Cystinosin (encoded by the [CTNS gene](/genes/ctns)) is a lysosomal membrane transporter protein critical for cystine efflux from lysosomes. Deficiency causes cystinosis, a lysosomal storage disorder with progressive neurological involvement.
Overview
Cystinosin is a 367-amino acid polytopic membrane protein localized to the lysosomal membrane, where it functions as an H⁺-driven cystine transporter<sup>[1]</sup>. 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](/mechanisms/autophagy-lysosomal-pathway-parkinsons), mTORC1 signaling, and lysosomal homeostasis<sup>[2]</sup>. [@festa2018]
Cystinosin has a distinctive topology<sup>[1]</sup>:
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Cystinosin (CTNS Protein)
Introduction
Cystinosin (encoded by the [CTNS gene](/genes/ctns)) is a lysosomal membrane transporter protein critical for cystine efflux from lysosomes. Deficiency causes cystinosis, a lysosomal storage disorder with progressive neurological involvement.
Overview
Cystinosin is a 367-amino acid polytopic membrane protein localized to the lysosomal membrane, where it functions as an H⁺-driven cystine transporter<sup>[1]</sup>. 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](/mechanisms/autophagy-lysosomal-pathway-parkinsons), mTORC1 signaling, and lysosomal homeostasis<sup>[2]</sup>. [@festa2018]
[Alzheimer's disease](/diseases/alzheimers-disease): Shared autophagy-lysosomal impairment; both show enlarged endolysosomes and impaired [Aβ](/proteins/amyloid-beta) clearance
[Parkinson's disease](/diseases/parkinsons-disease): Parallels with [GBA1](/genes/gba1)-mediated lysosomal dysfunction; [α-synuclein](/proteins/alpha-synuclein) accumulates when lysosomal function is compromised
Lysosomal storage diseases: Common pathway of lysosomal membrane damage → cathepsin release → neuroinflammation → neuronal death
Clinical Significance
Biomarker Potential
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
Therapeutic Targeting
[Cysteamine](/therapeutics/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](/mechanisms/mtor-signaling-pathway) inhibitors: May correct downstream signaling defects
[Town M et al., A novel gene encoding an integral membrane protein is mutated in nephropathic cystinosis (1998) (1998)](https://doi.org/10.1038/ng0698-120)
[Festa BP et al., Impaired autophagy bridges lysosomal storage disease and epithelial dysfunction in the kidney (2018) (2018)](https://doi.org/10.1038/s41467-018-03853-z)
[Kalatzis V et al., Cystinosin, the protein defective in cystinosis, is a H+-driven lysosomal cystine transporter (2001) (2001)](https://doi.org/10.1093/emboj/20.21.5940)
[Ivanova EA et al., Endo-lysosomal dysfunction in human proximal tubular epithelial cells deficient for lysosomal cystine transporter cystinosin (2015) (2015)](https://doi.org/10.1371/journal.pone.0120998)
[Trauner DA et al., Neurological impairment in nephropathic cystinosis (2010) (2010)](https://doi.org/10.1016/j.jpeds.2009.11.074)
[Bellomo F et al., Impact of atypical mitochondrial cyclic-AMP level in nephropathic cystinosis (2018) (2018)](https://doi.org/10.1007/s00018-018-2800-5)