Members of this family are all membrane bound proteins possessing a pair of repeats each spanning two transmembrane helices connected by a loop [1]. The PQ motif found on loop 2 is critical for the localisation of cystinosin to lysosomes [2]. However, the PQ motif appears not to be a general lysosome-targeting motif. It is thought likely to possess a more general function. Most probably this involves a glutamine residue [1]. Family members are membrane transporters since two members, cystinosin and PQLC2, transport cystine and cationic amino acids, respectively, across the lysosomal membrane [3]. The 2nd PQ-loop of cystinosin hosts the substrate-coupled H+ binding site underlying its H+ symport mechanism, suggesting that PQ-loop repeats have functional significance [4,5]. It is thus likely that PQ-loop-containing proteins act as a family of membrane transporters. Some transport cystine and cationic amino acids, respectively, across the lysosomal membrane. Others transport lysine and or arginine across the lysosomal membrane in order to maintain the acidic homoeostasis [6].

1: Novel protein domains and repeats in Drosophila melanogaster: insights into structure, function, and evolution. Ponting CP, Mott R, Bork P, Copley RR; Genome Res 2001;11:1996-2008. PMID:11731489

2: The targeting of cystinosin to the lysosomal membrane requires a tyrosine-based signal and a novel sorting motif. Cherqui S, Kalatzis V, Trugnan G, Antignac C; J Biol Chem 2001;276:13314-13321. PMID:11150305

3: Cystinosin, the protein defective in cystinosis, is a H(+)-driven lysosomal cystine transporter. Kalatzis V, Cherqui S, Antignac C, Gasnier B; EMBO J. 2001;20:5940-5949. PMID:11689434

4: Mechanism of proton/substrate coupling in the heptahelical lysosomal transporter cystinosin. Ruivo R, Bellenchi GC, Chen X, Zifarelli G, Sagne C, Debacker C, Pusch M, Supplisson S, Gasnier B; Proc Natl Acad Sci U S A. 2012;109:E210-E217. PMID:22232659

5: Heptahelical protein PQLC2 is a lysosomal cationic amino acid exporter underlying the action of cysteamine in cystinosis therapy. Jezegou A, Llinares E, Anne C, Kieffer-Jaquinod S, O'Regan S, Aupetit J, Chabli A, Sagne C, Debacker C, Chadefaux-Vekemans B, Journet A, Andre B, Gasnier B; Proc Natl Acad Sci U S A. 2012;109:E3434-E3443. PMID:23169667

6: LAAT-1 is the lysosomal lysine/arginine transporter that maintains amino acid homeostasis. Liu B, Du H, Rutkowski R, Gartner A, Wang X; Science. 2012;337:351-354. PMID:22822152