Heptahelical protein PQLC2 is a lysosomal cationic amino acid exporter underlying the action of cysteamine in cystinosis therapy

Proc Natl Acad Sci U S A. 2012 Dec 11;109(50):E3434-43. doi: 10.1073/pnas.1211198109. Epub 2012 Nov 20.


Cystinosin, the lysosomal cystine exporter defective in cystinosis, is the founding member of a family of heptahelical membrane proteins related to bacteriorhodopsin and characterized by a duplicated motif termed the PQ loop. PQ-loop proteins are more frequent in eukaryotes than in prokaryotes; except for cystinosin, their molecular function remains elusive. In this study, we report that three yeast PQ-loop proteins of unknown function, Ypq1, Ypq2, and Ypq3, localize to the vacuolar membrane and are involved in homeostasis of cationic amino acids (CAAs). We also show that PQLC2, a mammalian PQ-loop protein closely related to yeast Ypq proteins, localizes to lysosomes and catalyzes a robust, electrogenic transport that is selective for CAAs and strongly activated at low extracytosolic pH. Heterologous expression of PQLC2 at the yeast vacuole rescues the resistance phenotype of an ypq2 mutant to canavanine, a toxic analog of arginine efficiently transported by PQLC2. Finally, PQLC2 transports a lysine-like mixed disulfide that serves as a chemical intermediate in cysteamine therapy of cystinosis, and PQLC2 gene silencing trapped this intermediate in cystinotic cells. We conclude that PQLC2 and Ypq1-3 proteins are lysosomal/vacuolar exporters of CAAs and suggest that small-molecule transport is a conserved feature of the PQ-loop protein family, in agreement with its distant similarity to SWEET sugar transporters and to the mitochondrial pyruvate carrier. The elucidation of PQLC2 function may help improve cysteamine therapy. It may also clarify the origin of CAA abnormalities in Batten disease.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Amino Acid Sequence
  • Amino Acid Transport Systems, Basic / chemistry*
  • Amino Acid Transport Systems, Basic / genetics
  • Amino Acid Transport Systems, Basic / metabolism*
  • Animals
  • Base Sequence
  • Caenorhabditis elegans Proteins / chemistry*
  • Caenorhabditis elegans Proteins / genetics
  • Caenorhabditis elegans Proteins / metabolism*
  • Canavanine / metabolism
  • Cysteamine / therapeutic use*
  • Cystinosis / drug therapy*
  • Cystinosis / metabolism*
  • DEAD-box RNA Helicases
  • DNA, Complementary / genetics
  • Drosophila Proteins
  • Electrophysiological Phenomena
  • Female
  • Genes, Fungal
  • Humans
  • Membrane Proteins / chemistry
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Molecular Sequence Data
  • Oocytes / metabolism
  • Protein Structure, Secondary
  • Rats
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / metabolism
  • Saccharomyces cerevisiae Proteins / chemistry
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism
  • Species Specificity
  • Vacuoles / metabolism
  • Xenopus laevis


  • Amino Acid Transport Systems, Basic
  • Caenorhabditis elegans Proteins
  • DNA, Complementary
  • Drosophila Proteins
  • Membrane Proteins
  • Recombinant Proteins
  • SLC66A1 protein, human
  • Saccharomyces cerevisiae Proteins
  • Ypq1 protein, S cerevisiae
  • Canavanine
  • Cysteamine
  • Hel25E protein, Drosophila
  • DEAD-box RNA Helicases