An isoform of Pex5p, the human PTS1 receptor, is required for the import of PTS2 proteins into peroxisomes

Mutations in the peroxisome targeting signal (PTS) 1 receptor gene, PEX5, are responsible for complementation group (CG) 2 of the peroxisome biogenesis disorders (PBD). Of the two reported patients in this CG, cells from PBD018 (homozygous for the missense mutation N489K) are defective in the import of PTS1 proteins into peroxisomes, as expected. However, cells from PBD005 (homozygous for the nonsense mutation R390ter) are defective in the import of both PTS1 and PTS2 proteins, suggesting that the PTS1 receptor also mediates PTS2-targeted protein import. To investigate this possibility, we characterized PEX5 expression and found that it undergoes alternative splicing, producing two transcripts, one containing (PEX5L) and one lacking (PEX5S) a 111 bp internal exon. Fibroblasts from PBD005 have greatly reduced levels of PEX5 transcript and protein as compared with PBD018. Transfection of PBD005 cells with PEX5S cDNA restores PTS1 but not PTS2 import; transfection with PXR5L cDNA restores both PTS1 and PTS2 protein import. Furthermore, transfection of PBD005 cells with PEX5L cDNAs containing the patient mutations (which are located downstream of the additional exon) restores PTS2 but not PTS1 import. Taken together, these data provide an explanation for the different protein import defects in CG2 patients and show that the long isoform of the Pex5 protein is required for peroxisomal import of PTS2 proteins.