UniProt ID: Q15078
Gene: CDK5R1
Molecular Weight: 31 kDa (p25), 35 kDa (p35)
Subcellular Localization: Cytoplasm, membrane-associated (p35), cytosol (p25)
Protein Family: CDK5 activator family
Key Domains:
CDK5R1 (CDK5 activator 1, p35/p25) is a regulatory protein that activates cyclin-dependent kinase 5 (CDK5), a kinase essential for normal neuronal development and function[1]. Under physiological conditions, full-length p35 activates CDK5 at membranes, directing it toward normal substrates involved in neuronal migration, synaptic plasticity, and survival[2]. When cleaved by calpain to p25, it generates a constitutively active, mislocalized CDK5-p25 complex that phosphorylates pathological substrates including tau, contributing critically to Alzheimer's disease and other tauopathies[3].
p35 is a 307-amino acid protein with distinct functional regions:
| Domain | Residues | Function |
|---|---|---|
| N-terminal p10 | 1-98 | Regulatory; contains myristoylation site |
| Linker | 99-147 | Calpain cleavage site at Cys98 |
| CDK5-binding | 148-307 | Direct CDK5 activation |
| Myristoylation | Gly2 | Membrane targeting |
When calpain cleaves p35 at Cys98, it generates:
The p35 → p25 conversion causes[4]:
CDK5 is unusual among CDKs because it is not activated by cyclins. Instead, p35 (and its paralogue p39) serves as the exclusive activator in neurons[5]. The p35-CDK5 complex:
The myristoylated N-terminus of p35 targets the CDK5-p35 complex to[6]:
This localization restricts CDK5 activity to specific compartments, preventing aberrant phosphorylation of nuclear substrates.
With p35, CDK5 phosphorylates[7]:
The cleavage of p35 to p25 by calpain is a critical event in tauopathy pathogenesis[3:1]:
Several neurodegenerative insults promote p35 cleavage[8]:
| Trigger | Mechanism | Evidence |
|---|---|---|
| Aβ oligomers | Calcium dysregulation | AD models, cell culture |
| Oxidative stress | Membrane damage | Multiple models |
| Excitotoxicity | Glutamate receptor overactivation | Neuronal cultures |
| Calcium dysregulation | ER stress, mitochondrial dysfunction | AD/PD tissues |
| Ischemia | Energy failure | Stroke models |
The CDK5-p25 complex is a major pathological tau kinase, phosphorylating tau at multiple AD-relevant sites[9]:
Proline-directed sites:
Non-proline-directed sites:
This hyperphosphorylation:
Elevated p25 levels have been documented in[11]:
Unlike p35, p25 allows CDK5 to enter the nucleus, where it[12]:
In dopaminergic neurons[13]:
| Approach | Agent | Status | Notes |
|---|---|---|---|
| ATP-competitive | Roscovitine (seliciclib) | Preclinical/clinical | Pan-CDK inhibitor; toxicity concerns |
| Peptide inhibitors | CDK5i (TAT-p35-CIP) | Preclinical | Selective; blocks p25 binding site |
| Calpain inhibitors | MDL-28170, calpeptin | Preclinical | Prevents p25 formation |
| Gene therapy | p25 knockdown | Research stage | RNAi approaches |
| Partner | Function | Disease Relevance |
|---|---|---|
| CDK5 | Kinase activated by p35/p25 | Tau phosphorylation |
| Tau | Major substrate | NFT formation |
| Calpain | Cleaves p35 to p25 | Pathological switch |
| GSK3β | Cooperates in tau phosphorylation | Synergistic toxicity |
| Munc18 | Synaptic vesicle release | Normal function |
| P53 | Nuclear substrate | Apoptosis |
p25 transgenic mice[14]:
Conditional p35 knockout:
Dhavan & Tsai. A decade of CDK5. Nature Reviews Molecular Cell Biology. 2001. ↩︎
Hisanaga & Endo. Regulation of CDK5 by p35. Brain Research. 2014. ↩︎
Patrick et al. Conversion of p35 to p25 deregulates Cdk5 activity and promotes neurodegeneration. Nature. 1999. ↩︎ ↩︎
Kusakawa et al. p35 cleavage and p25 stability. Journal of Neurochemistry. 2000. ↩︎
Tsai et al. p35 is a neural-specific regulatory subunit of cyclin-dependent kinase 5. Nature. 1994. ↩︎
Asada et al. Myristoylation of p35 and Cdk5-p35 membrane localization. Journal of Biological Chemistry. 2002. ↩︎
Su & Tsai. Cdk5 in synaptic plasticity and memory. Current Opinion in Neurobiology. 2011. ↩︎
Lee et al. Cdk5 and neurodegeneration. Trends in Neurosciences. 2000. ↩︎
Cruz et al. Cdk5 mediates neurotoxicity in p25 transgenic mice. Neuron. 2006. ↩︎
Patrick et al. p25 accumulation in Alzheimer's disease. Nature. 1999. ↩︎
Takashima et al. [Cdk5 activity in tauopathies](https://doi.org/10.1016/s0304-3940(01). Neuroscience Letters. 2001. ↩︎
Fu et al. Nuclear CDK5-p25 and DNA damage. Journal of Neuroscience. 2011. ↩︎
Smith et al. Cdk5 in Parkinson's disease. Neuron. 2003. ↩︎
Ahlijanian et al. p25 transgenic mice: a model of tauopathy. Genes & Development. 2003. ↩︎