Identification and Characterization of the Putative Human Peroxisomal C-terminal Targeting Signal Import Receptor

To identify proteins interacting with the C-terminal peroxisomal targeting signal (PTS1), we screened a human liver cDNA library by means of a Saccharomyces cerevisiae genetic system, known as the two-hybrid system. We isolated a cDNA encoding a protein that specifically bound the PTS1 topogenic signal in the intact yeast cell but also in vitro after bacterial expression and purification. Sequence analysis of the full-length cDNA revealed the presence of an open reading frame encoding a 70-kDa polypeptide that belongs to the tetratricopeptide repeat family and that is homologous to the PAS8 and PAS10 gene products, which are required for the formation of normal peroxisomes in yeast. Subcellular fractionation of human liver and immunofluorescence studies on HepG2 cells demonstrated that this PTS1-binding protein is present exclusively in peroxisomes and that the PTS1-binding domain is located to the cytosolic side of the peroxisomal membrane. All available evidence indicates that the PTS1-binding protein is part of the peroxisomal protein import machinery and most probably is the long sought after human PTS1 import receptor.