Identification and characterization of a lysosomal transporter for small neutral amino acids

In eukaryotic cells, lysosomes represent a major site for macromolecule deg radation. Hydrolysis products are eventually exported from this acidic orga nelle into the cytosol through specific transporters. Impairment of this pr ocess at either the hydrolysis or the efflux step is responsible of several lysosomal storage diseases, However, most lysosomal transporters, although biochemically characterized, remain unknown at the molecular level. In thi s study, we report the molecular and functional characterization of a lysos omal amino acid transporter (LYAAT-1), remotely related to a family of H+-c oupled plasma membrane and synaptic vesicle amino acid transporters. LYAAT- 1 is expressed in most rat tissues, with highest levels in the brain where it is present in neurons. Upon overexpression in COS-7 cells, the recombina nt protein mediates the accumulation of neutral amino acids, such as gamma -aminobutyric acid, L-alanine, and L-proline, through an H+/amino acid symp ort. Confocal microscopy on brain sections revealed that this transporter c olocalizes with cathepsin D, an established lysosomal marker. LYAAT-1 thus appears as a lysosomal transporter that actively exports neutral amino acid s from lysosomes by chemiosmotic coupling to the HC-ATPase of these organel les. Homology searching in eukaryotic genomes suggests that LYAAT-1 defines a subgroup of lysosomal transporters in the amino acid/auxin permease fami ly.