Cystinosin (encoded by the CTNS gene is a lysosomal membrane transporter protein critical for cystine efflux from lysosomes. Deficiency causes cystinosis, a lysosomal storage disorder with progressive neurological involvement.
Cystinosin is a 367-amino acid polytopic membrane protein localized to the lysosomal membrane, where it functions as an H⁺-driven cystine transporter[1]. It belongs to the PQ-loop family of transporters and contains seven predicted transmembrane domains. Cystinosin is essential for preventing the toxic accumulation of cystine crystals within lysosomes and plays broader roles in autophagy, mTORC1 signaling, and lysosomal homeostasis[2].
[^2]
| | |
|---|---|
| Protein Name | Cystinosin |
| Gene | CTNS |
| UniProt ID | O60931 |
| Molecular Weight | ~42 kDa (predicted), ~55 kDa (glycosylated) |
| Subcellular Localization | Lysosomal membrane |
| Function | H⁺-driven lysosomal cystine transporter |
| PDB Structures | AlphaFold: AF-O60931 |
¶ Domain Architecture
Cystinosin has a distinctive topology[1]:
- N-terminal domain: Located in the lysosomal lumen, contains 7 N-glycosylation sites
- Seven transmembrane helices (TM1-TM7): Span the lysosomal membrane to form the cystine translocation pathway
- Two PQ-loop motifs: Conserved Pro-Gln sequences in TM2 and TM5 essential for transport
- C-terminal tail: Cytoplasmic, contains the GYDQL lysosomal sorting motif
- Intralysosomal loops: Large glycosylated loops between TMs protect against lysosomal proteases
Cystinosin operates as a cystine/H⁺ symporter[3]:
- Protons from the acidic lysosomal lumen (pH ~4.5-5.0) bind to cystinosin
- Cystine (the oxidized dimer of cysteine) binds in the transport channel
- Conformational change translocates both cystine and H⁺ to the cytoplasm
- In the cytoplasm, cystine is reduced to two cysteine molecules by the thioredoxin system
- Cysteine is used for glutathione synthesis and protein translation
The canonical function of cystinosin is exporting cystine from lysosomes to prevent crystal formation[1]:
- Lysosomes generate cystine through proteolytic degradation of disulfide-containing proteins
- Without cystinosin, cystine accumulates to 50-100× normal levels
- Cystine crystals physically damage lysosomal membranes, triggering cell death
Cystinosin interacts with components of the mTORC1 signaling pathway on the lysosomal surface[4]:
- Binds to the v-ATPase/Ragulator complex that recruits mTORC1 to lysosomes
- Loss of cystinosin leads to delayed mTORC1 deactivation upon nutrient starvation
- Hyperactive mTORC1 suppresses TFEB-mediated lysosomal biogenesis
- Contributes to the defective autophagy observed in cystinosis
¶ Autophagy and Vesicular Trafficking
Cystinosin deficiency broadly impairs the endolysosomal system[2]:
- Autophagic flux: Reduced autophagosome-lysosome fusion
- Chaperone-mediated autophagy: Impaired LAMP2A-dependent CMA
- Endosomal trafficking: Delayed endosomal maturation and cargo sorting
- Lysosome biogenesis: Reduced TFEB nuclear translocation impairs de novo lysosome formation
Loss of cystinosin in neurons and glia causes[5]:
- Neuronal cystine accumulation: Particularly in hippocampal and cortical neurons
- Oxidative stress: Depleted glutathione renders neurons vulnerable to ROS
- Synaptic dysfunction: Impaired vesicle recycling due to endolysosomal defects
- Astrocyte pathology: Reactive astrogliosis with cystine crystal deposition
- White matter disease: Progressive leukoencephalopathy and demyelination
Cystinosin dysfunction highlights conserved pathogenic mechanisms[6]:
- Alzheimer's disease: Shared autophagy-lysosomal impairment; both show enlarged endolysosomes and impaired Aβ clearance
- Parkinson's disease: Parallels with GBA1-mediated lysosomal dysfunction; α-synuclein accumulates when lysosomal function is compromised
- Lysosomal storage diseases: Common pathway of lysosomal membrane damage → cathepsin release → neuroinflammation → neuronal death
- White blood cell cystine levels: Gold standard for cystinosis diagnosis and treatment monitoring
- Corneal cystine crystals: Visible on slit-lamp examination, pathognomonic for cystinosis
- Brain MRI: Cerebral atrophy and white matter changes correlate with neurological progression
- Cysteamine: Enters lysosomes and forms cysteine-cysteamine mixed disulfide that exits via PQLC2 transporter, bypassing the need for cystinosin
- Gene therapy: AAV vectors delivering functional CTNS show promise in animal models
- HSCT: Hematopoietic stem cell transplant provides cross-correction through tunneling nanotubes
- mTOR inhibitors: May correct downstream signaling defects
| Interactor |
Type |
Functional Consequence |
| v-ATPase |
Physical |
Regulates lysosomal acidification and mTORC1 signaling |
| Ragulator complex |
Physical |
Modulates mTORC1 recruitment to lysosomes |
| LAMP1/LAMP2 |
Co-localization |
Lysosomal membrane partners |
| PQLC2 |
Functional |
Alternative exit route for cysteamine-cysteine mixed disulfides |
| TFEB |
Regulatory (indirect) |
Cystinosin loss inhibits TFEB nuclear translocation |