Caenorhabditis elegans has a single pathway to target matrix proteins to peroxisomes

All eukaryotes so far studied, including animals, plants, yeasts and trypan osomes, have two pathways to target proteins to peroxisomes. These two path ways are specific for the two types of peroxisome targeting signal (PTS) pr esent on peroxisomal matrix proteins. Remarkably, the complete genome seque nce of Caenorhabditis elegans lacks the genes encoding proteins specific fo r the PTS2 targeting pathway. Here we show, by expression of green fluoresc ent protein (GFP) reporters for both pathways, that the PTS2 pathway is ind eed absent in C. elegans. Lack of this pathway in man causes severe disease due to mislocalization of PTS2-containing proteins. This raises the questi on as to how C. elegans has accommodated the absence of the PTS2 pathway. W e found by in silico analysis that C. elegans orthologues of PTS2-containin g proteins have acquired a PTS1. We propose that switching of targeting sig nals has allowed the PTS2 pathway to be lost in the phylogenetic lineage le ading to C. elegans.