of specific sites as a rate-limiting step.
  • Linker length and isoform-specific properties modulate CaMKII’s response to Ca2+ signals.

Summary: CaMKII functions as a molecular integrator of calcium signals, sensitive to the timing and frequency of Ca2+ oscillations.

6. ATP and CaM Cross-talk in Activation

  • ATP binding enhances CaM affinity:
    • Kd for CaM: 150 pM (with ATP), 2 μM (without ATP).
    • ATP stabilizes CaM-CaMKII interaction, promoting activation.
  • Reciprocal effect: CaM binding facilitates ATP interaction, cooperativity in activation process.

7. RD’s Reflections

  • Foundational paper for understanding CaMKII structure-function — a must reread!
  • Key realization: Autophosphorylation is NOT required for initial activation but provides “memory” via autonomous activity.
  • Switch-like behavior = register for prior Ca2+ transients, integrating multiple signals.
  • The dual mechanism of autoinhibition (pseudosubstrate + ATP mimicry) is so elegant.
  • Frequency decoding resonates with ideas in CDPK/CDPK-like kinases — worth comparing in a “Signal Decoding” series!
  • Question I keep thinking about: How do plant CDPKs/CDPK-RLs compare in terms of this pseudosubstrate and register function?
  • Love the mention of subcellular localization regulation via isoform-specific targeting motifs (e.g., NLS) — essential for thinking about signaling specificity.

Take-home Messages

  • CaMKII’s multifunctionality arises from a modular structure combining catalytic, autoregulatory, and hub domains.
  • Ca2+/CaM binding fully activates CaMKII; autophosphorylation at Thr286 generates autonomous, Ca2+/CaM-independent activity.
  • Isoform diversity allows precise subcellular targeting and signal specificity.
  • Self-association and holoenzyme assembly regulate activation and potentially enable subunit exchange.
  • CaMKII acts as a molecular decoder of calcium oscillations, integrating signal frequency and duration.
  • “Switch-like” behavior enables CaMKII to function as a “register” of past calcium events — foundational for learning and memory.